RyanPerrella

Ive checked the Rennlist forums 928 & 944 group about the install process when doing rings and pistons.

I know some of you here had some good posts on the subject and i would love if you could share your experience's here. I know we have some racers that have no doubt put these engines together many times over and i would love to get any tips or hear from you about anything in relation to the bottom end.

I have my heads ready to be reassembled and am now turning my attention to the bottom end. I want to pull the pistons, clean them up, re-reing and replace rod bearings. Leaving the block halves alone and thus leaving the main crank bearings as ive heard not to touch them on a 100K miles engine. I would like to add some value to the engine though by doing the rings and rod bearings.

My questions and concerns relate to the rings and the cylinder bores. I am pretty sure the procedure is as follows: The bores are not to be honed, not touched, just cleaned up and coated in a mineral based oil for lubrication when reinstalling the pistons and rings. I have looked pretty closely at mine and when i first saw them i was somewhat shocked as they really looked excellent. Thus no need to touch them. I guess i am a little curious on how new rings will react with a block thats got 100K miles on it. Will the new rings put extra wear on the block or vice versa? Or is this just a complete non issue? What ring manufacturer do you reccomend if any, conversly who would you stay away from?

Am i wrong in my thinking that i should leave well enough alone with the crank and main bearing's? Or should that also be done. Engine has 110K miles on it? That being said should i just leave the rings alone?

Thanks in advance guys

Zero10

Did you do a compression test before disassembling the motor? This will tell you the condition of your current rings.
You will find that there are varied opinions on what to do at this point. Many will re-ring without having the cylinders lapped/honed, others will have them cleaned up by a machine shop, and the rest will advise you to leave your current rings in.
My advice is to re-ring without touching the cylinders. If you take it in and have the bores cleaned up so the rings will seat better, then you are removing cylinder material, and increasing the piston-wall clearance ever so slightly. If you don't then your rings will take longer to seat, but IMO it will result in a tighter piston-wall clearance. Since you are already in this deep, my opinion is to change the rings.

Stick with OEM rings, which IIRC are made by Goetz (sp?). I have a set for a 951 kicking around my house, but that wouldn't help you much

As for what bearings to do, I am a big fan of 'while I am in there....' work. Since you are in this deep, you might as well pull the crank and do mains/rods. 110k miles is still young, so this would be strictly preventative maintenance. Gives you a good chance to take a look at what is going on in there. Also, if you didn't do them, and they failed later.......

RyanPerrella

Yeah i was looking in the WSM and heard others tell me to pull the pistons and clean out the bore but not to touch it otherwise

Tom R.

think about the silicone lining in the cylinders.

RyanPerrella

on a 928 and i assume this is the same for the 944 and 968 blocks, the silicon is impregnated in the aluminum alloy blocks. This is not a lining, or a coating, is my understanding.

Oddjob

Yes, the silicon (definitely not silicone....) is alloyed with the aluminum in the entire block casting.

I have heard the same about the crank main bearings - last forever.

Compression tests are not that accurate to determine ring condition, leakdown is more telltale - but also prone to error and inaccuracy.

I agree that if you have the bottom end opened up, do the rod bearings. And probably not a bad idea to do the rings. You can check wear on the rings after pulling the pistons: check the end gap and see how it matches the factory spec range, and look for any marks, lines, defects on the edges.

Goetze is a pretty common brand of rings that can be bought from most aftermarket p-car suppliers. OE rings from a dealer may be prohibitively expensive (944T rings are over $400 for a full set from a dealer), but may be worth pricing out just in case. The set of Goetze rings I put in my turbo motor did not meet the factory new end gap specs - but were better than the original rings that I removed. My belief is that the OE Porsche rings may be of a higher quality or tighter tolerance than other aftermarket suppliers (even aftermarket rings made by the OE manufacturer).

M758

While they may last forever it is a good idea to replace the original ones. Rod bearings do fail and loose main bearing clearances will cause less oil to flow to the rods. So while the mains are "ok" the rods are starved for oil and go bang.

If you are in there will everythign out it makes sense to replace the mains and the rods. My rule is crank comes out new bearings go in.

PorscheDoc

Replace the mains while you are in there, they do not last forever.....

Here is the thrust out of one of the engines I am rebuilding right now. The other engine i am rebuilding, it's mains looked the same.

Oddjob

I would not consider main bearings to be "ok" if they are worn enough to reduce oil pressure.

Have you had been able to confirm rod bearing failure due to worn main bearings?

Thats an interesting point - you typically hear guys talking about cross/perp. drilling the crank at the rod bearing, but I have not heard much discussion about the oil flow (and possible pressure loss) through the main bearings as being a potential cause of rod bearing failure.

M758

I have not confirmed it, but looking at the system it stands to reason.

Main bearings are supplied oil through the galleys in girdle and block. They get plenty of oil. The rods get their oil through the crank and that oil is supplied from excess to the mains. So the "leftove" oil from the mains goes to the rods. What this means is that looser main bearing clearances allow more leakage and will in reduce the oil supply to the rods. So this means the rods will have less "extra" oil capacity. It is my feeling that the #2 rod has the least excess capacilty to flow the bearings. So an interuption in oil or hot thin oil shows up as spun number 2. The other area have enough extra oil flow to survive a momentrary air pocket or other. The number #2 followed closely by the number #3 is weak. So they spin first. All it takes is once minor hiccup and the number #2 fails to float. Metal to metal contact results with spun bearing seconds later.

So anything that weakens the flow of cool high quality oil will result in greater risk. The same way changing rods helps changing mains helps too.

RyanPerrella

Joe, I agree with you that if i pulled the crank i would replace the main bearings. I however have not pulled the crank and am looking at pulling pistons, and rods, obviously replacing rod bearings also, but i have not split the crankcase halves and dont intend to, unless main bearing wear were an issue. On the 928 the Trust bearing goes out on the auto cars but not on the manual trans cars as mine is so equipped. I will probably check crank end play and if thats within the low scale of specification then i will probably leave well enough alone. If i do pull the crank i suppose i would have to find a machine shop to polish it and replace the main bearings which are another pretty large expense.

Jim,

Thanks for the heads up on the Goetz rings, i was probably going to get a set of OEM rings as the OE Porsche ones are $113 per piston, where as the aftermarket has what I would think are the same quality for 1/4 the price.

I thought of buying a couple sets and maybe 12 ring sets and taking the best 8 from them. The rings are too loose, to large a gap with the Goetz ones you purchased, as opposed to being to tight, in which case you would use a ring file on them.

M758

On a 944 or 928 engine to pull the pistons you really close to pulling the crank. You are only 8 bolts away. I would not go that deep into the engine and not do the mains.

Polishing the crank is not required, but on any 944/928 engine I would do those bearings.

RyanPerrella

actually the crank is held together with closer to 20-22 nuts. Perhaps the bottom ends of these are different. The 928 has no crank caps, but an engine girdle that makes up the lower portion of the block. Similiar to modern F1 block designs, and as used on the new BMW M5 V10. I figured the 944 had the same design but perhaps it dosent.

Zero10

8 bolts away in a 928?.. He was talking about splitting the case. I've never worked on a 928 motor, but that sounds like a lot more than 8 bolts.
I agree with checking the end play on the crank. This is a pretty accurate way to diagnose the condition of the thrust bearing.

Oddjob

944s have the similar crank girdle setup, and there are a series of bolts, along with the nuts for the main bearing studs (probably 20+ nuts and bolts) - 2 step torquing sequence using the torque angle method, and it also requires the magical (and expensive) loctite 574, and a bunch of screwing around wet stoning the oil pump mating surface.

I went through this on my turbo engine rebuild: After discussing with a local p-car shop (very reputible within the region, these guys build a lot of 944 engines for club racing), I planned on only doing the rod bearings, since it was a relatively low mileage engine.

Unfortunately, I had to have some welding done on the block (the AC bracket mounts were cracked). The welding shop did not cover the bottom end when grinding, so there were metal shavings in the engine - so I had to pull the crank and pistons to clean it up. So then I of course replaced rings and main bearings. Had I not had to clean the entire block, I would have left the original main bearings in place.

The factory 951 rings are chromed on the top side of the top ring, so the OE rings are over a $100 per piston. I went with the Goetze rings, for $100 for a full set of 4. Interestingly enough, the Goetze box was marked for use with a 951 and 928. However, the Porsche part numbers are different for 951s and 928s - so again, who knows what, if any, difference there is with the factory rings. The end gap on the Goetze rings was too large per the factory specs, and I recall the problem with just with the 2nd compression rings, but all 4 of them were equally out of spec.