Pushrod Length

[steek]

Hello,

The reason I'm posting this is I've been trying for some time to find out the difference in length between R65 pushrods and other BMW air-cooled bikes.

I needed to know this because I'd bought some lightweight cam followers/tappets (made by Wank Racing) thinking all BMW air-cooled twins were the same (a stupid assumption in hindsight).

I had found out through my searching that R65 tappets were different in that the cup contacting the end of the pushrod was placed nearly at the bottom of he follower as compared with the "standard" ones.   This was to compensate for the shorter stroke and keep the pushrod angle correct.

The ones I had definately had the cup bit at one end which was quite prominent.

When I asked the place I bought them from (I can't remember now why I thought I needed to do this) they told me they would work fine with shortened pushrods, but didn't tell me by how much.

None of the reference material I could find listed a length, but usually suggested this was "not available".

I thought this was a bit odd if not unhelpful.

To finally get to the point, I ended up buying some second-hand cam followers/tappets and pushrods off EBay so I could directly measure them.

Anyway, they're 288 milimeters long, confirmed Today by a chance finding of a set of pushrods (or Stößelstangen) on EBay.

Hopefully this might prove useful to someone in the future.

Cheers,
Keith

[Barry]

I have a road test from back in 79 that mentions the hollow lifters used on the type 248 engines in order to maintain pushrod length but it doesn't mention if the pushrods were the same length as in type 247 engines. 

Then I remembered there was service bulletin that explains the various differences for the type 248. It doesn't give a length but states The pushrods have been slightly lengthened

I take that to mean they are longer than the type 247 pushrods.

The bulletin won't post as a pdf but if anyone wants a copy pm me with a e-mail address.

[Tony Smith]

The R65 cam followers are different not to maintain the length of the pushrods but to keep the angle of the pushrod as it transmits the force from the lifter to the rocker within reasonable limits.

If the pushrods were "attached" to the upper end of the lifter like other BMW engines then the pushrod would contact, and potentially bind, both the holes in the head and the pushrod tubes themselves.

Getting pushrod lengths, cam followers and rockers into harmony is a frequent problem with special build VW engines. A long time ago I used to make my own pushrods for modified VWs, I had a set made up with a threaded section in the  middle so that I could experiment  until I found the correct length - the it was just a question of cutting tube to length and pressing new ends into them.

The difference between standard pushrods and those made for a stroked crank and ratio rockers was quite surprising. I used to use steel seamless tube for the simple reason that the one and only time I tried to use alloy ones, they bent.

Anyway, if someone rally wanted to make their own pushrods for a BMW engine I would not anticipate many difficulties.

[Justin B.]

Barry - I checked and pdf is an allowed file type.   What size is it?