Forum bmwbike.com

hallo,
kann man einen 25/3 Zylinder+Kopf für einen R26 Motor verwenden?
Gibt es Probleme(außer geringere Verdichtung )?
Is it possible to use a 25/3 cylinder+head
for a R26 engine?
Are there problems(except lower compression)?
Rolf

Servus Rolf!

No you cannot use an R25/3 cylinder head on an R26, but you can use an R25/2 head at your own risk. The R26 and R27 heads have 60% more cooling area (finning) then an R24 - R25/3 head casting.
The R25/3 head has a combustion chamber (Brennraum) which is made up of two hemispheres (halbkugel) instead of one, giving it an oval shape. Due to the oval shape, the R26 piston will hit on the sides of the combustion chamber. All other heads have a round and single hemisphere combustion chamber.

The spark plug will also be on the left instead of on the right side, but that is not really important. If you decide to carry out this conversion, please refer to "zap" in the restoration tips.

Best regards,

Mark
Mark Huggett GmbH

Hello again Rolf

I just read your question again and saw that you want to use the cylinder as well. The answer is again no, unless you change the conrod and piston as well. The R24 to R25/3 cylinders stood higher above the base gasket as they also had a much longer conrod (135mm) then the R26 (125mm) and R27 (120mm). That means that the R26 rod and piston will reach 10mm short of the cylinder head gasket.

Best regards,

Mark
Mark Huggett GmbH

hallo mark
thank for the tips, first I"ve to explain the reason for this conversion:I want to tune a little bit my 25/2(without too heavy optical changes) , because if we (my/2 and I)
get heavy wind from the front, the speedmax sinks to about 70 km/h.So it could be a long journey to (to example)Kopenhagen.
Never the less: if I carry out this conversions
1. I"ve to shorten the cylinder (10mm)
2. I"ve to cut away the sides of the combustion chamber to get clearance for the piston. A 25/2 head doesn"t work(after 5 changes of the headgaskets,I"ve noticed why )
it has another way for the oil for the rockers?(Kipphebel), the 25/3 cylinder has opened holes (downside) for the M12 bolts, easy way for the oil to see the crankcase from outside.
Is 1.-2. right?
At last: I don"t understand your last sentence:"refer to zap"
sincerly
your teutonic rolf

Hi Rolf

It sounds as if you have a cylinder head from an R25 mounted on your R25/2. The R25/2 head is the same as the R25/3 with the exception of the shape of the combustion chamber. If you are looking for more power, keep the head and cylinder that you presently have and rather use a different piston and cam shaft. You can have the small end of the conrod worked to take a 20mm gudgeon pin, and fit an R50S piston, or you can order a R25 "Sport" piston from us which has a higher compression. You can combine the sport piston with an R25/3 cylinderhead which will give you even more compression. Higher compression with modern fuels actually means cooler burning (which I only learned 2 years ago) as the combustion is faster and heat is dissapated quicker. Slow combustion (low octane and low compression) causes overheating.

At the top of this page there is a blank space where you can type in a subject, and then click on the search button. There was a fellow who gave the subject as "zap" and it concerned overheating, power, lubrication on the R25 motors. This info is very important for your conversion.

Best regards,

Mark
Mark Huggett GmbH

I may be wrong, but the/2 head has an extra oilpipe for the rockers and so 2 extra holes in
in the head, or is this(with the holes) a R25 head?.
The R25/2 head, has it rockers with a oilpipe or without(like /3)?
Thanks
rolf

Hi Rolf!

You are quite correct. The R25/2 cylinder head went to the new rocker system like the R25/3, but still maintained the old oil pipes like the R24 and R25.

Regards,

Mark
Mark Huggett GmbH