bebbetufs

The block I'm building up is brand new so the cylinders are very close to 100mm. One of my pistons only gives me 0.008mm clearance when measured according to the manual. The piston came from a running 62k miles engine which had no damage to the skirt or the cylinder walls

According to the manual this is the minimum clearance. Still, is it enough?

The minimum clearance is interesting, it indicates extremely similar expansion rates for the cylinders and the pistons.

I mic'ed the piston and transferred the measurment to my bore indicator. I repeatedly get a pointers width less than 0.01mm.

blade7

It has been posted here about the tolerance groups of blocks and pistons, I think it was something like 0, 1 and 2. Can you measure the block the pistons came out of ? I may be a little wary about the condition of the coating on a used piston in a new block with tight clearances.

bebbetufs

Both blocks are tolerance group 1. The one they aim for when machining and the most common.

blade7

If I had a brand new block I would have put a new set of Wossners in it.

Dare

Any chance it's a measurement error? I found if I held the frame of the mic with my bare hand while I was transferring the measurement to the bore indicator the heat from my hand would skew the result. Essentially it makes the mic expand which sets the zero point on the gauge too big, making the clearance seem smaller.

Voith

It is interesting how little temp difference makes all the difference. My pistons didn't even fit the bores because pistons were indoor and block in car trunk.

8 microns is a bit tight for turbo application I think.

Ski

I agree, I would not run that tight for me. Factory was .020mm new? The last few engines I've done I've been running .0012" or .030mm with the Mahle slipper skirt ferroprint coating. Good luck with what you decide.

bebbetufs

The other three are at 0.012mm clearance. They are worn pistons so it seems odd, but I get the same result with two different mics and two different bore indicators. The temp in the garage was 18degrees C.

I don't think I can buy a new set of pistons now, and still make the start of the season. I have a few other pistons I can look at, but this set has been balances to 0.1 grammes.

I hate the constant setbacks.....

blade7

Maybe you have to hone/lap a little out of one cylinder then, I believe there is possible variation in each tolerance group anyway.

bebbetufs

I have the lapping paste, but I was under the impression it will only remove the aluminium between the silica crystals. I may be mistaken but I think one needs a special diamond stone cutter to actually cut the silica.

blade7

You would have to lightly hone the cylinder to increase the clearance and then lap with the paste.

Voith

It can be honed with regular CBN inserts, but it will only work if olive oil is used for lubrication.

http://www.enginebuildermag.com/2008...and-abrasives/

Dare

I believe that's right. The change in diameter from the lapping process is miniscule, less than .001mm. You'll need to hone the cylinders to change the clearance. And from what I've read this does require a specific type of stone, along with a particular honing speed and angle.

Ski

The manual is specific with the cut, hone, Finish - the last which IIRC, is done with Felt pad and the paste on the last pass.

Now, not smoke and mirrors but, I was also told a long time ago by a guy who actually did some time at the factory engine shop, that the last of the 944/968/928 cylinder finish process was the chrome plated Porsche rings actually completed the very LAST finish of the Alusil bores. Microscopically, the Alusil, even after the paste and pads, left an unfinished bore, where the piston rings flattened the exposed Alusil and fully seated it all together. ?? who knows but his advice has served us well so far.

I can tell you that if you have a shop here in the US that you don't fully trust, Duffin Engine service in San Antonio knows Alusil and the process if you can afford to ship and return. We've gone a tiny amount over stock clearance with past builds just to give a small window for thermal expansion, because - even good engine shops may not have the tolerance that Porsche did at the factory. So far so good...

Nice little exert from Sunnen
http://www.sunnen.com/NewsDetails.aspx?NewsID=11

bebbetufs

Problem is I live in Norway, and there are no such shops in this area. Besides. The price would equal having sleeves installed in the US, so I'm not going that route.

I'll double check my measurements and check my spare pistons as well.

Does anyone have one good turbo piston group 1, and very close to 99.990mm Ø?