What you do with the gearing on an M20 should be governed by what you use the bike for and how you like a bike to 'feel'...
The 18T gearbox sprocket fitted to a WDM20 lowered the gearing for convoy and off road use and everyday road speeds in those days were generally lower than today anyway...
If you ride your M20 mainly on the road then the fitment of the 19T gearbox sprocket used on the 'civvy' bikes may be a better choice...It's then a matter of changing the engine sprocket to raise the gearing if you like it that way...
However, you must bear in mind that the M20 has very limited power and thus a limited ability to pull taller gearing...Big sprockets don't generate power essentially, they just move what power you have up or down the (limited) performance range....So, bearing that in mind, very tall gearing will soon adversly compromise the performance of the bike up hills and you'll then spend more time in the lower gears in hilly conditions...So even the road conditions where you live might affect your gearing choices...
Also, with so little power any reduction in what's available will have an effect...Thus a freshly rebuilt and run in engine will be a bit more capable than one that has racked up a lot of miles and is down on power....Personally, if I had an M20 with an engine in standard spec. I'd go with standard civilian gearing on both engine and gearbox initially and see how you find it on the road....It's then an easy job to adjust things on the engine sprocket if you feel you want to...
I've found various owners claims of the capabilities of their standard M20s dubious to say the least and I've had plenty of experience of the type against which to judge them...I'd make any decision based on what you and your own bike feels comfortable with...
If your inclined that way, or if you intend to do a lot of touring a change to 600cc or beyond makes a lot of difference but that's another subject!...Ian
I agree with Ian on this. I would also advice standard civilian gearing on an M20. When I upped my engine to M21 spec, I tried a 21T engine sprocket, but found that was to tall for my liking. I've been running a 20T for years. Ron
Thank you , from what you have said , I will stick with the 19t one for now , not a big job to change it , once I have the bike on the road , and feel a need to !
I too tried a 21
In my case it eas because the club always seemed to end up on an expressway
Fie on the flat & I could keep up with the A 65's but anything that loked like a hill and it was back to 2nd if not 1st
