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Old 11-30-2012, 06:34 AM   #1365
SS Blowhard
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Joined: Oct 2008
Location: Gold Coast
Oddometer: 2,690
Originally Posted by Stagehand View Post
I SInce the short width of the R65 stems from the short con-rods, and one is required to shave the cylinder bases down anyway, is it feasible to simply put short con-rods in an R100? With short headed pistons, (JP's?) one should be able toget he same volume out of the jug... I doubt it, I bet the conrod bores are the wrong size, it would be a custom con-rod deal. Maybe thats ok?
Then there's making the pushrods do right. Use R65 pushrods?

15mm shorter conrods are available from a number of sources including Carillo
Not cheap and neither are std BMW con rods.
30mm overall width reduction and the changed intake characteristics look appealing.
The modification is not unusual on road race airheads.

I've also wondered if short top / short skirt pistons designed for a longer rod combined with a shorter rod an cut down barrels would result in a 60mm or better than 2" narrowing.
Whether the piston skirt extends past the end of the barrel at BDC and the increased angle of the conrod clearing the barrel end at half stroke wouild seem to be important.

Once upon a time I might have tried it.
I find the width of the 980 motor not so much of a burden that I want to throw $$ at it just to slim it down. It is rare that I need to be one inch closer to the edge of the track and the inch less still doesn't get the bike flat enough on the ground to get it under the gates they use on forestry tracks around here to keep the 4x4s out.

Maybe at rebuild time when I need to replace barrels pistons and conrods I will also feel like adding an amount equal to the parts cost to do the mods to slim it down. I'd be looking at $4.500.00 - $5,000.00 plus new headers and some more for have Paul Rooney or Ray Peake do the work using new parts.

I believe I could have picked up Rays 860 motor complete for under 5.

It may be cheaper where you live.
My bike is slow but the earth is patient.
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