997 Manual Trans Swap into 996

Gents, My 996 C2 6-speed Getrag 96.00 trans was having gear pop out and grinding problems. I was looking at over $5K minimum on a rebuild with tax and shipping. One builder quoted me over $9K. A friend offered me a used but in great shape 997 C4S 6-speed Aisin 97.31 trans for $2K and he said it would "bolt up." It is said the 997 Aisin is stronger, has less tendency to pop out and the gearing is slightly more track favorable. The approximate $3K savings, the ready now availability, and potential for an improved unit was enough for me to take my untrained, backyard skills and give it a try on my beloved 996 track car. I will try and tell the story with pics best I can. Forgive me for the typos, the formatting, etc. This is the first time making such a post and it is late at night so I am tired. My track car identifies as a GT3 but is a stock C2 with stock running gear dressed up for track duty. 996 Track Car at Daytona Dec 2023 Many have talked about how to take out the trans on the 996. I'll just say be organized, drop your engine down on the mounts as far as you are comfortable, and support the back of the engine for when the trans support comes off so you don't have all that weight hanging as a big lever off those loosened up motor mounts. I had the car up on a lift and put the trans jack under the trans. I unbolted my axles and underside covers. I then removed the trans support mounts opposite the engine. Next I took off the trans "bell housing" bolts and pushed the bolts into a piece of cardboard in the same relative position in which I took them off. Don't forget to take out your clutch slave cylinder. It is easier when the engine has been lowered on the mounts and you are dropping the trans on your jack a bit. The hoses will flex so you can get a socket in there. The extra space by dropping the engine down a little on the mounts helps a lot. Bolts in their relative position. I had already started reassembly when I snapped the pic thus the "missing" bolts You will need lots of extensions on your sockets for the various fasteners. The "wobble" extensions are very handy when you do not have a straight shot at the bolt. About the only way to get the bolt at the very top of the transmission off is with a very long and bendy set of extensions. Long and bendy extension to get top bolt Had the trans out in under 2 hrs. This was the easy part Drivers side 996 trans. The shift linkage is on the opposite side on the 997 Trans out. The 997 C4S trans might rest right up against the coolant line circled in yellow. Be prepared the fit might be snug! I also removed clutch and pressure plate because I was there...both looked good. RMS and IMS changed Sept 21, both still good. The Air Oil Separator (AOS and much anger at the yellow arrow) resides here. I changed out the AOS because it gave a few puffs of smoke on start up before the trans gave up the ghost which is a clear indication the AOS was on its way out. I put back in a stock style because is was 9x less expensive than the motorsports one. I know the motorsports one last longer but I got 3.5 years of hard driving out of the last stock unit so I'll take that risk. The AOS took two hours to remove. Was not pleasant as I could only get one hand on it, one hand to work the tools, and the tools themselves barely fit. I had to be creative on what tools to use and how come at the clamp or fastener. I found leaving the very top hose (yellow arrow) on and detaching it from the engine bay under the coolant tank was much easier. When I reinstalled the new one hooked up the hose to the AOS first then fished it through. Note the path that this hose takes. When you install the new one, you'll need to do it the exact same way. Note that when you start removing the 5 attachment hoses you will get coolant pouring out of a hose or two. Have a towel ready as the overflow reservoir will empty on you...ask me how I know. Here is the little bugger. There are two Allen head machine bolts (6mm?) that hold the AOS in place. I managed to get the new one back in, in 1 hour. It is a URO model I got from AutohausAZ. I have purchased a few things from them before and they seem to have good prices and relatively fast shipping. One of the Allen head machine screws is totally in the blind and in a hole. You have to do this by Braille. I taped the screw on to an Allen key so I would not lose the screw in the abyss. It worked and I highly recommend this technique for such applications. Painters tape seems to hold well enough but also comes off without too much of a fight. Use what you have and say a little prayer as you go. Also, it is super hard to get the spring clamps off given the tight location. Hard to get pliers in there and grab ahold of the tabs. After wrestling this for way too long and finally succeeding, I bought a spring clamp kit and used it to install the new AOS the next day. The kit helped a lot. The most useful tool is the one with the compression device on the end of the cable. I found a couple of the other pliers helped also depending not the location and angle. I got this kit delivered in 1 day from Amazon for about $40. Again, I found it super helpful in removing and replacing the clamps. Spring Clamp Kit Sorry for the lack of 997 trans photos. I was just caught up in the work and had dirty hands so was not thinking about taking photos to document this for posterity. I will do my best to explain with what pics I have. First thing I did was enlist a friend to help me take the 996 trans off the jack and put the 997 on. After that I was on my own again. I believe each trans weighs about 170-180 lbs. That is how much I weigh so I was not wrestling that up on the jack myself. I have been the orthopedic surgeon several times already. I now know better to get help with heavy things. Only took me 56 years to learn that. Having some technical difficulties with some pics. I am going to stop now and hopefully pick up tomorrow night with Part 2, the thrilling re-install!

Looking forward to part 2. I am very interested in doing this swap as well.

Subbed!

Love this type of stuff. Nice work...

I purchased a motorcycle jack from HF.. they are pretty low profile, wide and basically give you a 12"x12" flat area for the trans to sit on, a motorcycle strap around the trans held it in place. Most of the trans jacks were big $$ or too tall for what I wanted to do. It also doesn't hurt to put. a couple straps under the trans while your getting it unbolted so the jack isn't in the way till needed. HF also sells a hydraulic lift cart that works perfect for dropping the engine if you'r using a lift.

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Just for reference the 996 Turbo/GT cars have the shift linkage on the right side, and use the same cables as the M96 powered cars.

Interested in this, nice work keep us posted

Before getting to the trans I had to swap the shift cables. It would be so much more difficult with the trans in place. I got a ladder and climbed up into the car and disconnected the cables from the shifter. I don't have a pic but you push up on the spring and rotate counter clockwise and the cable end will appear. Just lift up on the cable while holding the connector fast. The cable end looks threaded but you do not twist the cable in the holder to adjust or remove it. Think of the cable end more as serrated and it matches up with the serrations in the cable end holder. You simple place the cable end in the slot where you want it. You can adjust the cable throw and gear shift lever positioning to some degree this way. Now that the cable ends were disconnected I had to get the cable assembly and the body plug they passed through to pop out of its hole. I got the plug and thus cables out by using two screwdrivers as wedges between the sheetmetal hole in the bodying the rubber plug so the edge of the plug would not grab the edge of the sheetmetal hole. I then got under the car and tugged and rocked the cable site to side while pulling fairly hard and the plug popped out. Next I had to slide one cable towards me while holding the other cable in place. This was necessary as the cable sheath ends could not pass through the hole at the same time. You have to stagger then to get one out at a time. Reverse the process to put the 997 cables in. Make sure the cables are on the drivers/trans shift linkage side of the plug so you don't accidentally lengthen the cable path. Note that there is another line of some sort that runs through the plug also. You have to work this line into the hole meant for it in the rubber plug. There is a slit on the side of the plug that you can open up with some difficulty and get the line in. As you begin the rest of the trans install, try and keep these lines as direct to the linkage location as possible. Cable length really matters so be cognizant of this as you work. 996 cables, white is the side to side movement and green is the north/south 997 cables passing through the attached body plug. White is side to side and red is north/south movements With the flywheel back in, the clutch back on (aligned with the clutch tool...essential to get it centered), the shift cables in and the trans on the jack I wheeled it into place and began jacking it into place. Eventually I got it to where it seemed the main shaft on the trans aligned to the center of the clutch and pilot bearing so I started shoving the trans into place. Sounds easier than it is. You have to be lined up near perfect top to bottom, side to side, and at the right angle to get the engine and transmission to mate. What complicates matters is this 997 C4S just seems taller and is bumping up agains the bottom of the car and the coolant pipes and hose that run there. It feels a bit longer too but it is not. Everything is tight but I managed to shove it into place. I loosely place in two bell housing bolts while I still have maybe 1/2" gap between he trans and motor. I next begin working on obstructions and clearances onto tail end of the trans/.the side opposite the engine. I don't have a good pic but this water hose was touching the front drive output. I cut about 2" off the hose and reattached the hose to the coolant pipe towards the front of the car/away from the front drive output. There was enough flex in the hose to do this w/o pinching the hose. Make sure you have a bucket to catch the coolant that will come out. Prep your cutting and do it quickly so you loose less in the process. This is the hard pipe above the trans where it nearly impinges on the trans. I used the cut piece of hose to from the previous step to wrap this pipe to give it some protection. I did not see a good way to move this pipe. Maybe w/o the trans in place you could find some way to loosen up and move this pipe to the side slightly. I was too lazy to pull the trans back out and see if there was a better way but to be honest this looked OK. What is not shown is I later pushed things around little and put a small thin block of wood between the trans and the pipe to add another barrier between the two. I had hoped to remove it when I got the bell housing bolts and trans mount in and the motor mounts retightened, however it ws good and stuck in place when I tightened everything up. I reasoned that this will keep the edge of the trans from being a point load on the coolant pipe and while help distribute the load. I will keep an eye on this and all potential impingements as I start tracking the car again. After all this swap will not totally be proven until I put the trans through its paces on the race track. I cut the bolt holes or "ears" off the front output as they were touching or almost touching hoses and cables. Cutting these off gave be comfortable clearances. Note the hose at the top of the picture. This was hitting the "ears" until I cut the 2" off the hose and moved it up on the pipe. Now there is no way they will touch. The next action was to put the clutch slave cylinder in place. This is best done before the trans is totally in place as you need the space to work on it. It is actually better to bolt this up before you get the trans and engine totally aligned and mated. It is way easier to get to the bolts, the lower one especially. There is enough play in the hydraulic flex line and it does not hurt to bend the copper line a little. You can bend that back easy enough as needed after. I next put the trans mount in place. The piece that goes over the tail of the trans was hard to get in place. It is best to do this before you get the trans full mounted to the engine. The little bit of slack helps. In general it was helpful to keep most all the mounting bolts of various kinds loose until everything is in place and then work your way around tightening up the bell housing bolts, motor mounts, transmission mount, etc. You will need the little bit of slack to have access and make adjustments. In my case the transmission mounts fit perfectly. It was a bit of a challenge getting the bolts through the rubber and aluminum trans mount. Being able to wiggle things around was critical to getting the bolts back through the two sheet metal mounting pieces and the mount imbedded on the trans. 996 trans but the mount circled in yellow is nearly identical to the 997 one. Bolts go through the two holes as well as the horizontal and vertical trans mounts. Getting the holes on all three to align is a bit tricky. Put the other mounts in place but leave the very loose. Once everything is in place then tighten all the bolts/nuts. Almost there. It is after midnight so I'll finish Part III tomorrow if able.

Interesting. Thanks for the bit of knowledge theprf!

Gents, thanks for hanging in there with me...almost there! With the trans mated to the engine, the mounts in place, I tightened up all the bolts, including that one on top that requires the long set of socket extensions. Recommend doing this one bolt early while you have the most space to do so or the tight fit will become even more difficult. Last thing was tightening up the motor mounts before the last big hurdle, the shift cables and bracket. The collars on the end of the cable sheaths are supposed to slide into forked openings and lock into place. Because of the coolant lines and the vertical trans mount, the cables did not have a straight shot to the cable bracket. They had to make a few little bends which put tremendous pressure/stress on the cables once in the bracket. I almost could not get them there but with all my might I pulled them into place. However this did not feel right. On my 996 the cables had a little play and were not stressed at all.. Surely the with the movement of the car on road/track something was going to brake or disconnect with the cables being so taught. It may not look like it but the cables were super taught when in their bracket fork or holder (yellow arrows). After studying the cable routing, I could not find a better route. The biggest issue was the vertical trans mount that I had to go under. If the cables did not have the big plastic collars around them I would have gone through holes in the mount but I was unwilling to cut this mount or make holes big enough for the cables to pass through. Therefore I decided to modify the bracket. It looked to me like I could re-bend the bracket such that I could free up maybe 1/4" of displacement that would take some pressure off the cables. After two hours using a large vice, various hammers and adjustable wrench all I accomplished was to beat the heck out of the bracket and make it impossible for the two bolt holes to line up. Maybe with a metal brake and proper shop tools I could have made it work but I was only making matters worse with my tools and apparent lack of skill. Therefore I needed a new plan. The new plan was to cut the bracket in half so one bolt hole and one "fork" remained on each half. I then bent, cut, and ground each half until most of the tension was out of the cables when mounted. I also put a washer behind each bolt hole to move the bracket further out from the transmission body as this seemed to align the cable end snap fitting best with the shift linkage ball mount. The new problem was each 1/2 bracket only had one bolt hole and the brackets would loosen and rotate/move over time. Therefore I needed to weld them back together. Oddly there was what looked like an extraneous piece of metal on the original bracket. May have served as a shield of sorts for the cables in the bracket? I could find no impact to removing that piece - which turned out to be just long enough to be the spline or connector between the two brackets! Yay...a little bit of luck. I then put that strip of metal between the two brackets which were secured in their bolt holes and tack welded the pieces together with a little MIG welder. I then unbolted the bracket, removed the cables, and welded up the bracket more fully on the ground Note shift linkage on the trans that does the north/south movement is long and has a long movement. The modified bracket could easily impinge on the linkage movement so always be checking that clearance is important. if you feel you need to do this cable stress relief process at all. Your car, trans, and cable routing will probably vary so this might not be something you need to do at all or you will find a better way. I felt it necessary to modify the bracket in my application. Modified bracket Arrow is relief cut and circles are the weld locations. I ground the welds a little to make it look a little better. I then painted the bracket with a bit of left over high temp orange paint so my work did not look so sloppy. I am not a fabricator so not pretty but functional. The cables once mounted were under much less stress. I also put two zip ties on each cable collar to keep the cables in the bracket. The 997 cables were used and their collar snap locks were worn on partially missing. I also invariably changed the fork shape a little so zip ties were a little useful insurance. It is also just a good idea to put the zip ties (or wire in place) if you are going to drive hard or track your car whether or not you do a swap. My "beautiful' bracket Cables mounted, zip ties helping insure they stay in place and cable ends snapped on the linkage ball mounts. The rod from the cable sheath to the snap mount is fairly well aligned now and the cables are not pulling too hard. You can see where the cables go under the mount. This is the bend that threw everything off. I tested the linkage and the sifting with the gear shift inside the car and then I re-checked the movement under the car. Everything seemed to be moving properly. The gear shift movement is not quite a free moving as my 996 set up was but I think that is due to that last bend under the trans mount. It is not bad but not as buttery as the original. The numeric cables might be better but they were 2.5 times more expensive and I was not sure they would not have similar problems with the bends. Supposedly the cable and sheath is bigger so might have been more difficult to bend, so it was not worth the risk. Maybe some day it will be. Last thought on the cables is if there was an easy way to pop the sheath ends/collars off then going through the existing holes on the trans mount might have been perfect. Now as I look at the pics I wonder if I should have attempted that. They just might come off w/o too much trouble. I am sure there are better ways to solve the problem than what I did. You do you and find a better path or more accommodating cable set. Next time I have the car in the air I will probe that collar removal idea. If that works it would be important to protect the cable sheath going through the trans mount bracket holes otherwise you might cut your cables on the sheet metal over time. Remember to check your clutch slave cylinder for proper operation. If you have not done so in a while bleed your slave cylinder and the hydraulic line. Get fresh fluid in there and remove any trapped air that might have been introduced. Also don't forget to fill up your coolant tank for whatever you lost if you cut water hoses to make shorter or if you replaced the AOS and got a shower from the overflow bottle. If you didn't reconnect the small coolant line that connects under the overflow bottle, you will want to do that. I have run the car up to temp three times now and I think I have finally got the trapped air out or so it seems. I have not had my car on the street but the clutch is engaging nicely and the trans is going into each gear. I putted around my backyard and could test reverse, 1st and 2nd to some degree and everything is working. I engaged 3rd - 6th and the gears seem to slide in fine. I will not know the true status until I get it to the track but it feels like everything is working. Maybe I'll "sneak" the car around the neighborhood enough to test each gear and engagement. Either way I will report back once I have more experience operating then trans. I took the opportunity to change the gear oil (75W90) - about 3 quarts. I used Mobil PTX (Porsche Transmission?) that I got from the dealer. Everyone has favorite oils but the parts guy they said they use this in all their racing transmissions and Joe Cogsbill from Cogs Cogs transmission said this was designed for these transmissions and worked best in his experience. The gear oil I drained out looked pretty good and was only slightly darker than the new oil. I put a kitchen green scratcher pad in my oil collection and filtered the oil that came out. I did not find any particles. The oil I pulled out of the 996 did have a bit of "glitter" in the oil so something definitely went bad. Bottom line is he swap can be done! I bet the AWD/front drive on this trans made it slightly taller and that is why I had extra interferences to deal with. Not sure if the 997 C2 or C2S box is dimensionally different. Thanks to Matt J in the parts department at my local Porsche as it was his idea for me to do this write up. Sadly this was after I was done with the job otherwise I would have taken more photos along the way. Hopefully you will find this helpful or at least instructive! Best, Gary L. If I can save a few more pennies I think I will try and have the 996 box rebuilt along with its LSD or a new one depending on pricing. It would be great to have an extra box should I encounter problems again. Honestly though I ran this car HARD for 3.5 years and the guy before me said he only ran it for 15 or so hours. I find that highly doubtful unless he was a wuss. This car is no Cup car but a blast to drive and except for this and the AOS has been relatively cheap to run. Guys with more power and fancier cars often pass me but not all of them. I surprise a lot of people and usually get some props by the end of the day. They are surprised then I give them the pass and then they cannot shake me off their tail except for in the long straights. While I fantasize having a Cup car, I love my little beastie. A Cup will likely never be in my budget to purchase or maintain. Drive it, brake it, fix it, repeat!

Gary this is awesome. Thank you. Can you clarify which clutch Parts you used, 996 or 997?

I kinda want to do this swap. Of course i cant on my car but i would be very willing to work with someone who was in my area to make the labor very affordable. Im 20 min from philly.

I kinda wish I had gone this route instead of building my box… big props to the OP for the write up and best of luck it shreds hard at the track.

A vacuum lift is priceless if your cracking open the cooling system. It will pump the bubbles out of the system nicely on my 996 both times I drained it it was painless to put the coolant back. Great writeup!

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C Didy, I retained all the 996 clutch parts, the flywheel (kind of clutch'ish), the EPS clutch and pressure plate I was already using, the pilot bearing and throwout bearing as well as the lever arm (sorry...not sure of the same) and even the slave cylinder. I had the "new" 997 Sachs slave cylinder on the 997 trans but it looked like an identical Sachs factory unit to the one on my 996 and since that was still attached to the hydraulic lines and had no visible wear or leaking I decided too not swap them out. I did not want to bust ope the system and have to bleed it. I have the Motive pressure brake bleeder so it probably would not have been an issue but why fix what was not broken. It turns out the clutch and slave cylinder appear to be working fine. Great question!

pdxmotorhead...yes, I need to get one of those. I have been running the car up to temp and filling what gets drawn down. I will be watching this carefully the first time I get this out on the track. HOD (Hooked on Driving who I coach for) is have a Coach Appreciation Event at The FIRM in North Central Florida in a few weeks. It is a small, low threat track about 3 hrs from me so that might be a good venue to test at w/o a large audience or big impact to other drivers if I have an anomaly on track. How do you use the vacuum lift? Attach it to the overflow/fill bottle and pump down the pressure and run the car up to temp and it draws out the bubbles? Do you have to repeat several times as you add more water/coolant?

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Drain all the coolant you can, attach vacuum unit to the reservoir and remove all the air from the system. Make sure it holds pressure and then prime the fill line and attach it and put it into your coolant bucket. Turn the valve and allow the coolant to be drawn in until there is no vacuum left and then the system is full. It's a must-have for any water-cooled Porsche DIY owner.

Hey guys! I went up to a small track in north central Florida called The FIRM and ran the car hard and it was GREAT! Shifted smoothly, the heal and toe was clean and nicked right into gear. Loved it. 2nd gear required a quicker shift than it did before. I am up against the redline faster due to the gearing. Sadly 1/2 way through the day the trans rubbed a hole in a rubber coolant line. I believe I mentioned the trans was a tight fit with some of the coolant lines. In one case I shortened the hose which straightened it out and left good clearance. That hose was fine and the gap remained. I had installed a small block of wood to make the gap slightly larger between the trans and the metal part of the other coolant line. Unfortunately with all the aggressive driving the block came loose and the hose started to rub occasionally against the front output shaft and it rubbed small hole so I was done for the day. The good news is the fix is easy. I'll take a pic when I get the new hose. Basically I put a pair of hose clamps around the metal part of the coolant line and another hard line under the car in close proximity and forcibly moved the coolant line over with the slack/flexibility that exited in the coolant line system. I tested with the broken hose and looks like it will maintain a safe gap. Honestly, I should have done this the first time around and I was intellectually too lazy to see my bumper block idea was not really going to work for the long haul. Chalk that up to another lesson learned and the tuition was a shortened track day and the purchase of a new hose. Best, Gary

Originally you thought the shifting mechanism was going to be more resistant in comparison to your 996. Do you still feel like the routing of the cables contributes to the stiffer shifting action?

I was able to relieve much of the stress to the cables by modifying the bracket that holds the cables to the transmission. Initially I felt a little more resistance to shifting but after settling in the shifter was fine...smooth with the stock cables. I am sure the Numeric cables would be more smooth but I am satisfied with the stock ones. The did not draw any attention to themselves while on track. The downshifts were all good. I will say the spring mechanism that centers the shift lever in neutral between 3rd and 4th does feel stronger in the Asin trans of the 997 than the Gtrag in the 996. That took minute or two to get used to but no big deal. In fact I think I prefer it so that I always know where I am when in neutral w/o looking. Does that help or answer you question?

Did you take a photo of how you modified the cable to trans bracket?

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There r detailed pics on the 1st page, scroll up?

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Thanks for posting on your project. Let me know if you have another 997 transmission, haha. I agree some of those spring clamps are not accessible with standard pliers. I got a spring clamp tool but it petered out after one project. Do you remember which kit you got on amazon?

Nothing special, the DAYUAN brand. I have used it a few times now and is doing OK. None of these look like super quality but it is working! Amazon Amazon When I save up a few bucks I might send in my 996 to be rebuilt so I have a back up.

I did some calcs on the 996 and 997 trans gear ratios and theoretical max speeds. In sum with the 997 trans I have to shift on average about 5% or 6 mph sooner. I noticed at the track my shift points seemed to come sooner. I also learned that with the 3.4L or 3.6L at the max RPM is 7,300 but I generally try and shift by 7,000 for a buffer and to preserve the engine. I found the calculation formulas here: https://www.jegs.com/tech-articles/g...las-resources/ I found the 996 gear ratios here: https://blog.fcpeuro.com/the-definit...-transmissions I found the 997 gear ratios here: https://blog.fcpeuro.com/the-definit...26167373824li0 I found the 996 car/engine specs here: https://www.excellence-mag.com/resources/specs/63 Through tire seller sites I found the tire diameters. Generally the slicks I run are 25.2" diameter. Spreadsheet attached if you want to see the comparo!

Guys, as one of my coolant hoses rubbed the output shaft during hard driving and got a hole I made a few tweaks to the set up to solve the problem. See the pic below. Everything seems fixed and sturdy. One hose clamp was probably enough to move the coolant pipe the 1/2"=3/4" that I needed but I put a second one on for safety and then a tie wrap to encourage the clamps not to move. I also wrapped the new hose closest to the output shaft with some of the old hose as a buffer. I will be check this next track day to see if anything moves or gets rubbed but I don't think I will have an issue. When I was done I painted the output shaft and the hose clamps flat black to help them hide and protect against corrosion. Hose clamps to fixed point to relocate coolant pipe. Also put some of the old hose around the new hose as a buffer against the output shaft

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Great write up! After 3 years and about 10k miles, the edge of my 997 transmission rubbed a hole in the hard pipe. Leaked out most of the coolant during a cross country drive. Looking at aluminum brazing to hopefully fix the pipe. Otherwise, might need to drop the engine to replace the hard pipe and maybe bend it to accommodate the longer transmission.

What I described did not work as well as hoped but what did work and is still work...just tracked my car hard at Daytona today...is using hose clamps to pull these pipes out of the way by about an inch. Don't know what at the small hard body mounted tubes were that were under the car and near by but I put the hose clamps on those and around the pipes and tightened them up and it worked and continues to work.