M50 Overbore

Mike Pemberton is the man to ask, he has much experience  in overboring and also stroking these engines.

He made me the parts to convert mine into a 85x88 499cc.

3% is nothing.  Even the published recommendations vary quite widely.

Sorry Tommy...misunderstood..anyone else know?  It's a featherbed so should be like other featherbed bikes?

Normally are best held in check by cross referencing your idea and calculations with others of similar interest.  As little is coming from this quarter maybe this link will help. 

 http://www.brufnut.de/SS100/ENGINE/CRANK_BALANCE/crank_balance.htm

Whilst balance measurement was part of my remit during working years, the designers produced the factors we were measuring.

Jon

Hi Tommy,

   To obtain the perfect balance with a reciprocating single cylinder engine is impossible and the best you can expect is to obtain a smooth period at your most used rev range. I guess you are looking for more speed which will need a higher balance factor. Unfortunately what works well at 70mph will probably be rough at 40mph in the same gear. If you intend to use more revs, 60% is a good starting point, but there are so many other factors involved it's all a bit of guess work. Unless you want to do it all yourself, it might be best to get your flywheel assembly dynamically balanced and tell the operator what sort of rev range you are looking for. By the way, did you consider the extra weight of the thicker little end bush needed for the BSA piston ?

Why not just get a piston and cylinder for an ES2?  Or even a whole ES2 engine while you are at it

Mike?

Unfortunately this is not something that can be achieved just through calculation, there are simply too many variables involved, and probably the biggest is the chassis, when Nortons put the ES2 and M50 into the featherbed chassis they had to change the crankshaft balance to suit and seem to have done it again when the slimline frame came in, likewise the 750 Commando has a radically different balance factor to the Atlas, a neighbour of mine who put a Commando lump into his wideline broke two cranks before he realised this.

The BMW R69S had to be fitted with a crankshaft torsion damper which the very similar R69 never had because they would literally snap the crankshaft off behind the flywheel if held at a certain rpm, which was found to vary considerably between otherwise identical bikes.

Ariel Red Hunters which had been tuned with HS parts needed to have the bottom front engine bolt left out to cure the resulting vibration, contrary to all accepted wisdom.

If I were you, I would try it with the crankshaft as is and the engine sprocket that you find best before doing any balancing, because firstly it might just be ok, and secondly it is always easier to take weight off a flywheel than to put it back on,

Be prepared for a lot of trial and error, and good luck!