Hi Drew...I have the pistons made for the 720 engines. (I'm UK based). To do the 720 conversion you will also need an unlinered M20/21 cylinder. These were fitted postwar to both models.
I have just used the last piston from the previous batch and will have to get a current price for the next lot. The pistons are manufactured in America and exchange rates and the current state of the economy tends to affect prices from batch to batch.
The pistons are forged as opposed to cast and a 'straight fit' with no crank rebalancing required. They are made with the correct gudgeon pin size etc. and a 'modern' ring set....Ian
email (option): ian@wright52.plus.com
Hi
Sorry to jump in on the thread but could you put my name down for a piston please Ian and what is the best way to get in touch as I have tried to mail
Regards
Mike
email (option): Michael.stewart@hotmail.com
Jeff, I'm afraid you cannot buy a single piston...they are not a stock item but a custom made product produced to my specs. and minimum batch quantity is six..They can also be utilised to 'oversize' the B33 engine.
Mike...Mail me again on the address shown....Ian
email (option): ian@wright52.plus.com
Hi Bram..it is not necessary for me to have six customers before ordering pistons...though a couple are beneficial in keeping the initial costs of the batch down.
I have just used the last piston from the previous batch in a rebuild so I will post shortly with an up to date price for anyone who is interested..Ian
email (option): ian@wright52.plus.com
Hi Mark..An M21 flywheel assembly is needed for the 720 motor to get the 112mm stroke as the engine has to be bored and stroked.
A look at any cylinder will tell you if it is linered or not..the standard liner is of the 'top hat' type with a larger diameter section at the top which is recessed into the gasket face to make a flush surface.
This larger diameter part of the liner can clearly be seen running around the edge of the bore. Externally the barrels look pretty much the same..M21 barrels have a slightly bigger inlet port..1 1/16" as opposed to the 1" port of the M20. Later barrels also have a thicker base flange and if you get one of these you may have to change to the longer holding down studs that go with them....sometimes the originals are just long enough though.
A solid copper head gasket is used and this may need to be eased out around the cylinder bore to match the larger size...again though, there seems to be some variation in gaskets.
Work on the ports,valves sizes and the carb size used are all optional..Most people will go with the M21 sized carb and port to help things along a bit and (so far) all of the engines I have built have retained standard valve sizes. Skimming the head to raise the compression ratio is also an option, which I did do on one of mine. I ran a 1 1/18" B33 carb..a 289...with a matching port on the 720 I sold a couple of years back...another option.
There are no implications in terms of reliability, my other 720 is just as reliable as any of the standard ones I have had and I have quite a high mileage on it now in this form, as it was the first one I did.
It should be noted though that this does not turn an M20 into a Gold Star..the inbuilt inefficiency of the head and port design of a sidevalve is still there and this manifests itself in a limit to top end performance.
So with a standard set of components, apart from the piston and crank,top speed only increases a limited amount. Cruising speed also goes up a bit and this is achieved at lower revs.
Where the conversion really pays off is in the large torque increase up to cruising speeds..The practical effects of this are better acceleration, vastly improved hill climbing ability and all at lower revs...so fuel consumption improves as well.
Personally I think it is well worth the trouble as I use my M20 a lot and frequently do fairly long journeys on it.
There are other, cheaper options which won't deliver quite as much but do markedly improve over the standard engine...just a straight M21 conversion for instance to name one.
Anyone contemplating altering their M20 should firstly consider what they want from the bike, then look at all the options, and then go with the most cost effective change that provides what is required.
To date none of the people running a 720 seem very keen on going back though!...Ian
email (option): ian@wright52.plus.com
The inlet port is only 1" for a short distance at the carb end and quickly opens up to, and beyond, 1 1/16" nearer the valve, so alteration is very easy. The castings are the same, but I could well imagine the M20 and 21 carrying different part numbers (for ordering spares) if they had different port sizes. However, I have never checked this. I have had barrels with both port sizes as intended (assuming that wasn't just a variation resulting from production tolerances) but as they are interchangeable, over time any engine could end up with either barrel fitted.
I also don't know if port sizes were ever 'standardised' between the two models at some point..again a possibility...
All of the above are questions I have never needed to resolve as I have enlarged the ports when necessary and ignored the slightly larger port when fitting them to M20s. I guess if you have the correct port size that is a bonus..Ian
email (option): ian@wright52.plus.com
Apparently, 'Silmocrom' liners were a new invention at the time, patented on 15-9-1938. They were 'spun' cast iron liners but with chromium, silicon and manganese added at the casting stage as hardening elements. Combined with alterations made to the cooling rates and quenching treatments this produced a harder liner material with enhance wear characteristics....Ian
email (option): ian@wright52.plus.com
So after all this expense, no one has tried my trick of filling the head in for about 10 cents of mig wire? Oh well do it the hard way! I'm surprised Tan hasn't experimented? 
hi,just pondering on the questions asked.has anybody tried going down the route of changing the crank for a b31 unit,which in theory would give more rpm ?running a standard m20 piston would lower the swept volume to 465 cc.however running a 90mm piston combined with the 88mm stroke of the b31 gives an over square configuration of 560cc,has anyone any thoughts?,has it been tried?cheers rick
email (option): richardholt@rocketmail.com
Hi Douglas...I have also welded the heads as you have, to get a better compression ratio etc. I started with a ratio I knew to be more than the figure that 'received wisdom' said was the upper limit and then progressively worked backwards by testing, measuring the ratio and then grinding some out and repeating the procedure.I wanted to determine for myself what was the practical upper limit. The main problem with the high ratio was 'pinking' which soon appeared when the engine warmed up and was under load. After a while I knew exactly how far I could get from home on my favoured 'test course' before it would occur!
I found that about 7~1 was the highest practical figure...have you measured the ratio in your engine?
The problem with fitting a B31 (or B33) crank to a sidevalve involves a number of problems, both physical and theoretical.
Firstly the timing side mainshaft has to be changed for the longer sidevalve one and the side of the flywheel has to be machined back. Secondly, there will then be a problem with the existing conrod/ piston combinations..there isn't one that will put the piston at the top of the cylinder at TDC. So some custom parts will be required or alternative parts from other engines.
The resulting set up, if you did it, would then provide a crankshaft assembly more able to rev. This, however, could be achieved simply by re machining a set of M20 wheels.
That in itself would not necessarily make a faster or more powerful engine without serious alteration to the porting and valve layout, though it might get there more quickly. The inability to pass more gas is ultimately the limiting factor in these engines and without that possible power increases are limited.
'Oversquare' characteristics are desirable in OHV designs as this allows more room in the cylinder head for bigger, or multiple valves and that means more valve area and consequently more gas flow, if the cams and ports are matched to the layout. In the sidevalve it is of no benefit..in fact arguably it is a disadvantage because it increases the size of an already inefficiently shaped and overlarge combustion chamber..though admittedly this is all in the squish area..Ian
email (option): ian@wright52.plus.com
hi,just been reading some of this web-page concerning a 6-stroke engine,a bit beyond my comprehension,but interesting http://www.sixstroke.com/specifications.htm take a look,cheers rick
email (option): richardholt@rocketmail.com