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The Fabricator

R1150GS Light Weight Wrist Pins

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The Fabricator

For the adventurous.

Currently, I ride a 2000 R1150GS.  I also have a 1977 R100/7, 160,000 miles.  

When I bought my R100, there were light weight wrist pins offered by San Jose BMW.  I bought and installed them and I can say it made the engine noticeably smoother.

After logging 10k miles on the R1150GS, I started to think about making it smoother.

Both engines have a sweet [smooth] spot around 3200 to 3400 rpm.  I think it is inherent in the design of the opposed twin and the balance factor BMW chooses.  The only imbalance inherent in the design is a rocking couple due to the connecting rod off-set.  

I looked for lighter wrist pins offered for BMW R motors by anybody, but did not find such.

I scoured motorcycle engine parts catalogs for wrist pins by diameter and length and wall.  The stock pins are 22 mm diameter.  It was easy to find something thinner wall and longer.  

One problem is BMW fits circlips in a groove in the wrist pin external to the piston, while everybody else uses a shorter pin with a  groove in the piston for a wire clip.  This means an groove would need to be machined in the piston.

I could not figure out how to hold a piston in my lathe.  If I had, I would have done it myself.

I took it to a machine shop.  For $200, the machinist cut the groove and shortened the wrist pins.  I used a Honda wrist pin clip.

 

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The machinist was able to make an arbor to hold the piston in his lathe.  The arbor is now my property as I paid him to make it.

I think the engine is smoother in the 3,000 - 3500 RPM range, it was hard to gauge because the engine was pretty smooth there already.  Now it is ultra smooth.

Incidentally, I re-used the head gaskets.  I sprayed Copper Coat on both sides.  Now 10k miles later, no leaks.

I think the project cost around $300.

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