Forum bmwbike.com

I have recently bought a R26 to restore and have the following questions.

1) Motor has come with a R27 Head(Has frame mounting bolts just above carb) - does this creat any problems other than with originality?

2) Cylinder is standard bore (68mm) how can I tell if it was from a R26 or R27 or are they the same?

3) I know there is a difference in Connecting rod lengths/piston combinations for R26/R27 how can I tell the difference?

4) Since I need a rebore and a new rod kit, what combinations of rod, piston, cylinder and head will give me the best performance?

5) On a previous R25 project there was a thin band that went around the top of the cylinder and mated into the head - does the R26/27 use a similar item and if so, why?

Thanks in advance

1. doesn`t matter, the late R26(/2) had exact the same casting, only without the bolts
2.the same
3.? I don`t know (especially in english)
4.rod+piston of the 27(naturally),18hp to 15hp
5.If we mean the same thing, the 26/27 had the same "metalband"(it was part of the cylinder and stuck into the head).
It is to protect the headgasket of the "fire and pressure" of the detonating gas
greetings
rolf

1) I don't know. They have different compression ratios but this could be done with the pistons.
2) The R26 and R27 use the same part number for their cylinder.
3) R26 connecting rod is 125 mm center to center, R27 is 120 mm. R26 and R27 pistons have different part numbers, but I don't know how to tell them apart. I have a photo of what is said to be an R27 Mahle piston which I will try to post here.
4) You must be referring to a lip on top of the cylinder that fits into a recess in the head. I think my R27 had this lip because I remember fitting the head gasket over it . The parts manual and the shop manual for both R26 and R27 show a raised lip on top of the cylinder.

Hey Allan,
what is about your depolarising electric system?
3. the difference between 26 and 27 pistons is the distance between the piston-bolt(?) and the top of the piston, in german "Kompressionshöhe".
greetings
Rolf