Porsche Enthusiasts Club Forum

Just started checking things out on the new toy (1991 951 with 155k on the clock.) and there was quite a bit of oil around the input to the turbo, I tracked it down to the breather and to cut a long story short, the head is now off and and although the bores are very clean with no scores there seems to be a fair bit of wear, especially if you read the Porsche manual (these things appear to have pretty tight tolerances.) pre strip cold compressions were all around 125 and apart from an issue with the knock sensor that was messing up the boost the engine ran surprisingly well, oil pressure was 4 bar hot.
My question / dilemma, do I just re-ring it or go for a full rebore etc, Ive read that pistons are almost impossible to get hold of unless you are very wealthy.
Your opinions and experience would be greatly appreciated

I think you can get the bores re coated with Nikasil. If your gonna do that you may as well replace the big ends and all the belt related gubbins on the front. If you are going to keep it at stock boost and not track it then I'd Skim the head, stick a good HG on and be done with it. Of course you would be wise to do the water pump and belts/tensioners too.

Just my 50pence..
Ben.

If it's not actually scored and you are positive your oil problem was from the breather then I would leave it as it is, or maybe re-ring it if you really want to. Going for a 0.5mm oversize rebore is going to involve finding the right pistons (which can be hard) and also a machine shop that are knowledgeable about our Alusil cylinders which requires a specific honing process.

For a start the engine is an Alusil one not Nikasil & it isn't as such a coating. Linky here http://en.wikipedia.org/wiki/Alusil

A few people can bore & treat the Alusil to provide the correct surface on the bores. Mike is one at Boosted. Link here http://www.chevroletls1.com/news.html

I suspect given the compression figures a re-ring would probably resurrect it to a pretty good standard.

Thanks for the replies, I spoke to Mike at Boosted earlier today as he is just down the road. He said for minor wear he could hone the bores and fit oversize rings or if the wear was excessive then its really another block or fit liners, he has not fitted liners before but didnt see any problem if thats what was required. He advised getting an accurate measurement of the bores and then to take it from there so Im going to take his advice and measure the bores, hopefully i will get away with rings but somehow I dont think so. I expect it will probably be liners as I dont want to play pot luck trying to get a second hand block.

Beware of scope creep ... you have a small oil leak (any other symptoms?) and you are ending up looking at a rebore/new liners.

Sounds excessive. Particularly as you have good compression figures there. (A cynic would say that the solution was no further away than a rag and a bit of gunk, but then I don't know how big the oil leak was.)


Oli.

If it ain't broke don't fix it! A full rebuild will be 4K+ If oil consumptions a bit steep then check the valve guides first. A head overhaul will cost very little (About the same as a new gasket set) & since you have it off anyway you might as well while your there. Measure the bores if you must but if it runs ok...........Be cheaper these days to sell it & buy another. Which is exactly what I should have done with mine.

Be very careful with liners as this block is all alloy with very thin walls and unless you use someone with specific knowledge there can be all sorts of problems with different expnsion rates. Speak to Jon Mitchell at http://www.jmgarage.com he is the clubs technical advisor and very knowledgeable regarding these engines.

I'd love to take JMG up on their january checkover offer but they are 150 miles away from me.

I've done a bit of research on liners and unless its a track car and you keep a spare set on the shelf to swap between meetings then I don't think its the way forward. Not for me anyway.
I've had a chat with a guy up north who seems to be very knowledgeable about these engines and he can do a rebore and fit 100.5 pistons for a reasonable price, he can also do rebores / pistons and sort out S2 engines.
I will have a chat with Jon Mitchell (thanks Paul) on Monday and take it from there, although everything may be ok when I measure the bores,, I wish!!
I have checked the head and turbo etc and all is well, no problems there.

Bit confused here! The liners will be cast iron & shrunk into the block, no way of changing "between" meetings without a major engine rebuild & if installed properly have to be machined out normally! I think someone is getting confused

Yes I was, the last liners I played with were o ring sealed and quite easy to remove but that was a 19 litre diesel not a Porsche.

Check the prices on Lindsey racing. They have J & E pistons at 100.5 plus over size rings etc. Parts come in 2 weeks & even with VAT & duty often cheaper than anyone here. If you need another block to reuse stock pistons they can souce one for you & it may be cheaper. Lots more '44s over there! If nothing else will give you a bench mark.

hi sorry to jump in, i have a 86 turbo clutch totally gone.150 thou on the clock,while the clutch is being done toying with having the engine re ringed as well,it was using a bit of oil before the clutch gave out, if you have any result could you pass me on info please as nobody seems to no about the alusil proccess around here thanks bill

Sorry to hear of your problems, I have spent a few weeks now researching what can be done, during this time I have spoken with quite a few "experts" who have ranged from excellent to not so good!
Not sure where you are but there is an excellent guy in Birmingham, seemed to know everything and offers some pretty good advice, he can completely remove the cylinders and fit new liners. There is also a guy just off junction 24 of the M1 who will bore the existing cylinders and fit liners into them (a lot cheaper), he can bore Alusil but the problem is oversize pistons are very difficult to get hold of in the UK.
It appears that the rings take the wear and you may get away with reringing as long as there are no serious score marks, drop a new ring into the bore and check the end gap, 0.8 mm is the max limit according to the Porsche manual. I have seen reports that if you rering you should have the bores etched to make the new rings seat correctly, It does not sound correct to me but maybe some other members can answer this.
As a few members have mentioned "be careful" once you start and its in bits you probably should do the water pump, belts, top hats, all the seals, big end shells, valve guides, oil pump drive etc etc, maybe even some racing rods and head studs ready for an increase in power later on All this will produce quite a big bill especially if you pay a specialist to do it, If youre going to do it yourself and intend keeping the car then I highly recommend it, its great fun but still a big bill.