Vintage BMW Motorcycle Owners

Hi everyone. Me again,
On to the cylinder of my 61 R27. So, I'm trying to figure out which way to put the piston onto the rod. While I can see 'Vor' stamped on the top of the Nural piston there is no arrow or anything to say which way is Vor. So, anyone know if, with the piston in my hand and 'Vor' is right side up should the arrow be pointed to the right or left. I am also assuming "Vor' means 'to the front'.
Thanks.

Vorn means front or forward. My R25/2 workshop manual says that arrow points in the driving direction. The reason for the orientation is because of an offset that is in the wrist pin relative to its location in the piston. If there is no offset, the wrist pin centerline is exactly in the middle between the outside edges of the cylinder wall. With an offset, the centerline is slightly closer to one side or the other. AFAIK, this has to do with the angle that the rod makes when going through its rotation and the offset helps with the lateral forces caused by the rod pushing the piston towards the cylinder wall.

My R69S pistons had no markings of any kind...not even a Vorn. Turns out the piston has no offset so it didn't make any difference. I installed the piston so that the markings were readable.

My guess is that if you can read the letters Vor(n), the arrow should be pointing to the right.

If it says "vorn," there is probably an arrow although it may be hard to see if it is a used piston. If there is an offset, it should be measurable, however.

IIRC (BIG IF, please check with Vech), the small side faces the direction of rotation. i.e., if you are facing the rear and the crank rotates in a clockwise direction, the small side would be on the left of a vertical cylinder.

Bruce

I have looked long and hard and can't see an arrow.. I'll see if I can measure a difference. Where is the arrow typically? Above, below, beside 'Vorn'. There MIGHT be an arrow looking like thing NEXT to 'Vorn'. It does seem to point to the right as Kurt thinks might be right. See photo. It would be quite small. Sorry for the lousy photo!
Any help?

more reading for you and your piston

http://www.beemergarage.com/documents/203.pdf

http://www.beemergarage.com/documents/220.pdf

http://www.beemergarage.com/documents/231.pdf

http://www.beemergarage.com/documents/230.pdf

Thanks--I'll add them to my ever widening notebook! I love the statement on the 3rd page "a deplorable mistake has crept in.....".

Now, can I send my questions for my E-Class also??????

I asked the German Einzylinder forum and got the following info...I'm still trying to verify what is meant exactly.

"the offset is to the right side if you sit on the bike. You can measure it using a caliper - ca. 1,5 mm from the middle."

I take that to mean that the wrist pin will be shifted 1,5 mm from the middle of the piston to the right as you sit on the bike.

Another possibility...are there pockets in the piston dome for valves? If so, wouldn't one pocket be larger than the other to accept the larger intake valve?

There are indeed pockets and they look identical to me. I'll digest that offset info and see if, by multi-circumstantial evidence, I can feel safe in one orientation.
Any opinions on that possible arrow in the photo?

Any opinions on that possible arrow in the photo?

It's possibly an arrow! I really can't hazard a guess as to exactly which way it's pointing.

the repair manual states " make sure the piston top
mark "vorn" is ahead (to the front) .
The arrow points into the driving direction.
I suggest you continue to clean the piston top
to reveal any remnents of a arrow".

Does the "vorn" mark favor one side of the piston top
to another ?

also- to press the wrist pin back into the piston,
heat the piston up to 176 ...210*F