Engine Balancing 101

I'm shopping around for a good machinist to do an engine balance for my 3.6L rebuild and want to be armed with the correct information before I select a shop. Since the boxer engine is a different animal from a V configuration that 99.999% of shops are probably used to seeing 99.999% of the time, I want to be able to ask the "right" questions to find the right shop. So far, as I understand it, the basic premise of the boxer engine design is such that the two sides of the engine roughly cancel themselves out, making it "perfectly" balanced in theory . Because of this, a machinist doesn't use bob weights to simulate the rotational mass as they normally would to balance a V6 crankshaft for example. So a rough procedure for a Boxer design would be as follows: Balance pressure plate on it's own. Balance the dual mass flywheel on it's own. Balance the crankshaft on it's own (no bob weights, but perhaps with crank pulley attached?). Balance the crank, DMF, and pressure plate together. Ensure no run-out on DMF. Weight match pistons and pins Weight match rods. Total, big and small ends as well as center beam check. Gap rings Ensure rings, pins, and clips are weight matched with pistons. Index all components so they are assembled in a balanced manner. Does that sound right to any experienced engine builders or machinists out there? Thanks in advance!

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Sounds about right. The only thing I think you left off is the heavier rod/piston assemblies want to be located closest to the flywheel. Might add since you are in northern CA Jerry Woods' shop is not too far from you. IIRC he's located down in Campbell CA. Give him a call and see if he'll share with you any tips on balancing these engines. Here's his shop's address: 491 E Mcglincy Ln # 1 Campbell, CA 95008-4901 [phone removed] Here's a link (though the web site is not fully fleshed out...) http://jerrywoods.com/ Also, I note that Jerry and Bruce Anderson (of Excellence Tech Notes) hold a Porsche engine rebuilding class... Might be worth checking to that to see what's offered, covered and how much it costs. Start here: http://www.pelicanparts.com/techarti...ruce-jerry.htm Or drop Bruce a note asking him the questions you posted above. Sincerely, Macster.

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Logray, that all sounds right, but the rings & clips should be within weight tolerance already & I would be surprised if they are used in the process. Once all rods,pistons & pins weigh the same the only consideration of which cylinder to install them in is bore clearance & the pulley & flywheel are already indexed to the crank so marking pressure plate orientation to the flywheel is all that's needed.

On my 914 engine (4 cylinder boxer), I had Jake Raby balance it. To my knowledge, he just balanced the crank, pulley (really a fan), flywheel and pressure plate as an assembly. all of the pressure plate bolts were labeled so they could be reassembled in the holes the were balanced in. Then he weight matched the connecting rods - both by balance point and by overall mass. Pistons were also weight matched.

Is it also possible to reduce the weight of various components ie. pistons, rings, valves etc. with aftermarket engine parts, in addition to weight matching?

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Yes you can install lightweight components for example aftermarket JE pistons that LN engineering supplies with their Nickies rebore.

Ok, thanks everyone for the replies, I think I have it mostly visualized in my brain. And thanks Macster for the referrals, I will give those guys a call. Dumb question time, this is engine balancing 101 class after all. My flywheel has 75k miles and the spring checks out per Porsche's TSB. The surface does not look that abused. Since I have my engine apart and am having it re-bored, should I apply BOAT logic (bet on another thousand) and buy a new flywheel? (I own a boat, and know how easily the $1000's can flow). My clutch has about 35k miles on it, and while there is some pad left, it's probably about 60% ish gone. Should I invest in a new clutch kit as well? Here's where I'm a little foggy still. Let's say I DON'T invest in a new flywheel and clutch/pplate and have my existing parts balanced with the crank/pistons/rods/etc. Life is happy and balanced for a while until those parts wear out. Then say in about 20-40k I invest in a new clutch and/or flywheel. Does that mean complete engine dis-assembly again to achieve proper balance? OR can I just take the new flywheel and pressure plate to the machine shop to have them balanced (AND) would that require "remembering" how the prior pressure plate was indexed to the rotational assembly and then applying that logic to the new pressure plate/etc.??? Am I making sense here? I hope so. Looking for some practical advice and thanks again!

Balancing is somewhat motor specific. Different types of designs require different methods of balancing. Your motor should not require balancing, no matter how many miles it has. The factory balances the crank, flywheel and the rods, pistons and related parts are balanced closer than needed for the factory redline. If you were building a very high rpm race motor and wanted to lighten the reciprocating weight, balancing would make sence. Also if you were to machine the crank, knife edge the counterweights, stroke it or otherwise modify it balancing would be necessary. The 996 motors are really a very low rpm and very smooth at max revs as is. I'd say that balancing is not needed at all. In V motors where opposing forces are less than 180 degrees apart a balance factor is used. This is reciprocating weight as a percentage of the rotating mass. The center balance point of the connecting rods by weight determins the division point. Changing the balance factor up or down with make the motor smoother at either a greater or lesser RPM. So there is no absolute best factor even in the same type motor, it is application specific. In a flat motor the crank is generally balanced by the component manufacturer for that application. If you put a new crank in your motor it woud already be balanced. Same for rods and generally pistons. In most modern motors, pistons are of the same weight as OE. This is so even with an oversize, the original weight is not changed and does not affect engine balance. If you are fitting oversize pistons, compare the new to the old. Weigh as an assembly with rings pins and clips. If a difference arises the larger pistons should be made to weigh the same as OE by generally drilling or removing material inside around the boss of the pin. If fitting new rods, they should be weighed as well to be sure. If using the original, I'd recommend inspection, magnaflux or simular. New bolts/ nuts/studs. Polishing the crank is an option. If you have the cash and there's plenty more where that came from replace the flywheel pp and disc. If you don't mind replacing them again in xxx miles, I'd use up what you have. New or old flywheels should not require balance. I have balanced motors for over 25 years, have all the bobweights, fixtures, scales etc. If you want to improve your motor I'd pay most attention to the heads. Have someone help you with a precision valve job and an honest no BS clean up and matching of your ports. It can be really tough in that regard as many are less than honest and really don't understand what they are doing and realize their customer certainly does not. Good Luck

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These are good questions regarding the clutch to ask the techs doing the balance, however I would replace the clutch assy and at minimum check that the pressure plate spring tension is within tolerance.

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Nick, thank you for the lengthy response. A little bit of background might change your mind? I've always thought anytime you change the weights of pistons/rods/bore size/etc. the engine should be balanced. Perhaps over time the crankshaft has shed some metal and could use a few 0.XX grams trimmed here or there. In any case, I',m having a 3.4L M96-01 rebored to 3.6L by LN engineering, along with new larger and lighter JE pistons. Shouldn't I worry about balance because of this? At the very least weight matching the pistons and pins (or verifying the factory did their job). Re-using old crank. But I suppose a new question, since all new pistons, pins, clips, rings are going to be lighter than the OE pistons, should that mean I need to do something different? Or just make sure the crank itself is balanced, and then that the pistons themselves are weight matched? Magnaflux is on the shopping list for the crank and rods. Old rods will be re-used. New ARP hardware is going in. The cash is certainly a problem, so if it's not a problem to "use up" what clutch and flywheel life I have left, and have it all balanced now and then replace it later (but perhaps balance the new???) I would certainly rather go that route. As for the valve job, that was done 1000 miles ago. I checked valve seat seal after pulling the heads last week and it is 100% spot on after the valve grinding. Regarding the valve guide measurements (the provided) and valve guide replacement (5 of them), and all new valve seals, I can only trust the shop that did the work. Thanks again.

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Thanks, I certainly will ask them and have my eyes on a clutch assy, but certainly can't spend the money unless it is needed.

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Whoops meant to say check spring tension on dual mass flywheel or that it breaks away freely and not seized . As in part min and max tolerances that also applies to the wallet.

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Ok, yep NP, I certainly tested the old DMF and it takes quite a bit of force to spin it by hand left or right then it stops after 15mm or so, and it does return/spring back to the original position like it is supposed to according to the Porsche TSB 1360 for DMF checking.

Logray, if you are reusing the OE crank it should need no attention providing you don't remove it and disturb the bearings. Don't remove it unless you must. You can damage the bearings by doing so and create a lot more work and expense. The crank needs no re balancing to compensate for wear. If it was worn enough to need it, it is junk. If you pull the crank, you will likely find either rod or main journals that need minor attention at best and a few bearings than could use replacement to make it all perfect. This will require new crank and rod bearings which will be expensive, but more so rod big end refirbishing to make them round again, and a crank regrind to remove any minor scoring. This will also require a very lengthly process of choosing the correct bearing sizes. This proceedure varies depending upon the motor but can get very costly. So, leave the crank in the motor intact or be prepared to do it all. More later...

Before I had the engine apart, I had noticed a possible issue with run-out on the crank pulley. After some measurements with the dial indicator, the run-out did not appear to be excessive (.020") IIRC. When I removed the pulley and tested the radial run-out on the crank nose, I found even less (.002-3"). What was a little troublesome though was I couldn't find the run out on the pulley itself (sans expensive machine to test it)... anyways sort of resolved that issue in my mind already. Anyways, this wasn't the reason for the rebuild. Horrible compression (150psi) and loss of oil seal. Maybe curiosity got the best of me for the crank and the play earlier. Well, I've gone this far and am going to be replacing all of the other internals, cylinders, pistons, ARP main and rod bolts, etc. etc. etc., why wouldn't I open the bearing carrier and have a look at least at the bearings? If they are perfect perhaps I should put it right back together then. Long story short, I've already split the bearing carrier and have ordered new main and rod bearings (total about $125). I don't have the new bearings yet to measure old vs. new, but from a visual there doesn't appear to be much wear at all on any of the main or rod bearings. In any case, I'm not about to spend this kind of money rebuilding an engine without having the components balanced (or at least checked) before I put them back in. At the very least I would like to verify the crankshaft, dmf, pressure plate are as balanced as they can be before I put them back in. If they are already perfectly balanced, then so be it. And for the new pistons, they are probably machined to very tight tolerances from JE, so I am guessing there won't be a lot of weight matching to do. And again on the rods, I intend to weigh or have them weighed, and again, if they are already perfect then so be it. big rod bearing small rod bearing main bearing carrier & crank

[QUOTE=nick49;8980864]Logray, if you are reusing the OE crank it should need no attention providing you don't remove it and disturb the bearings. Don't remove it unless you must. You can damage the bearings by doing so and create a lot more work and expense. The crank needs no re balancing to compensate for wear. If it was worn enough to need it, it is junk. If you pull the crank, you will likely find either rod or main journals that need minor attention at best and a few bearings than could use replacement to make it all perfect. This will require new crank and rod bearings which will be expensive, but more so rod big end refirbishing to make them round again, and a crank regrind to remove any minor scoring. This will also require a very lengthly process of choosing the correct bearing sizes. This proceedure varies depending upon the motor but can get very costly. So, leave the crank in the motor intact or be prepared to do it all. Now, I'm wondering if you can remove the crank with the girdle and bearings intact from the case? I'm thinking so. I didn't know you could bore the M96 motors unless you are sleeving it, then fitting the oversized pistons. If these are made for a 3.4 then you can be assured they were designed for a drop in without any major work. You may want to check with JE re needing balancing. If LN is is doing the sleeving, they will likely bore each cylinder to each piston or check to make sure all pistons are within a couple of tenths of each other. Ring gap should be ok as is and not need to be fit but be sure to check. They will know what to do. Sorry, I keep jumping around from the machinist to the customers POV. How far down are you going to take the motor yourself? Bottom line, study everything, ask lots of questions. Know exactly what everyone involved in the process is doing, know what you need to do. Good Luck!

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Yes the pistons are made by JE specifically for the 3.4L->3.6L rebore that LNE does with their NSC-billet sleeves. Although they are mostly likely going to drop in, they do advise some preparation: "Pistons as well as wrist pins should always be balanced and cleaned thoroughly prior to installation." "Piston diameter must be measured at the gauge point, which is .500� up from the bottom of the skirt unless otherwise specified, as with all FSR type forgings. Maintain a minimum of .001� and do not exceed .0025� total piston cylinder clearance unless otherwise specified with JE Pistons when used with Nickies cylinders." I believe they install the sleeves creating 99mm bore. Then the JE pistons are provided, but they do not match them to the corresponding cylinder. As you can see above they do recommend weight matching the pistons, even supplied from the JE factory. Since I'm doing the assembly (they don't assemble), they do advise checking the gap. "The rings supplied with your JE Pistons and Nickies cylinders have been selected for optimal performance with your Nickies cylinders. Most ringsets will not require gapping, but it is absolutely imperative that ring end gaps are checked and gapped if necessary." It's pretty much 100% apart at this point. I even had to extract the IMS bearing again to replace the center support stud (stripped the slotted end badly while removing the 12 pt nut). The heads will not be dismantled, as the valve job was already done a few months and 1000 miles ago.