david fracolli

So I have been looking through the archives trying to figure out what to do about pistons for my new engine and a few things just do not make any sense.
According to what I have read Porsche specs the piston to cylinder wall clearance at .008 to .032mm. If my math is correct 1mm is aprox .039 inches. Then .032mm would be .00126 inches.
If this is the case then how can you apply a coating of .004 inches to build up the skirts? Would this not have a thickness of aprox. .1mm?

Thanks

david fracolli

wow 50 views and not one person can tell me if my math is correct.

BC

People do it all the time. If you are using used pistons, part of that tin/ferrous coating would have been worn off as it is indeed a sacrificial coating, not really a permanent one.

For piston skirt and crown coatings swaintech (by your side) is a great place.

david fracolli

I know people do it all the time. Just trying to rationalize the math.
Really just trying to figure out if I can take tolerance group 0 pistons have them coated and put them in a tolerance group 2 block as I really do not want to try and find some tolerance group 2 pistons.

BC

Swain does high build coatings, but I think the max is .004.

http://www.swaintech.com/store.asp?pid=10971

What motor is it? I had a problem finding Tol2 as well. My whole motor is tol2, (some are indeed mix and matched between bores, or at least it mentions it in the books).

david fracolli

The engine that is currently in the car is an 88 non s 944 turbo that is tolerance group 1. The engine that I plan on using is out of an 87 N/A and is tolerance group 2. I would like to use the pistons out of the original engine so that I don't have to try and source a set of tolerance group 2 pistons.
Which brings me back to my original question.

BC

Well, since nobody else is responding, you get my answer "to your original question"

You can build up .004 with Swain, which on diameter would add .008. At that point you have a loose piston sure as hell less loose, but now you may wish to figure out if the ring back spacing is too large - the situation where the piston is possibly too small to support the ring. Maybe not a big deal.

Good Luck.

david fracolli

How do I have a loose piston?
Acording to Porsche specs the diameter of a tolerance group 0 piston is 99.980mm
The bore diameter for a tolerance group 2 piston is 100.020.
If the bore is not worn then the piston to cylinder wall gap would be .020mm which is well within the
.032mm spec set by Porsche.
Yes, or am I missing something?

Barry Johnson

How were you getting .02 as the difference between 100.02 and 99.98? My math says thats .04, unless I'm not taking something else in to factor?

I'm kind of in the same boat as you, Dave, and I have a set of scratched turbo pistons to go into my N/A block, but I can't find out how to tell the difference between the tolerance groups. Going to send my pistons off to get re-coated. Heard someone (maybe Swain) can do 4 of them for $200. Sound right?

Ski

.0254mm = .001"
1mm = .039"
.004" skirt coating = .1016mm

So, with the tolerance on the rings being from .21 -.46mm IIRC new, the above coating is as thick as I would go, because you don't want to get your rings out of spec after all your hard work, file to fit, this being done after your have double checked the bores. The mean average that we've previously worked off of ( just us doing our own work and for friends) has been a clearance on stock pistons of .020mm, which we've strechted to .0254mm, easy to do the math. The Mahle slipper skirts, Mahle suggested .040 but everyone pretty much used .038mm, this based on expansion rates of the forging.

Just remember to get your projected clearance, based on the measurement about 5mm from the bottom of the skirt.

In answer to the question, it seems doable but you'd be at or near the maximum, based on a non egg shaped bore. I think you'd be wise to have a finishing pass done on the bores, which may open it up .010mm, then your at the limit. I'm sitting here doing three things at once. Not trying to sell you anything but I've got an excellent set of group 1 pistons and new rings for sale on evilbay, measuring 99.94 on the smallest and 99.99 at the largest, again, this based on 5mm from the bottom of the skirt.

david fracolli

I am looking at swain to coat my pistons too. I found a set of tolerance group 1 pistons for dirt cheap so I went ahead and bought them before I even measured the bores.
As far as piston to wall clearance goes, unless my math is flawed, if you had a piston that is 99.98mm in diamater and put it inside a cylinder that is 100.00mm in diameter then you would have a gap of .010mm around the piston.

Ski

yes, total clearance .020mm, measured on each side .010mm - correct.
Here's a pic of the Swain coating on jwl's 2.8L Tom C pistons, with gold thermal coat.

Barry Johnson

Know if that PC-9 skirt coating will work in our blocks as-is?
The gold coating sounds great, but I think for myself at least, hoping for 500hp or so, the TBC stuff would probably be fine. Stock pistons didn't have any ceramic coating anyway

Ski

yes, it will. Those above are in their 3rd year of operation, not sure how many miles but the car runs like a scalded A$$ ape!

Stock pistons had Ferrostan iron coating, which that process is no longer approved today, for environmental reasons. The new stuff being used by Mahle is Ferroprint; still an iron based coating but eco friendly.