2nd IMS bearing after 36k....

Hi all, just had the clutch and DMF replaced on my C4S which afforded a look at the condition of the IMS (which had already been replaced 36k previously). Lots of grease and oil pushed out past it so went for further inspection and replacement. Assuming I have attached the photos correctly you can see the existing bearing is an SKF explorer bearing with the seals flipped off which I will presume have been like that since install. There is noticeable play in it compared to the new one that's gone in. Not adding any opinion as there is already lots out there... Just thought someone may find the info useful. [/URL][/img] [/URL][/img]

I am no expert on bearings, but I note that the bearing pictured is a 'C3' specification which means that, when at room temperature, there will be a noticeable 'play' between the inner and outer races. It is designed to deal with situations where heat build up can cause short term expansions that would over stress a standard bearing. Typically they can be used in high temperature and/or high speed environments. Others, with more knowledge may be able to comment whether the C3 specification is the correct spec for this bearing. I would be really interested to know the answer. If your new bearing was 'standard' then there would be a noticeable difference When I first started looking at the whole ims issue, as a newbie, I couldn't help thinking that there didn't seem to be anything special about the bearing, that required the multi-hundred pound solutions being offered by certain companies, despite their best attempts to persuade otherwise. Forget the roller bearing arguments and so on, what was needed most importantly was a new replacement bearing, if there was the slightest sign of wear. Checking for this especially if the gearbox was out, is a no brainier. A top quality 6204 standard or C3 spec bearing (FAG or SKF, say) is less than £10. (I know the single row bearing is only applicable to later models etc) Anyone solved their ims issue at this level (along with OP, by the looks of it)? I certainly know of one indie who keeps a 6204 in the freezer box of a fridge in readiness for this job.

The bearing in this example is basically very close to the design limit for the loads and speeds applied but premature failure is usually caused by the interference fit in its housing being too tight and resulting in high initial wear rates depositing metallic particles inside the trapped grease and turning it into a type of grinding paste prematurely wearing the bearing and the dust seal (which we have physically uncovered). Ball bearings run reliably on miniscule amounts of oil and in the original bearings the heat softens the grease and eventually it leaks out of the seals after which some small amounts of oil get in (but sufficient to keep the bearing running) - and often last for very high mileages indeed. Removing the old bearing and replacing it often results in a tighter fit for the new bearing because the housing can distort with stress relieving over the years of use and/or damage the housing surface finish. As a result it is important that the "fit" allows for this and to compensate for the variations a C3 would be the right bearing in all those circumstances (which it has been correctly explained is not as tight as others). The bearing sits at a level below the oil bath an anyway the chains inside running at up to 40mph at peak revs spray oil everywhere that lubricates the bearing, keeps it cooler and flushes away any wear particles. The loads from chain flap (exacerbated by the poorly designed chain tensioners that have no oil seals) both radial and linear can still wear out a bearing of this size - while others last almost indefinitely. The original dual row bearing had a slightly lower rating than the single row bearing and created more wear particles and ran hotter but if they got through the initial wear situation and survived probably lasted slightly longer after that than the single row bearing. Roller bearing replacements offer more radial support but less axial support while plain bearings with a pressurised oil feed are probably the best solution if the overall size cannot be changed but are much more expensive. Ceramic bearings can suffer from shock loading. The larger later ball bearing is the best solution if the engine is being rebuilt because but it needs an engine rebuild to fit it. Replacing an old small bearing can damage the housings and so if the original is running OK (even if there is some play) may not result in longer life than leaving the old one in place - but this can only be judged by experience and there is always some risk involved whatever you do. 35K is typical of the lowest lifespan to expect from an engine running on the original thinner viscosity oils recommended for new cars. Running with thicker oils as the car ages and with a LTT (to increase viscosity even further) can extend lifespan. As always there is an element of random influences that can result in the very same solution working better and longer in some engines than others. It is very rare for there to be any problems with the larger bearing but the standard fir is still grease filled and running with low oil levels or thin oils for too long can very rarely result in failures. As always with this engine type advice cannot be specific about expectations but experience has shown which outcomes have the best overall reliability and value even though very occasionally one of the best solutions might still cause problems - it is all a question of following the proven best solutions and hoping that your engine falls into the category of the majority with that solution that will never experience a problem again. Baz

@baz Was there ever any 3.4 litre engines that came with the larger (only replaceable in rebuild) type IMS's?

The new 3.4 short blocks have them.

I think the exchange short 3.6 engines from Porsche have a latest spec bearing as well.

Do you know from when Alex? Steve

From when they staryed doing them a few years ago deffo. Maybe since they went to the larger one on the 997? :dont know:

Mine was a 3.4 2010 replacement engine and has the larger non replaceable one I believe.

I'll check what the engine number is at the weekend and look into it