Clearence when reboring cylinders

[malmac]

Hi Folks


What clearence do you allow between new pistons and the bore.
I have measured the clearence between new standard R69s pistons and new %69s barrels (thank you Ulis) and the clearence is .03mm.
So that is based on the diameter, so .015 when the piston is central to the bore.

In the workshop manual it talks about Piston Skirt clearence for R69s being .08mm+.1mm.
Seems like .03 is a tight fit?


Any insights would be appreciated.


Mal

[mcsherry1328]

I just had my R69S cylinders bored and pistons fitted. The engine builder used the specs from the original BMW manual . The cylinders were bored and the pistons are at .0035". When I installed the cylinders onto the new pistons (from Ulis) it didn't seem at all tight.
I remember from my Motocross days that I was always told that air cooled engines need a tighter fit to transfer heat. I'd like to explain why I did this work. I bought the R69S engine several years ago by itself and really had no history of its former life. I bought this engine to use in my R67/2. The engine worked perfect for the 2000 miles I rode it last year. When out for a ride a friend told me that that engine was smoking a little under acceleration. So this winter I took the engine apart and discovered that one piston was standard size and the other was first over. At that time the piston clearance was at .006". That was nearing a point when it was time to have this work done. I'm told that back then it wasn't unusual to repair one cylinder and leave the other side. Again the engine ran perfect and was very smooth, so it didn't seem to affect anything. I had the cylinders bored to second over. The engine is finished and once again it works perfect and runs smooth. I have only put on 25 miles so far.
Regards, Michael

[malmac]

Thank you for sharing your experience.

Was the clearance measured at the skirt of the piston on the thrust side?

At the first measurement I only had "002" thou between skirt and barrel. That all seemed a bit tight to my liking but Ulis supplied both the pistons and the new barrels so I guess I had better remeasure and confirm before I contact them to ask.

Looking like an idiot is an easy task unfortunately, so had better try and get all the numbers right before I contact them.


Thanks again


Mal

[malmac]

It is a voyage of discovery opening up some of these old engines.
Mine has been worked over and I am not sure which parts have been changed but some definitely have.
The timing gears dont match, the flywheel has been skimmed for clearence and it looks like the valve lifting bores have been resleeved.

So it is time for some tender loving care.

Thank you for sharing your experience. My pistons are also from Ulis.
I had better go back and remeasure the clearence and see if I have mis read the guages.
I have a set of internal micrometers but not a dial style bore guage.

Thanks again


Mal

[schrader7032]

.0030" for "Solo use" or .762mm.

Which is it? 0.762mm is 0.03".

[Twocams]

I used benchmarkworks : metric/USA volume, measurements, torque. etc. for 1955-1969 Earles Fork BMW Motorcycles. On his site in 2014. This was the best information I had at the time/used it. Solo should have been .0762 / .0030. I took my new pistons and old jugs to a engine rebuild shop that has worked on BMW bike before. Using a print out from Bench Mark with information / got'er done. I used the .0030" measurement. And there ya go.

[schrader7032]

OK, in your first post, you dropped a zero on the MM dimension. Might want to go back and edit it just so there's no confusion later on!

[malmac]

Just for people's reference.

here is the page out of the original BMW workshop manual.

I assume that the R69s clearence is higher than the the R50 and R60 because it was anticipated the S would be ridden harder and have more piston expansion. But maybe the pistons, with the high dome were manufactured differently and had a larger mass to expand.

Anyone with better information?

Mal

[Twocams]

I set my R69S to Bench Mark Works. .0030" for "Solo use" or .0762mm. There is also a .0035 for "side car use" I haven't had any problem with the .0030 clearance. but this is only one man interpretation.

[Twocams]

https://w6rec.com/duane/bmw/piston/index.htm
Malmac Here is some information on the /2 pistons. Don't know if will help or not...but.