Tech Tips!

by Jonathan Teraji

The Crankshaft Story


YSR50 Crankshaft

The YSR50 crankshaft is infamous for the left crank arm breaking off of the left crank wheel.

This is a close up of a left crank arm that is ready to break off of the left crank wheel

This is one that has broken off

This is typical of the YSR50. After about 7000 miles, or 4-5 race seasons this arm WILL BREAK. Guaranteed!

It is not possible to predict the exact time it will fail, but it will fail.
This is the reason why when engines are sent to us for rebuild, we frequently recommend REPLACING THE CRANKSHAFT rather than rebuilding. (Yes we do rebuild crankshafts, however, first we inspect the crankshaft, try to find out the history, i.e., mileage etc of the bike, and determine the probability of breakage)
Also, FYI, it is not very much more to actually buy a new crankshaft, compared to rebuilding a crankshaft when you figure parts and labor into the equation.
PREVENTION???? No, all will eventually break, however, there are some things you can do to delay the inevatable.

1. Weld and balance the Crank

2. Change main bearings

3. Check integrity of rotor

4. Don't use synthetic oil

Why does this happen? Engine was designed for a red line of around 8K. As these bikes are more frequently than not run at around 12k rpm the excessive weight of the rotor really stresses the left arm. If the crank is at all out of balance this really stresses the arm. Result? Breakage!

That's it for now. Shoot me some E-mail if you have specific YSR questions and I'll do my best to answer them as time allows, and/or suggestions for topics in "Tech Tips!" Remember this page is for you!
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