BMW Einzylinder-Forum
TERMINE und FORUMSTECHNIK => International discussion forum in english language => Thema gestartet von: BSA Bob am 06 März 2026, 09:17:29
On my R26, cleaning the carbon off what I think is a Uli's piston I see that the directional arrow is facing to the rear of the machine. The valve pockets look identical & measure 26mm wide. How critical is this? The piston crown & bore are in excellent condition & I'd hope not to have to reverse the piston.
Hi Bob.
The pin is slightly off center for these pistons usually. This is the reason for the directional arrow.
As far as I know, this is to reduce tilting tendency intentional.
Hard to say, if this is critical at all, running the piston "wrong way round".
How many miles/kilometers did it run up to now ?
If you report "removing carbon" , I suppose it ain't "fresh" anymore.
What was the reason you removed head and cylinder ?
Did you had any noise from the piston ?
Some pistons are "de-centred" with the pin, some are not. You don't have to remove the piston, to see what is going on. Even by eye you should see it, when the barrel is off.
On our machines, the "smaller side" should be on the right side (ref. to the driving direction).
If there is an arrow, then the piston pin is decentered. The new pistons from KS (Kolbenschmidt) are NOT decentered!
For R26/27 the manual says "arrow in driving direction".
So just turn the piston and that's it. The piston does not run in with the cylinder wall. The piston rings do!
Check the cylinder wall if there are still honing marks. Otherwise measure cylinder and piston diameters to calculate piston running clearance.
If there is excessive piston clearance oder the cylinder is unround or conical, take the next oversize piston.
If it is ok, then you can do a slight honing with a hand honing tool, optional new piston rings.
best regards
Stefan, the one cylindrist
Don't trust the arrow over all!
When I repaired my father's R35 I got a new second source piston with obviously decentered piston pin (one could see it without measuring) and the arrow was on the wrong side.
So I turned the piston and installed it with the correct decentering.
Thanks for the comments.
I fed compressed air into the combustion chamber to find it leaking even though I'd never heard anything with the engine running . There's a bad crack around a previous repair, the cylinder head shop can't repair it & a lot of carbon has been forced into the crack. I'm hoping this may finally be the cause of the motor's sluggish performance, I can only get 100kph out of it no matter what. An R27 head is being worked on to replace it.
That's the reason for the head being off. I emailed Uli's & they recommend reversing the piston. The engine's done about 10 000km & I wonder if I should fit new rings. The honing marks are still present. Others have said the motor sounds good.
Well, 100 km/h is not bad. Where are the cracks? Between valve seat and sparkplug? Pics would be helpful...
It is hard to believe, that someone would mount the piston deliberately wrong. Do check the "smaller side", not the arrow!
Is it an Ulis-Tuning-Piston?
Only 100km/h is bad for an R 26...
...and an R 27 head will not be the solution. Yes, maybe without cracks, but after that please check if the ignition is adjusted properly and the carburator setting is more than correct. Brakes are completely released and, sorry for that, you are not more than 93 kg?
I don't have the failed head with me at the moment. From the outside air was escaping to the right of a previously repaired spark plug hole. Air bubbles were evident when air applied to combustion chamber & WD40 was sprayed on the site. Petrol also leaked through when the head was inverted. From the inside a crack was visible to the right of the plug hole.
It's definitely not a Uli's tuning piston, I think it's one of their pistons though.
The R27 head is being used because I have it on the shelf as a spare.