So, did the top-end teardown after the compression test showed a problem. Found a few interesting things:
- left cylinder and piston were stamped 'A', while right side was stamped 'B'
- Rings were definitely out of spec by more than I'm comfortable with
- Bore was definitely out of spec
At that point I had to make the call: new pistons and barrels or new pistons and rebore.
After I did my sums, the difference between the two options wasn't very much (about $200 more for the Siebenrock powerkit). In return I could look forward to more power and torque, lighter pistons, Nikasil lined bores that are supposed to never need reboring, etc. Just as important to me was the fact that I could do it in one day and wouldn't have the bike off the road for weeks waiting for parts, rebore etc.
So I went for the Siebenrock powerkit.
Kit arrived last week and I fitted it yesterday. Total installation time about 4 and 1/2 hours (including topend teardown, parts replacement and rebuild. Doesn't include two trips to the local auto parts store because I'm a doofus and didn't get all the things I needed before I started). Have done valve clearances but haven't yet checked timing and carbys (hey, it was 37 degrees C (100F) and I wanted a beer!).
Job was pretty straightforward and would have taken less time except for three things:
- the instructions from Siebenrock are in German (eventually found a translation on the web when my German proved not to be good enough)
- Standard BMW cylinders have got a good bevel on the base of the cylinder flange that makes fitting pistons with rings into the cylinders easy (you can do it with your fingers). The Siebenrock cylinders didn't have that bevel so you need to use a ring compressor. I don't have a motorcycle ring compressor and couldn't find one anywhere on a Sunday so I ended up jerry-rigging something out of a big airconditioning hose clamp.
- It wasn't clear to me if the cylinders were handed (ie. cylinders are matched to sides of the bike and should only go on the correct side). After a fair bit of farfing around and test fitting I was able to convince myself that either they weren't handed or I had them on the correct side by chance.
Took it for an initial 1mile test run and it went beautifully. Took it for a 20 mile test run and I couldn't wipe the smile off my face. Bike is running fantastically, a noticeable increase in power and torque (even with the very gentle riding I was doing), pulls higher speeds at lower revs and there is significantly less side to side vibration.
I'll do the timing and carbs on the weekend but I did notice two things that I need to address:
- Carbs definitely need to be done - idle speed is too low and bike keeps cutting out even after warm up. That may be a result of the higher compression or maybe even just getting back to compression that is more equal on both sides
- The starter motor is struggling. I suspect that the 41 year old starter isn't able to handle the greater compression. It seems to kick over fine most times but was completely unable to turn-over the motor a couple of times. I suspect it's struggling to turn the motor over when a piston is approaching TDC on compression.
I'll do the timing and carbs and see if those two issues can be sorted out. But I get the feeling I'm going to have to get the starter rebuilt or get a new one that has a bit more power.
If anyone else is considering this I'd recommend it. Assuming that there is no big problems that crop up later, this was a straightforward and worthwhile task for those who have a reasonable amount of motorcycle mechanical knowledge. I wouldn't recommend it for anyone who has minimal mechanical experience unless you're willing to take a lot of time or get some help.
If anyone wants more detailed instructions on how I went about it, just ask.