89 951 engine overhaul

[four0four]

Thanks guys, encouraging as always


I've got a TIG rig, but it's just DC...

HTS-2000 looks pretty cool, I don't know if I'll go that route but it's tempting. Could just stone down the new corner rather than use a dremel, in hopes of slightly tracking on the not-messed-up-surface.

Going to let it sit for a bit while I work on some other bits - Steering rack is getting new seals, and the pistons need to be cleaned.

re: pistons and bores - How do those numbers look wrt taper? Additionally, is it insane to mix-and-match pistons/bores to chase more even clearances?

I keep going back and forth on just getting new pistons.

[Tom]

Stone is a good idea. I was surprised how glass-smooth it made the oil pump mating surface.

Translating those clearances to metric, it looks like you are under the wear limit, but getting close. I'm sure there are a few good miles left in those parts, just hard to know how many. Was it burning oil before you took it apart? I'd check the ring grooves too before deciding. I'm a broken record I know, but I'd either suck it up redo the bores and get new pistons, or do neither. If you re-use those pistons in those bores, I wouldn't swap pistons and cylinders. It doesn't look like it would reduce the clearances in any meaningful way, and the mismatched wear marks might speed up further wear. Sounds like you have the specs, but here is a page from the tech spec book if helpful....

951-limits.jpg (496.42 KiB) Viewed 1687 times

[four0four]

Did you just use a sharpening stone?


Oh, duh. I should've looked for that...I actually didn't give you guys the measurements needed for that. Nor did I take them...Everything listed is perpendicular to the crank axis. Out of round requires I measure all those rotated by 90 deg! I should've done that today, ack. That plus the ring clearances will be the last bit of data I feed into whatever decision I make about the pistons.

re: ring clearances - Wasn't noticeably burning oil. #3 had a worryingly high leakdown number (somewhere around 20%?), but I meant to give a double check on the stand. Forgot in my excitement to crack it open, so here we are. It is the most egg-shaped cylinder of them all, no idea if that is related.


Actual updates: Tore down the rack. That castle nut thing is a massive pain in the ass! Took several rounds of heat and penetrating fluid to even get it started. A PO painted the rack at some point, so of course that's flaking all over the place. Planning to strip the rest, it won't be plated but it'll look a little cleaner.

edit: agreed on the wear patterns with swapping pistons. that's off the table now

[chris white]

So here is a ‘common sense’ check. You have your engine out and will need new bearings, gaskets and such to reassemble. Not cheap stuff. A set of pistons, rings and a nikasil block coating is well under $2k and will make it exactly back to factory spec.
Considering all the effort and expenses it doesn't make much sense to try and reuse old parts. The inexpensive route would have been to leave it alone. I’m assuming there was something that made you pull it out, so might as well fix it right!

[four0four]

Yeah, I wrote it all out and came to a similar conclusion. The scope creeps ever further...That said, there's a shop locally that claims they'll do alusil work. Going to at the least pay them a visit. This would let me go with the Mahle 0.5mm oversize pistons. They advertise a comp ratio of 8.6:1, however.

I’m assuming there was something that made you pull it out, so might as well fix it right!

"another damn oil leak?"

[chris white]

“Another damn oil leak” - almost always caused by too much crankcase pressure from blow by….so freshening up the block and new pistons make sense!!

[Tom]

Yeah, I wrote it all out and came to a similar conclusion. The scope creeps ever further...That said, there's a shop locally that claims they'll do alusil work. Going to at the least pay them a visit. This would let me go with the Mahle 0.5mm oversize pistons. They advertise a comp ratio of 8.6:1, however.

I’m assuming there was something that made you pull it out, so might as well fix it right!

"another damn oil leak?"

Have you sourced Mahle pistons? Keep in mind that the Mahle Motorsport pistons are not the same as the original factory pistons (at least the more recent one's I've seen), and the Ferro-print skirt coating is an eco-friendly version of the original Ferro-stan. They're also expensive, and I'm not sure if they have a 'standard' oversized 951 piston or if every supplier specs their own? Chris might know for sure? If you had someone with a track-record of success doing 944 alusil bores, however, I'm sure you could build a fantastic motor with Mahle pistons! I have their old-style (long skirt) 3 liter conversion pistons in my motor and they've been going strong for years. If you did nikasil (or sleeves) you'd open up a world of quality piston options much less expensive than the Mahles, which might more than make up for the cost of shipping the block around to have it prepped. Either way, it's worth nothing that a stripped down block is just light enough to send via regular UPS/FexEx in a big box (to have it coated/sleeved/bored by a tried and true shop). I've done it several times.

[four0four]

Grr...my last ditch effort at Molnars without paying scalper price fell through, oh well. Related: Why are all aftermarket rod bolts SAE?

Maybe it's my age showing, but I assumed I'd be able to grab a set from LN Engineering. Doesn't seem too expensive!

I'm chatting with two (relatively) local shops: Autosport Seattle and C&D Performance. I know the former has experience with 944s, I haven't got there yet with C&D. Autosport is nearly within walking distance though, so I'm biased

Worst comes to worst, I can ship it at work for a decent rate. They're used to just walk-ups though, so it might be a little Interesting to ship a block there. Good point wrt shipping amortizing the plating costs. I'm new to this: where would I go to find pistons with 944-friendly geometry but Nikasil plating compatibility?

[chris white]

Grr...my last ditch effort at Molnars without paying scalper price fell through, oh well. Related: Why are all aftermarket rod bolts SAE?

Maybe it's my age showing, but I assumed I'd be able to grab a set from LN Engineering. Doesn't seem too expensive!

I'm chatting with two (relatively) local shops: Autosport Seattle and C&D Performance. I know the former has experience with 944s, I haven't got there yet with C&D. Autosport is nearly within walking distance though, so I'm biased

Worst comes to worst, I can ship it at work for a decent rate. They're used to just walk-ups though, so it might be a little Interesting to ship a block there. Good point wrt shipping amortizing the plating costs. I'm new to this: where would I go to find pistons with 944-friendly geometry but Nikasil plating compatibility?

Getting aftermarket rod bolts is not really a great idea. They are not cheap and you will have to resize the big end after you replace the bolts - thats more than half way to getting an aftermarket set of rods!
Any piston is Nikasil friendly - just make sure you get the right rings (not chrome).
I really like the Mahle stuff....but its not cheap and custom pistons are taking over 6 months to get right now.

just for fun I took a look at my parts shelf and found some stuff that you might be interested in -
Two sets of 100mm wosners - one for Alusil and one for Nikasil!

Wossner wants $800 a set, I have had these here for a while (almost forgot I had them!)

Take your choice for $600.

[four0four]

Getting aftermarket rod bolts is not really a great idea. They are not cheap and you will have to resize the big end after you replace the bolts - thats more than half way to getting an aftermarket set of rods!

Oops, I didn't phrase that well. All hardware included with aftermarket rods seems to be SAE! Replacing the rods is what got me this deep to start

Any piston is Nikasil friendly - just make sure you get the right rings (not chrome).
I really like the Mahle stuff....but its not cheap and custom pistons are taking over 6 months to get right now.

just for fun I took a look at my parts shelf and found some stuff that you might be interested in -
Two sets of 100mm wosners - one for Alusil and one for Nikasil!

Wossner wants $800 a set, I have had these here for a while (almost forgot I had them!)

Take your choice for $600.

Ooh, well that's a pleasant surprise! I'll surely take you up on it, let me figure out what's gonna happen to these bores first though. Go ahead and re-forget about em for a few days

...I'm assuming these are from before Wossner had their little coating incident?