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Re: 1944 BSA WDM20 boat anchor and other gearbox hell

What makes you think it is overheating? A cylinder head temperature of 350 is not hot for a M20. My bike on a hot summer day, ridden fast will show 500 degrees at the head.

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

200 - 250°C measured under the plug on my M21 with alloy head under normal riding conditions.

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Chad the piston skirt clearance is 3.5 thou to 5.5 thou. I err on the higher side as despite careful running in, I've had two pistons nip up over the years at the lower figures. The ring gaps are 8-12 thou. Ron

email (option): ronpier@talk21.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

BSA factory service sheets give a bore clearance of .004"-.006"...Ian

email (option): ian@wright52.plus.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Useful Data from my Military WM20 Maintenance Manual. Ron
Scan-20220805-2

email (option): ronpier@talk21.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

There is, in fact, some variation in the quoted piston/bore clearances for the M20...Factory worksheets (post war) state .004"-.005", the 'Air Publications' workshop manual gives .0035"-.0055" and the REME M20 standards book shows .003"-.005"....So, .003"-.0055" if all those figures are taken into account..(Those tolerances apply to BSA solid skirt piston types)

Any of these clearances can be applied and will work effectively if the engine is set up correctly and suitable running in procedures are followed...However, in the case of the lower clearances great care should be taken not to load up the engine in the early stages (say the first 200 miles)...Do not labour up hills in too high a gear but, conversely, do not over rev the engine in the lower gears, just ascend the hill more slowly in a lower gear..
Accelerate gently from a standstill at junctions etc. and on level ground run on a 'rolled off' throttle with as little load exerted as possible...Beyond these early stages loads can be increased gradually in short bursts withh full loads being used in the 750-1000 mile range...I have routinely bored to lower tolerances, the only exception being on tuned competition engines where a shorter (or no) running in period is necessary...

As well as careful running in it must be ensured that no condition exists in the set up of the bike that will increase the operating temperature..For example, retarded ignition settings, a weak fuel/air setting for the carb or any air leaks in places such as the exhaust valve seat, cylinder head gasket or carb mounting flange...

If all of the above is adhered to there is no reason whatever why the lower tolerances should not be applied...That after all is the purpose of providing an upper and lower tolerance...I would imagine tens of thousands of M20s and other machines have had the lower end of the tolerance range applied entirely successfully...

Having spent years building scores of engines for other people to use, one of the things I rapidly learned is that peoples expectation of what speed and load an engine can stand during running in can vary enormously with some simply 'overdoing it' when they believe that they are treating it correctly...'Gently, Gently' is the golden rule early on in the running process with the 'low load in any situation' rigorously applied...

If, however, you don't have the patience for gentle running in, if you operate a machine with a sidecar that inherently is exposed to more operaing loads or if you live in an area with very high air temperatures the upper end of the tolerance scale would be my recommendation....Ian

email (option): ian@wright52.plus.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

My first bad experience was with my rebuilt 500cc engine which was rebored to a lower tolerance by your engineer Ian. After over 500 miles of patient running in, it nipped up and cleared quickly 4 times over the next 50 miles. The piston was ruined so a new BSA piston was fitted and at least another 1 thou honed out.

My last experience was about 2 years ago after I'd had a new liner pressed in for my 600cc piston. This all performed by my own engineer Ainsley. Again I'd ridden carefully for over 500 miles and the piston nipped and cleared about 8 times on a 30 mile ride home from Southampton. I just had to let it cool for a few minutes each time and I rode with full choke in case it helped. The piston cleaned up ok and Ainsley honed it out to 5+thou clearance.

You might know that I am a stickler for correct ignition timing. The bike has been fine now for well over 1000 miles, during which time I have hit 60mph three time :scream: Ron:slightly_smiling_face:

email (option): ronpier@talk21.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Usually when a piston nips up after 500 miles or more the problem is not the bore clearance but weakness of the needle setting and/or the main jet settings....Altering bore clearance may only be addressing and compensating for an underlying setting problem that is causing a measure of overheating...At 500 miles the piston/bore is half way through the running in period.The piston is therefore 50% bedded in and unlikely to nip up unless badly abused. However, as more performance is used it is moving into the operating range of the carb that employs the settings at something over half to full throttle...These settings are not used anything like as much, or at all in the case of the mainjet, earlier on in the process and hence no problem.....
As I said in my last posting the upper and lower limits are those deduced by the manufacturer that the engine will operate within well and there is absolutely no reason not to use any part of the tolerance range...Only an external factor will lead to problems if the bore tolerances are adhered to...I have always bored my personal standard engines to the lower limits without problems at the running in stage...As a general rule and assuming the timing is correct and there isn't an issue with air leaks, it is better to start the running in procedure with carb settings that are known to be a little rich and then to weaken them as required when the engine is nearly run in...
Particular attention should be paid to the carb settings higher up the range. It is also worth noting that standard Amal carb settings are not 'cast in stone' but a guide only, the intention being that settings are adjusted as required to suit production variations in the ports gas capacity, cam tolerances etc. and in older machines, varying degrees of wear in carburettors that are not brand new...

I believe a close initial bore tolerance followed by careful running in produces the optimum conditions for long engine life and that is why I have routinely followed that practice...Others no doubt disagree with that theory and may well apply a larger tolerance or different running in procedures for whatever reason....However, that does not alter the fact the lower tolerances can be used without problems given that all other factors are correct....Ian

email (option): ian@wright52.plus.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

You may remember Ian, that on a trip to Beauluie many years ago, You suggested raising my needle a notch....Which promptly snapped when you went to reinstall it. I had to nick Rob Miller's needle to get home. (Lucky for me, he had trailered his bike). Ron

email (option): ronpier@talk21.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

I remember that Ron...and the overheating problems on the way there!!....:stuck_out_tongue_winking_eye: ....Ian

email (option): ian@wright52.plus.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Ron Pier
You may remember Ian, that on a trip to Beauluie many years ago, You suggested raising my needle a notch....Which promptly snapped when you went to reinstall it. I had to nick Rob Miller's needle to get home. (Lucky for me, he had trailered his bike). Ron
I remember that, my M20 wouldn't go because both my rebuilt mag and Martin Bratby carb failed at the same time, and rather than helping me get it fixed my friends robbed it of donor parts for Rons bike, just as well I don't bear grudges and witter on about these things years later.

Rob:grimacing:

email (option): robmiller11(a)yahoo.co.uk

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

I read these last posts with interest because I'm currently running in a new piston & bore in my WM20 taken out to 600cc. About 100 miles so far.
Speed is kept to about 40MPH, allowed to 45 on downgrades & not lugged. The plug colour is a nice mid brown.
The carby is a 1" 276 as my correct 1 1/16" has issues. Main jet's a 170 & the needle's in #2 groove. Would it be a good idea to richen the mixture, & would I raise the needle, use a bigger main jet or both?

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Bob I presume you have fitted an M21 crank? The Amal book quotes a 160 main jet and needle position 2, but in a 1 1/16" carb. I'm not sure how it calculates with the smaller carb. Maybe raise the needle a notch for running in and see how the plug looks from then. Ron

email (option): ronpier@talk21.com

Re: 1944 BSA WDM20 boat anchor and other gearbox hell

Hi Ron, thanks for the reply. Yes, there's an M21 crank in the engine so for practical purposes the engine's a standard M21.
I'm glad you pointed out the 160 main jet, I'd thought it was a 170. So while the carb settings are a bit mixed up for an M21, it's not too lean. I'll raise the needle a notch for the running in.

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