16H crank balance factor

Balance factor for SV & OHV singles is 50%

Based on personal experience I would be wary of going below 50%. I once fitted a GPM Big 4 piston to my 19S (balance factor 48%) and the vibration was so bad that I suffered blurred vision. Is your piston an original item or a later aftermarket item? I would suggest checking the weight of your piston, including gudgeon pin, against an original (sorry, I don't have have any piston weight data for 16H). If it is more than just a few grams heavier I would consider rebalancing the crank.

Cheers, Ian McD

I don't disagree that balance factor is important but a couple of percent each way (50 to 48) cannot make the difference between comfort and disaster.  Vibration frequency depends on the square root of the mass, so it's always difficult to change it very much. And the size of the effect depends on the mass, so a couple of percent change would be hard to measure. A single will always be severely out of balance unless it's improved with such things as balance shafts etc.

There was a lengthy discussion on an Australian Yahoo Group a number of years ago around balance factors and people reported using factors as high as 64% on side valves - supposedly with success. I had mine done at 58% when it was last apart and it runs nice and smooth for what it is. I am sure that there are plenty of people happy at 50% too.

The most significant thing on mine was the mounting of the engine into the frame. The cases were found to have been narrowed from excessive polishing and the bolt holes had flogged out so that the whole engine became lively at open road speed no matter how tight the bolts were done up. In hindsight curing that was way more important that the balance factor.

The GPM Big 4 piston was 61g heavier than the original item. According to my calculations that changed the balance factor by just 3% (from 51% down to 48%). Maybe my calculations are dodgy, although the 51% for an original piston is in the right ball park. The vibration introduced by that piston change (no other changes to engine mounts etc) made the bike unrideable. The Big 4 piston was quickly replaced by a Matchless flat top piston and the crank rebalanced to 64% (again by my calculations). The vibes really smooth out from around 55mph in top.

The following was posted by Jim Comstock on the AccessNorton forum a year or two back, from his 30 year old training notes.

"The balance factor of about 65% creates the lowest peak imbalance force if the engine were mounted in a infinitely rigid frame. The balance factor of around 58% creates the lowest average imbalance forces in the infinitely rigid frame. The Commando engine with isloastic mounts creates the least amount of stress on its bearings and cases at 53% as that is where the engine motion creates a circle on its mounts."

Our frames aren't infinitely rigid so resonance effects then are tested via the seat of the pants and will vary somewhat from bike to bike.

Mine works nice and smooth at 58% but i don't usually ride much over 50mph - personal choice due to the limitations of a sprung seat and poor brakes. Ian MacD above seems to ride faster and so the higher balance factor works for him.

I would probably aim for a lower figure, around 58-60%. The decision to go for a higher figure was influenced too much by Phil Irving's "Tuning for Speed". Tuning for Comfortable Touring would have been more sensible! It would be preferable for the vibes to smooth out from 45mph upwards. If they were to increase again from 65mph it would be no bad thing as it would act as a reminder to back off a bit. As things stand the engine is still remarkably smooth at 70mph which is not a sensible rate of progress on such a long stroke single.

If original spec pistons were still available for the 19S I probably would never have changed the balance factor. I ran this bike for 30+ years with an original piston (51%) and the levels of vibration were acceptable. But I do prefer it with the revised balance factor.

Ian McD