Measuring piston ring wear

[GIBSON]

I've got my 73S apart for rebuilding (the bottom end is complete, new bearings etc.). The pistons have 98K miles on them. I can get a .004 feeler gauge in the top ring. Seems like I remember B.A.saying that's too much wear. If so, do I machine the ring land and have a custom ring made? I was burning a normal amount of oil before the rebuild, within specs anyway. Are the other rings to be measured as well?
How is Deves for rings if I have to have a special set made? Anybody have good luck with total seal? If there are previous threads on this please point me towards them.
Thanks

Bruce Herrmann

[john walker's workshop]

the measuring is made with NEW rings to be accurate. rings wear on the sides just like ring lands. a couple thou would be good.

[GIBSON]

Good point, never considered the wear on the rings. Think I get a new set of Deves and see what I get for numbers. Thanks

[911pcars]

I believe .004" is the maximum for ring side clearance. If you're measuring with used rings, new rings will be better. Measure the 2nd comp. ring the same way. Don't forget to remove the ridge (if any) from the upper end of the cylinder and to stagger the end gaps before installing the piston assembly (among the 25 other things to check/perform). I would like to have seen Leland's checklist.

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

[rvanderpyl]

If you still have >.004 with a new ring then you replace the pistons.
It isn't recommended that you attempt to machine and fit oversize rings. Apparently this is somewhat similiar to trying to repair a crankshaft, it might work for 40,000 miles and it might break in 2000. Depends on whether you like betting your rebuild on it.

Robert

[GIBSON]

OK, new set of rings it is. If the gap on the new rings is >.004 I think I may go for new pistons, pretty pricey however.
Anybody like anything better than Deves rings?

[Early_S_Man]

Bruce,

Do you have the '72-'73 spec book? The actual limit for the top compression ring is 0.112 mm, which translates to 0.0044"

Before you spring for a new set of rings, measure the thickness of all six of your top compression rings, and compare them with the spec. The new set of Deves rings I have for my engine top-end job measure 1.485 mm, just under the max of 1.490 mm.

The wear limit for the top gap is 0.2 mm.

If your pistons are indeed worn too much, why not get your Biral cylinders bored out to, say, 86.0 mm and get a set of JE forged 9.5:1 pistons with valve relief cuts and give your engine a slight displacement boost and a bit more OOMPH in compression?

------------------
Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler