89 951 engine overhaul

[chris white]

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

After the failures - the failures were on early production units.

[four0four]

Perfect!

Tore some more bits off the block. A stud I installed (see rennlist for the thread...) to fix a chowdered up WP bolt hole was a little proud, and wore a groove into one of the idlers (!!). The lower balance shaft housing seemed to be leaking slowly - teardown revealed I hadn't managed to get all the hardened old seal out (sub-millimeter bits left dried to the surface). Lots of things are more obvious when you're not on your back!

Finally, and most weirdly: The oil pump drive o-ring (999-707-283-40) was completely compressed into the drive gear (it's kinda got a beveled edge). There was corresponding wear and thermal stress on the washer. I'll be replacing it and the washer, but what the hell?

Edit: it's a red o-ring. I'll have to check my order history, but the other time I've replaced this it was definitely green. Different manufacturer, minimally.

[four0four]

Eh, was going to PM this but it strayed a bit into general questions:

@chris white -

I found this doc from Wossner...but 0.1mm clearance seems high (Turbo WSM specifies 0.08mm wear limit!). Do you have guidelines/installation specs for those pistons, or are they just that different?

Shop I was talking to isn't comfortable boring the block, but they're happy to hone it and swear up and down that they can process the bores after. Not sure how to interpret that, would've assumed the former is a good bit easier.

[chris white]

Eh, was going to PM this but it strayed a bit into general questions:

@chris white -

I found this doc from Wossner...but 0.1mm clearance seems high (Turbo WSM specifies 0.08mm wear limit!). Do you have guidelines/installation specs for those pistons, or are they just that different?

Shop I was talking to isn't comfortable boring the block, but they're happy to hone it and swear up and down that they can process the bores after. Not sure how to interpret that, would've assumed the former is a good bit easier.

that shop sounds kind of suspect to me. boring is the easy part, just about any shop can bore a block - alusil finishing is the hard part requiring specialty tools and chemical pastes.
Also - I don't really know how you hone a bore that is not square and straight back to factory specs. can't fix out of round with a hone.
The piston to wall clearance is dependent on the piston material and its expansion rate. the stock pistons are made of an alloy with expansion rates very close to the alusil block so you can have very tight piston to wall clearances. The Wossners are made of 4032 alloy which needs a little more room to expand. if you used 2618 alloy woudl would need even more room.
for example the sleeved engines I build have .15 to .18mm of clearance. depends on materials and applications. Most important is that the cylinders are round and not tapered.

[chris white]

Info on the side of the Wossner box of pistons.....0.08mm

[Tom]

Eh, was going to PM this but it strayed a bit into general questions:

@chris white -

I found this doc from Wossner...but 0.1mm clearance seems high (Turbo WSM specifies 0.08mm wear limit!). Do you have guidelines/installation specs for those pistons, or are they just that different?

Shop I was talking to isn't comfortable boring the block, but they're happy to hone it and swear up and down that they can process the bores after. Not sure how to interpret that, would've assumed the former is a good bit easier.

that shop sounds kind of suspect to me. boring is the easy part, just about any shop can bore a block - alusil finishing is the hard part requiring specialty tools and chemical pastes.
Also - I don't really know how you hone a bore that is not square and straight back to factory specs. can't fix out of round with a hone.
The piston to wall clearance is dependent on the piston material and its expansion rate. the stock pistons are made of an alloy with expansion rates very close to the alusil block so you can have very tight piston to wall clearances. The Wossners are made of 4032 alloy which needs a little more room to expand. if you used 2618 alloy woudl would need even more room.
for example the sleeved engines I build have .15 to .18mm of clearance. depends on materials and applications. Most important is that the cylinders are round and not tapered.

+1 on that shop.

With no proof or evidence, I've long suspected that factory piston-to-wall spec has been the root cause of several rebuild failures. The tech spec book say .0008 to .032, which is crazy tight. I guess that worked with perfectly matched piston materials, at stock boost levels, etc. but if I tried to build a motor like that, I'm sure it would gall and seize in short order. Even the factory motors start getting piston to wall interference (score marks, etc.) when you start pushing the boost/heat. Chris would know better, but if you plan to push the motor at all, I'd think even .1mm (~.004") is on the tight side. But all that's why I think it's best to work with someone who has built plenty of these motors successfully, and has a formula that works well after plenty trial and error. You don't want to be part of someone's trial and error.

[chris white]

more engine trivia - a modern F1 engine cannot be started until it is warmed up via an external coolant heater. It will seize up unless it is up to temp. They have a little bit more advanced tech than us....

with steel sleeves .005" is as tight as you can go with a 4032....I know, I have done the experimenting and subsequent rebuilding!
So don't worry about .08 to.1mm with alusil - I woudl be fine with either as long as the cylinders are round and without taper. if you plan on using it hard (lots of heat) stick closer to .1mm

[four0four]

Shame about that shop, I enjoyed talking to them otherwise...

Anyway, C&D performance in Kirkland are comfortable doing that work...I'll have them re-do all of my measurements first, but it seems like there's enough material to bring the clearance back to 0.08-0.1 and completely remove all the taper. Assuming I did my job right

Would there be any other gotchas about running this kind of clearance? I assume probably not - once the pistons are up temp at least. Off topic, but what do at-temp clearances end up looking like?

And Chris, assuming this comes back with an OK from the shop - I'll probably rid you of those alusil compatible Wossners

[Tom]

Would be a neat hat trick to bore and hone that block and still be able to use standard size 100mm pistons rather than one-overs. Based on your measurements, it looks like you'd need to go to at least 100.04 or so to straighten out the bore. Pistons are often smaller than the advertised bore (Porsche's group 0 are .02mm under), so I guess the questions are how precise can the machine shop be about removing 'just' enough to make it square (I honestly have no idea), and what are the actual dimensions of Chris's pistons? If the machine shop can't keep the bore that small (any score marks?), then you'd be looking for one-over pistons, or punt on the alusil and reconsider Nikasil...

[four0four]

I'm holding out a little hope there, yes! (edit: well....) I'm far from a machinist though, so it's possible I fouled up the measurements enough that it's not the case.

I was under the impression the 100mm pistons were 100.00mm, but writing this out I'm realizing that's the bore, and presumably those Wossner pistons are 99.92mm. Gah! Maybe this is why that shop said no to boring... I guess the next best bet would those Mahles from earlier.

I think I posted pictures of what scoring there was - Maybe 2 lines I can catch total. I'm not quite sure what the depth threshold is for being able to feel it with a nail though.