Help with piston and cylinder specs

shbop · 08-31-2013, 06:43 AM

Just got my pistons and cylinders cleaned-up and measured, but I've got a few conflicting bits of info. The Turbo Carrera manual Vol. Vll 1989 says pistons should measure: 96.981 - 96.991mm, while Vol. V 1984 says: 96.864 - 96.874.
These specs are for Cylinder and piston set (2).
Also, one lists a piston to cylinder clearance spec of .025 - .052mm, while the other says: .030 - .047mm.

My cylinders all measure within the specs, but my pistons seem to measure just out @ 96.953mm - 96.958mm. Sure would appreciate input from those who know the correct spec. This is a 1983 930/66 block. If, as I believe, the pistons are out of spec; are there coatings that would make-up the .028mm diff, or is it new piston time? Thanks in advance for any help.

Eagledriver · 08-31-2013, 07:16 PM

There are different size pistons and cylinders depending on the batch. The spec you need to worry about is the clearance of the pistons in the cylinders. I measure this with feeler blades. The new spec is .001 to .0025 inches. The wear limit is .004 inches. You get to decide how close you are and how much money you want to spend. The other limit is for the ring grooves. If you are going to re-ring, you'll need the top ring groove to have less than .004 inches of clearance from the ring. If you are keeping the old rings (I recommend this), then you can get away with a bit more.

-Andy

shbop · 08-31-2013, 08:35 PM

Thanks Andy. So if your cylinders are true, you go by feeler blade only? I guess I'm getting a little too far out in the weeds. My ring landings are good, along with the end gaps, but when I go by just raw measurement data, it seems like I'm on thin ice. I'll go for the feeler blade down the cylinder, and see what's happening.

Eagledriver · 09-02-2013, 10:33 AM

I don't have the tools to properly measure the cylinders and pistons, so I improvise with feeler blades. You can check the taper and roundness by moving a piston to the top and bottom of the cylinder and by turning it 90 degrees. I doubt that your pistons are worn much on the skirts. You can probably still see the machining marks on them. The only place I usually see wear is at the bottom of the skirts. You have to be the judge. If the set was working well before you took it out, it should still work well after you put it back together. It's just a matter of deciding how long it will last based on how far gone it is now. The wear will go faster as the pistons get looser.

-Andy

shbop · 09-02-2013, 01:14 PM

Unfortunately this set had been riding the bench for the past 12 years. I wasn't able to run the motor before tearing into it. It had suffered broken head studs, and the owner parked it.
The dreaded divilar battle. I lost. Three broke at the case. That's the bad news. Good news is there will be some new (good) ones going back in.