Piston and rings

[getset]

So I have a 78 R100/7 that was manufactured in 9/77. I have just disassembled the top end to replace the rings and gaskets and pushrod tube seals as the compression was low and I was leaking oil all over. in removing the cylinders I have found them to be 8.5:1 late model as shown here (http://moragafalconers.org/BMW_Pistons/). there are two numbers stamped on the top 9397, and 9H. I am now a little confused as to what rings to get, and is there a possibility that they have been overbored? I guess I need to check the cylinders to determine right?

thanks.
J---

[getset]

So itlooks like I am mistaken?

I believe that the late 77 production years moved to a non squish head piston with the full hemisphere domed top. In which case they could appear to be stock. That is at least what I think the microfiche is telling me.

Thanks for any help on confirming this.

[20774]

My /7 was also produced in 9/77. I can't remember what the piston tops looked like but I think you're right, they are not squish band. The 9397 is the OD at 93.97mm which should be stock size. I think the stock R100/7 was 9.0:1 not 8.5.

As for rings, you should do some measurements of the bore of the cylinder at 2-3 locations from top to bottom. Check for ovality and the size. That will tell you if you need to go to the next oversize in terms of a rebore, etc. Depending on mileage and use, you could get away with standard rings. Again, if you get them, put then into the bore and measure the end gap. If too wide, you'll be blow by and lose compression. It's a toss up whether you can go to the next size rings and then file the ends. Sometimes this doesn't work. Plus, if your cylinder is that big, then the piston will be have too much clearance around the sides and will rock and rattle inside the bore.

[keelerb]

Here's some good information on rings and ring gaps generally, as well as a technique for using a ring to measure taper (wear) in a cylinder bore.

http://www.hastingsmfg.com/ServiceTips/ring_gaps.htm

[lkchris]

As originally configured, 1977 models had 9.0 compression for 100/7 and 9.5 compression for 100S and 100RS. Both are premium fuel engines.

For 100 engines there is no such thing as "later 8.5 pistons" for the nonnikasil versions.

Pistons marked 93.97 are NOT overbore pistons. ETKA suggests the latter might be marked instead 94.21, 94.22, or 94.23 (i.e., add .25). If you're not boring, then you need rings for the 93.97 piston. I'd buy new piston-fitted nikasil cylinders before I'd bore iron cylinders and buy new pistons to match.

Frankly, I'd investigate cylinder heads first as regards low compression.