Forum bmwbike.com

My old bmw was smoking from the breather tube on the oil pan and running hot. I checked the compression and the clearence on the rings which were very worn. I replaced the cylinder, valves, guides, rings and had the barrel machined on my 1953 R25/2. I put it back together with new seals and oiled it up good. It started up and idles fine. But after driving under normal conditions for 40 minutes it starts to blow blue smoke from the breather tube. Its top speed is only 40 mph now. It used to get to at least 60 mph. Any ideas? Thanks, Trevor

It is not clear what you did to the cylinder. You say you got a new cylinder and had it bored (machined). A cylinder should be bored to fit its piston, and a new piston is usually used in this situation.

It seems you used the old piston and just replaced the rings. Maybe the fit is not so good between the piston, rings and cylinder.

Or maybe everything is basically OK and the rings have not seated yet. If the rings have a one-piece cast iron oil ring, it could take a while for the rings to break in and conform to the cylinder.

However, on my R27 I had the cylinder bored to fit a 4th oversize piston from Huggett, and it broke in quickly and perfectly. There was no blowby, the oil stayed very clean, and no oil was consumed.

Bonjour Trevor and Allen,

I had similar experiences as Trevor with my R25/2. My bike not only run hot but even seized regularly.

I applied a number of interventions and the situation improved a lot. Yet I do not obtain the maximum speed as mentioned in the instruction manual. My interventions :

1) I corerrected the alignement of the connection rod. Mine was 'tordue', I mean axes of bigend and smallend were not paralel. I was amazed how little torque is required to change the direction of both
axes. I took the crankshaft out of the motorblock. However by looking into the cylinder from above you should see the piston perfectly centered during a complete crank rotation.
2) I discovered the slot clearance of the rings of the new KS piston I had bought and installed was zero. Thus even on a brand new piston remove the rings and check that the slot clearance is some tenth's of a mm. (These KS pistons are far to heavy for our singles, but this is another topic).
3) I increased the valve clearance by 0.1 of a mm. I am sure the valves do not close perfectly on the running machine when I set the valve clearance to the officially recommende values. Perhaps due to some rocker wear or other wear the clerance is different on a running and a not running machine.

I hope these hints give you some new ideas. In any case your motor should never run hot. Keep the full throttle excercises to a minimum and only for very short moments. The noise of a seizing piston is terrible!

Cheers
Han Verhagen

I checked all the rings and they are all in correctly and have the proper gap. It has been suggested that the chrome compression ring takes a fair amount of time to work itself in. Anybody have any suggested duration for this to happen? Thanks, Trevor