Ronin951

I've searched the archives and found very useful information on this subject, but I still need some advice from experienced builders.

The background is I'm building out a 924S block to replace my previous badly scored 951 block. I lucked out on e-bay and found a cheap block with straight bores. The only problem was there were some scratches (not scores) that failed the fingernail test. It appeared the guy who disassembled dragged the rods down the bores. I was concerned a ring would catch, so I took the block to a shop that does Alusil bores. They did a "light hone", which consisted of steps 3 & 4 (Hone, Lap, and polish?) of the Alusil boring process.

As it turned out this took more out of the block than I anticipated. I was told there would be no significant change in the bore. I unfortunately don't have the specific before numbers, but 0.004" (current - factory) seems too much for normal wear. No? I'm hoping I can push back because of his statement.

Below are my current measurements. I've pulled the specs from the manuals and laid it all out. The bottom line is I'm about 0.0045" or 0.0755 MM outside of the wear limit. Or 0.0068" over the factory clearance.

One option I'm considering is getting Swain to PC-9 coat them, which will add another ~ 0.004" according to his website. This still leaves 0.0038" over factory and 0.0015" over the wear limit.

Questions:

Would the gaps including and/or excluding the coating cause too much piston slap?
What are the long-term effects of this over the next say 20k miles (5k'ish track)?
How much should this "light hone" have taken off?
Recommendations?

( Note that I rounded off the factory clearance of 0.008 - 0.032 to 0.020.)

Thanks for any help.

mark944turbo

.006 is right at the limit of what I consider acceptable for clearance. This comes from the fact that on an iron block engine this would be about the wear limit for our bore size. The factory is able to run small clearance on our engines because they are aluminum block, you wont run into slapping problems until you are WAY over their tolerance limit.

It would be a good idea (safe) to get them PC-9 coated, even thought that stuff wears off very quickly from what I have seen. You can get up to .004 added if you tell them to do a heavy coat, the problem is I dont know how controllable that number is. This is the reason why you are supposed to have the pistons too big and hone the block to size, but like always when doing a patch job like this it is too late. I have been there myself a few times. In the future don't trust anyone who says the words "light hone" unless you want to be in a compromise.

Most important in all of this is the ring gap, so it would be a good idea to get new rings and see if that is in spec before you do anything else. End gap has a huge effect on power and longevity, if it is too big you might want to consider some oversize pistons from Ski or the factory.

Good luck !

mark944turbo

As a sidenote, the turbo pistons look fairly overbuilt compared to the aftermarket stuff available, so they probably have very stiff skirts, which helps you even more. They are also forged so naturally require more clearance. A lot of things working for you, as long as that ring gap is ok.

Ronin951

Thanks Mark. I just want to do it right and hopefully not again in the near future. I saw somewhere a guy took a set of 100.5 rings and filed them down. I'll measure the gaps over the weekend, but I don't expect any good news there.

mark944turbo

I haven't heard about that with the rings. It seems like big ones filed down would be extra stressed when compressed into the cylinder, but I don't know if this is significant or not.

BC

Where did you see this?

mark944turbo

Taking apart lots of low-hour 944 racing engines was my old job. Sometimes the coating would be worn through in the middle of the skirts, right where clearance is measured.

I dont know how it would do on a street car where the load is a lot less/cycles more, but I imagine still not very good when you are talking tens of thousands of miles.

Matt Sheppard

I had to wet sand my pc-9 coating just to fit them in my turbo block on my rebuild - standard coat, not the extra build. It is safe to say that the engine has machined it down somewhat since then.

Ronin951

One thing I'm considering is a set of Lindsey's Wiseco 0.003" over pistons. They come with wrist pins and rings, which @ $800 is not a bad deal IMO. They are coated with PC-9, so from what you guys are saying this would not be a long term solution. :can of worms:

Mark, how did the bores look when you disassembled? Were the pistons coated specifically by Swain or someone else?

Ski

If budget is a serious consideration, then you can probably make it. But, to get it right, it's going to require oversize pistons and this will add at minimum $1200 - $1600 to the bill. The original bores, clearances, etc were based on the expansion rate of the internals. Some of the aftermarket items have an initial greater clearance due to their own present expansion rates; ie Tom C pistons or the pistons that we do with Mahle. The new Mahle pistons actually recommend twice the original factory spec, .040mm total clearance, but the rings come file to fit, with recommended clearance based on NA, nitrous, supercharging, etc. Several people have chosen to use .038mm total clearance and have been very pleased with the results of compression test, leakdown, and dyno results. I would have an extra set to offer you but UPS damaged a set in shipping so I'm giving my personal set to the buyer and I'll fight the claim with UPS.

Last time I checked with Sunset, there were 6 sets of OEM 100.5 pistons, with rings in Germany but they were $1700.

I do have a used set of OEM 100.5, Swain coated skirts over the standard Ferrostan coat, thermal coated tops, that have some scratches on the upper section of the piston(blown head gasket and carbon particle scratches) but have new rings. There are no indents in the top of the crown though. These were sort of an emergency set of for the track car.

evil 944t

Ronin951,

I bought a couple sets from JAB racing/ski and they look great. I will have two motors with their pistons, going together this week and dyno time is reserved. I will post my final thoughts after that but I suspect they will be good.

mark944turbo

The bores were usually untouched, but these engines were all sleeved, and iron takes abuse better than our delicate aluminum/silicon. It was pc9 by swain.

Ski's Mahle pistons are a great deal in my opinion, I would try to convince him to do another run if you end up needing pistons. The other options are not proven long term, even though they may work.

Ronin951

Update on this. More questions of course. I ended up going with the Wiseco's .003 over. They call for a .003" clearance. Right now I'm sitting at .0025. In the interest of "doing it right the first time" I'd like to take out the 5 10th's. So, the questions are..

What step of the Alusil boring process should my machine shop start with? Is the last step (felt pads?) good for .0005?

Is there any way to tell if the silicon is exposed as it should be? The shop I use is new to me and I know of no way to verify their work.

OT:

The exposed girdle stud loctite is very brittle. Replace studs? Re-surface deck/girdle while I'm at it?

When I place the girdle on the block it rocks a little bit. Maybe .5mm or so. Will the loctite 574 and final torquing flush things up?

Thanks for everyones help so far.

m42racer

I sent you a PM of who to call. They do this work in house and can answer all of your questions.

What I understand from what I have been told in the past, the material is removed by regular honing using 2 steps of stones. Rough and finishing. Then the bores are polished and finally the lapping which brings to the surface the silicon. I am not sure if these numbers are correct but this is what i can remember. The finishing (honing) leaves approx 0.0008'- 0010" to be removed by polishing. The lapping part removes no material at all.

mark944turbo

The exact amounts removed during the refinishing process (when done properly) are given in the 944 na shop manuals iirc. I dont have a copy or I would help you out, perhaps someone has them and can scan it for you.

I dont know of a way to check to see if the process has been completed, but better safe than sorry as far as time spent on the final step.

Dont refinish the girdle surface unless you want to align/hone the main bearing holes over again. It is not necessary, the brittle loctite and slightly rocking girdle are normal. Dont need to replace the studs.