Cylinder Height - What do I do now?

[rw7810]

I purchased a brand new set of 2.2 PCs from a reliable list member sometime ago in anticipation of an eventual rebuild. They are OEM with PORSCHE packaging. One out of the 6 have the marking for tolerance group 6 denoted by a 6 surrounded by a triangle. The rest all have 5's. The little spec book says only cylinders of equal height group may be installed in any given cylinder bank. What do I do?? The difference between the two groups is .025mm. Do I compensate with an additional shim (.025) on the two 5's that are paired with the 6??

Also, what is the max amount of diff in grams you want to have between the piston/pin and rod pairs?? Right now i have 3 that total 555 grams, two at 550 and one at 545.

[911pcars]

Bob,
There was a recent thread about cylinder trimming. Do a search for the differing opinions.

From your info, I would suggest trimming the one tall cylinder to match the other 5 (or 2 on the bank). Different cylinder base gaskets will work, but it changes the deck height (cylinder and comb. chamber volume) which isn't recommended.

As for the different piston/pin weights, I would consult an engine balancer. Don't remember if 10 grams is outside the tolerance range. If so, they'll remove weight to match the lightest assembly. Shouldn't cost much and the engine will rev that much smoother.

JMHO,
Sherwood Lee
http://members.rennlist.org/911pcars

[Wayne 962]

Hello from the Bahamas. Yes, you should keep similarly grouped height cylinders together. You can probably machine the tops of the cylinders to match, or you can cut one head as well. I would measure each one carefully (have a machine shop do it) to determine the exact difference.

There aren't any .025mm shims as far as I know - that is pretty darn thin...

-Wayne

[rw7810]

OOPS - Wayne, you're right again. I thought the base gaskets were .025mm and they are actually .25mm. Plan to take set over to ProTecnik in Houston to see what Sam says. Also plan to have him balance the pistons & pins.

[5axis]

Thin is right. 0.025mm comes to a little more than .0009 " Not much material to remove to match the set. I would do some very careful measuring before going too much further.

[john walker's workshop]

you could drag it across a sheet of 220 sandpaper on a piece of glass a couple of times and pretty much match it to the others. machining would be tricky, being that it would be very difficult to set the cylinder up in the machine with less runout than that to begin with. we're talking in the 100ths of a mm.