[speedysprocket]

So I bought this 968 drivetrain to drop into my 87 S over the winter... Anyways, one thing led to another and the engine is apart. Had 155,000 miles on it. Crank looks clean, visible lines in the surfaces, but nothing you can feel. Bores have slight scoring, but nothing deep.

New bearings are a given. Should I just have the crank polished? Is it fine to use the old pistons with new rings and a hone? And what is this about replacing rods? I understand the 92s were not as stronger, but the engine will be mostly stock, and the most it'll see is autocrosses.

Finally, are the windage trays/crank scrapers worth my money to put in while it's apart?

Thanks, and if anyone knows good machine shops in the Hampton Roads, VA area, I'm interested for the head, block and crank.



Alex

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[Dubai944]

The bores can't be honed in the traditional way, they need special treatment. If you do it you will likely need oversize pistons. If the bores are not damaged you can fit new rings without a hone and still get a good seal. I fitted new pistons and rings to an S2 block recently without touching the bores and still got less than 2% leak down on all cylinders. The early 968 rods are technically weaker in design, but in practice I ran a set for two years in a race engine that constantly revved to 7500rpm and they were fine when I eventually replaced them. Maybe I was lucky, but it would seem for a low stress application new bearings and rings on a reasonable engine will be fine unless you want to spend extra money. Sounds like you don't need to waste money on aftermarket windage trays either for your application.

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[speedysprocket]

How do the bores need to be treated if honing or boring?

Thanks for the reply!

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[jfrahm]

Boring and re-lapping the cylinders, plus oversize pistons, will cost more than just getting a later motor with nicer bores that is already put together. The lapping process uses a caustic paste to etch out the aluminum and expose more of the silicon in the bores. New pistons with a tin coating to prevent galling in the bore need to be fitted. If you treat the bores and run old pistons they might gall and scuff as this plating is not there to transfer to the cylinder walls and allow the parts to wear together in a friendly manner.



If the leakdown was not bad before I suggest leaving it be. Light scratches that you cannot catch a fingernail in are not going to be a problem. If you have ever torn down a dirtbike engine that ran OK you won't worry about trivial scratches anymore :-) Plus the alusil blocks last a long time and in normal use I would not be motivated to re-ring or bore without obvious issues under, say 250K miles. This is not your father's Oldsmobile.



Getting early 944 rods or 951 rods are the inexpensive way to put strong rods in your motor. Search out the threads here for more detail. Also not a big deal IMO unless you do some crazy stuff. I can recall exactly zero cases of a bent or broken early 968 rod in a stock motor on the street (but there might be some out there.) The rods were an issue in the 928GTS which had many failures. If you pull the pistons you should re-ring and make darned sure you get quality rings and gap them properly.



While you are in there replace the oil pickup and reinforce the support for it so it does not crack.



Cheers,

-Joel.

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[speedysprocket]

This is the first I've heard of the oil pickup; what's the deal there?

And what are good rings?

Thanks!

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[RS Barn]

I agree to almost everthing. I don't like to brace the oil pickup tube. It is very thin and welding a brace is going to cause a stress crack at some point. I do add skirt to pick up if using at DE;s

All Alusil bores have some scoring. If scoring appears to be at the surface, I would lightly score bores with scotch brite and WD-40 and install new rings. To break in rings you must use some sort of mineral oil. only change after 500 miles or so to current choice of oil.

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[flash]

reinforcing the joint is fine, and not a bad idea at all, but you should not weld that joint, but rather braze it - big difference - welding will crack - brazing likely won't - it won't be cheap though, as brazing is all but a lost art, and if you do it wrong, it's worse than not doing it at all



most oil companies will tell you to use a break in oil, which is high in detergent amongst other additives, and change it in no more than 20 hours or 500 miles - i just used amsoil break in oil for that very reason, and there was a very definite difference in its composition - until lately, i never did this , and broke in every motor on the track, with regular oil (though lighter weight than we usually ran - never had one fail either, but then i never dealt with alusil bores, and any aluminum block automatically got steel sleeves, as will this one when i need to get into it

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[jfrahm]

I suggest a brace that mounts to a mounting bolt and is hoseclamped to the tube above the reinforcement area. Even a wide clamp or two over some soft metal would act as a strain relief in that area. Maybe wrap it with a strip of dead-soft copper and hoseclamp the sucka. It cracks at the reinforcement, generally. A crimp-on spring Oetiker clamp would be good for this application IMO.



As for the rings, do some searching. Some say the aftermarket rings are not of the same quality as from the dealer. Maybe there was a bad run, or maybe some builders just did not know how to file rings or clean ring grooves. Maybe people who pay full pop for OEM rings install them more carefully. Some say the gaps are really wide:



http://forums.RL.com/rennforums/944-turb...rings.html



Likewise there is a debate about the bearings but I figure if you plastiguage them and (naturally) use new nuts for the final torquedown you should be OK. If anything feels weird when you torque them down odds are a rod bolt just failed. Rod bolts are basically very stiff little springs and can fatigue and get too easy to stretch. Ignoring this is an expensive and oily lesson.

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[speedysprocket]

So new nuts on the mains, you saying new rod bolts too?

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[jfrahm]

IMO the rod bolts should cycle a few times but you never know. Likewise the main studs (but I would not pull the crank if you can avoid it.) Odds are bad bolts and studs will loosen up before reaching full torque if they are failing. Nowadays I'd be as worried about getting counterfeit new hardware as I am about re-using old rod bolts and main studs.



For a street/autox motor I'd have my druthers with a running pullout from a tippy with some fresh variocam ramps and maybe a head gasket. It is very hard to replicate the quality and conditions of a factory build and I trust a motor with 80 thousand miles on it more than one that has been disassembled and reassembled as a project by someone who has not assembled a lot of motors.

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[Cloud9...68]

"I trust a motor with 80 thousand miles on it more than one that has been disassembled and reassembled as a project by someone who has not assembled a lot of motors."



Gulp. <img src="/forum/images/smilies/968/sad.png" class="smilie" alt="" /> I resemble that comment....

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[speedysprocket]

Well this motor had 155,000, twice your 80. I didn't want to go through all the trouble of putting something in that would just have to come out later to get freshened up...

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[speedysprocket]

So are rings really going to cost over $100 per piston? Where should I be looking to get rings?



Thanks for the help!

Alex

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