Hello all. Does anyone here know if I can get more than 48 mph out of my 1954 M-21 by changing to z larger drive sprocket? Has a 19t on it now.

If this is a viable way to gain speed, how many more than 19 teeth should I try?

Thanks, Wayne.

email (option): wzuccarello@yahoo.com

I've tried both 20 and 21t primary sprockets on my m21 plunger. The 21t was impossible with the slightest incline, the 20 is good solo but is difficult with a pillion on hills.

Thanks for the imformative reply. Was that sprocket on the primary drive or the secondary drive?
Thanks, Wayne.

email (option): wzuccarello@yahoo.com

When I built my WM20 engine into M21 (600cc) spec. I upped my standard 19t engine sprocket to the standard WM21 21t sprocket. It pulls like a train on hills with much more acceleration and higher top speed. I run with the standard WM20/21 gearbox sprocket (19t from memory)
I would have thought a 19t engine sprocket on an M21 is for side car work?
Ron

email (option): ronpier@talk21.com

I think the Standard M21 engine sprocket is 19T and they go down to 16T for sidecar use, presumably you select a sprocket that appropriate to the sidecar and local conditions (hills!).
I am running a 21T on my M21 and it goes very well and has no problem with hills at all, but then I have put one of Ian's Magic Pistons in to take it up to over 700cc!

Cheers
Pete

email (option): petercomley@web.de

19 tooth is std, 16 tooth for sidecar work, I would not use bigger engine sprocket because you will lose power on hills, but your top speed down hill will be greater ,my own WM20 has 18 tooth sprocket and will cruise all day at 55....however my tuned 1956 M20 has 21 tooth sprocket,gas flowed barrel, higher compression, Goldie clubman cams, 389 amal and will cruise at 70 on motorway!.....check your speedo with sat nav, mine reads 10mph slow.

Would it not be so that an M21 which only attains 48mph with a 19t sprocket is down on power rather than undergeared ? It can't be running out of revs at that speed.

email (option): 79x100@gmail.com

My thoughts also Rik.

Ron

email (option): ronpier@talk21.com

Sometime in 1969 I changed my M21 powered WD M20 to 21 tooth sprocket on the engine shaft (along with other mods) and have never regeated it. All MD M20's are under geared for milatary service and therefore not suitable for "modern" road. You won't regret the the change!

For what it's worth, my overall experience with an M21 spec engine and 21 tooth primary sprocket is the same as Ron describes; absolute top speed under optimal conditions is 65mph, with less downshifting on hills. I would like to change the primary sprocket back down a tooth and increase the trans. sprocket up a bit, but larger c/shaft sprockets beyond 19 tooth for the old boxes seem to be unavailable...

JDE

email (option): teladelujo@msn.com

Yes I find my 21t engine sproket is great for ordinary road work. But I have wondered about going down to a 20t to make the bike a bit more buzzy for convoy work and New Forest/Dartmoor lanes. Sometimes you just can't win Ron

email (option): ronpier@talk21.com

About 3 years ago I changed my std wd engine sprocket to a 20t

My engine is a std m20 so no extra power on tap.

My bike is quicker for sure and at these "higher" speeds less resonance occurs which must be good, also if and when I need to travel on faster routes, that extra few mph makes a big difference.

It does require extra shifting in the hills as I found in the "mountains" surrounding Buckfastleigh in July & other outings previous to that

I would not go back to a 19T again

Just my experience however, we all differ

Best regards

Jo'b

email (option): Jonnyob1@googlemail.com

Hi Wayne,

Having run many miles with a 1951 M21 (and still doing daily!) and 19 tooth engine sprocket and 19 tooth gearbox sprocket and mostly with a massive sidecar, I can definitely say that solo with those sprockets you should be good for around 65mph with that set up.

If you're only getting about 48mph is that because your engine is revving as fast as the engine's maximum - i.e. are the engine revs as high as you get flat out in the lower gears? – if this is the case then you can afford to try a 21 tooth sprocket (which, I think, by around 1954 was standard fitment anyway) - But I suspect (unless you have some large non-standard sprocket at the rear wheel) that in your case the engine is not rotating anywhere near those maximum revs, which indicates an engine well down on power rather than a gearing issue, or maybe even a binding brake?

I have been down to a 16 tooth engine sprocket when I first put a 'normal' size sidecar on, but quickly reverted back to the standard solo 19 tooth item as it pulled perfectly well enough without the unnecessary higher engine revs.

In fact with a 19 tooth sprocket on both engine and gearbox you should be able to achieve over 40mph in 3rd gear if you're a bit (lot?) merciless on the throttle!

Hope this is of some help!

Regards,
Steve

email (option): steveplumb1@sky.com

Have checked the partslist and found that 18 teeth for the M20 gearboxsprocket was standard till 1948, it was then changed to 19 teeth with the redesign of the gearbox.

But lots of literature only stated 19 teeth for the gearbox, so if you are very undergeared you probably have the 18 teeth sprocket.

I always fit the 19 teeth gearbox sprocket as the nearest hill is 200 miles from here. But on the other hand we do have strong winds which have almost the same effect on the engine as riding up a hill.

Regards, Michiel