This may be a little off topic as my question relates a 1936 Velocette MSS that I have recently acquired and rode in the 700km Durban to Johannesburg rally. (Bikes in this event must be pre December 1936)
After the ride, I stripped the engine down to determine the source of a rumble type knocking noise. I found a worn timing side main bearing, 0.012” crankshaft float and a balance factor of 0.84. The big-end bearing was fine and piston clearances were within limits.
While the main bearing and end float are easily fixed, should I be concerned about the balance factor? All literature I have suggests that this should be between 0.65 and 0.70?
Will the off-spec balance factor result in forces that may lead to premature failure, or is it simply a question of comfort?
I will be most grateful for any views and opinions from members on this forum.

email (option): pvlietstra at gmail.com

Balance factor varies for different engines and was originally determined by the manufacturer...

The 'ideal' factor is a compromise dictated by the structure of the machine and the perceived requirements of use...In other words as the engine can't be fully balanced anywhere due to its design a workable compromise is sought..

The engine vibration produced by the out of balance forces can be felt at different points in the rev range (sympathetic vibration) dependant on the design features of both the engine AND the chassis. At some point the chassis will vibrate 'in sympathy' with the engine...

Thus an engine with a given balance factor may be fine in one chassis but not in another (Triton owners take note!)...

So, a commuter machine, for example, that is ridden primarily in the low to mid section of its rev range will have a balance factor that moves the sympathetic vibration point to the upper end of the performance range where it is rarely used and thus is not so intrusive...

As many British single cylinder machines of 350cc-500cc have basically similar engines and chassis designs and were intended for similar uses a balance factor of 55-65% is common...BSA used 58% for M and B series engines in all chassis types I believe...That is certainly the only figure I have seen quoted for these models...

On the other hand I am currently building a 1970 T120R Bonneville and that has a balance factor of 80% as standard...and Triumph did alter that during the Bonnevilles fairly long production run due to engine and chassis changes that were made...

In conclusion therefore I would suggest you dig a little deeper to determine the original balance factor for your engine before drawing any hard conclusions about what is right or wrong...There are too many variables to draw exact conclusions from what is correct for other machines...Ian

email (option): ian@wright52.plus.com

Hi Ian
Thanks for your reply. Since my last post I did some digging. No one is quite sure what it should be but the Velocette owners club in Australia had figures for nine assemblies of this model, ranging from 61% to 68% and an average of 64%.
To reduce my balance factor, plugging holes means stripping the crankshaft. I have done this many times on various BSA's but the Velocette is a little different and I had no other need to strip the crankshaft. So I opted to drill holes in the counterweight. 2 x 12mm holes in each counterweight reduced the balance factor to 65%, so I am quite happy.
Once again, thanks for taking the time to respond.
Best regards
Peter

email (option): pvlietstra at gmail.com