I'm in the process of 'refreshing' an R27 that I bought a few years ago. I bought the bike barely running from an owner who had basically given up on getting it to run properly. He said he had adjusted valves, completely tuned up the ignition and had re-built the carb several times, but couldn't get it to run well.
Once I got it home I did a compression check and found it extremely low. So low that I was actually surprised that it started. I removed the head to find numerous problems. The exhaust spigot was completely destroyed, the exhaust valve seat was totally knackered, the exhaust valve burnt and the spark plug thread insert loose. The top of the piston looked fairly new (I could almost read the markings on the top) so I didn't suspect a piston/cylinder problem. Here's a picture of the exhaust spigot as I found it:
I had a very experienced BMW head specialist rework the head with the following result (clickable images):
After re-working the head I found that compression was still low. I subsequently removed the cylinder/piston and measured tolerances. The bore was .0045 and the piston ring gaps around .020 (new spec is .002 and .002). The piston was the first oversize and the bore was very concentric from top, middle and bottom, leading me to believe that a previous owner had fitted a new piston but that whoever did the machine work over-bored the cylinder and mis-fitted the rings. A new Kolbenschmidt piston has been sourced (second oversize) and the cylinder/piston sent to Bore-Tech for fitting and carbide treatment.
Since I had the engine removed and partially taken down I decided to completely tear it down to check the oil slingers. I also took the remainder of the bike down to check bearings in the steering head, swingarms and wheels. I'm glad that I did as all were in very bad condition. So, I'm in the middle of tearing the engine down, awaiting the return of the piston/cylinder, and starting to re-assemble the rolling chassis. I did have the frame powder coated and some of the tinware re-painted so it's starting to look pretty nice.
Here's the bike before teardown:
Here's the re-assembly so far:
Here's the engine in mid-teardown: