Just picked up the July edition of Classic Bike to look at the (brief) article on buying M20s...Both Mick and myself were asked to give our views and say a little about our own bikes. Sadly for Mick they have got the pics mixed up and my battle weary mug has been attributed to Micks comments, whereas I have shed a few years...thanks Mick!
As is the nature of magazines these days the article is short and not very detailed but on the whole reasonably accurate....but burning out exhaust valves in 2-3000 miles if driven hard? Where do they get these ideas?.Certainly not from me!..It seems perhaps some of the sidevalve myths are still alive and well...Ian

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

Ian, my exhaust valve burned out at around 4000 miles. Can't see that I did anything wrong. Correct plug colour. Timing correct. Valves adjusted correctly. Ridden hard. Unleaded fuel, but these bikes were "happily" running on pool petrol. Comments?

email (option): dannydefazio@sumpmagazine.com

Hi Danny..BSA valve steels were, and are considered to be top notch. The sidevalve engine though is a harsh environment due to the higher 'normal' operating temperatures of the type.
The valve seat and face will, therefore, require attention at more frequent intervals than would be the case with a cooler running OHV unit.
The 'margin of safety' between normal running temperatures and valve failure is also smaller as a consequence.
Valve failure takes one of two forms. Either the lip of the valve splits or burns at the edge of the valve seat or the valve seat on the side of the valve nearest the cylinder, which is exposed to the highest temperatures, collapses (deforms).
The exhaust valve lip is modified slightly in the SV to help it cope with the higher temperatures in this area. Above the valve seat and below the top face of the valve is a pronounced 'land' that results in a thicker edge to the valve. Refacing of the valve and/ or 'grinding in' the seats over a period of time reduces the width of this 'land', reducing the valves resistance to heat and leading to early failure once the limits are reached.
Premature collapse of the valve seat is also directly attributable to excess heat. So, anything that raises the operating temperature of the engine beyond 'normal' can cause the problem.
The usual candidates are 1) air leaks leading to a weak mixture...possible causes are worn carb, leaking head gasket, leaking around the timing plug in the head, leaking around the carb mounting flange and ironically a worn ex. valve seat/valve face.
2)Also possible are insufficient tappet clearance, incorrect setting of the valve lifter mechanism and incorrect carb settings.
3)Another potential cause of overheating is incorrectly set ignition timing..particularly if it is set retarded. This can occur even when the timing was initially correctly set if the contact breaker gap has not been maintained at the correct setting...a reduction in the gap retards the timing.
4)Finally is the question of pattern valves..if these have been manufactured from the incorrect steel the potential for failure is self evident.
In conclusion there are multiple conditions that can affect the valves performance and lead to premature valve failure. Of these conditions examples can be routinely found in many of the sidevalve engines currently in use. If all the specified conditions and settings are correct exhaust valve failure will be an unusual occurence and only the higher level of valve/seat maintenance dictated by the SV layout will be required...Ian

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

Ian, is that the long or the short answer?


Seriously, I don't know why mine failed. I guess I must have done something wrong, but I can't see what - except possibly the points. Will have to check them more often. Thanks for the info.

email (option): dannydefazio@sumpmagazine.com

There's nothing worse than half a story!...I wouldn't hazard a guess for your particular bike Danny, I know nothing of its history etc.
Here's an interesting thought though...if the average army bike covered 30,000 miles during it's career and used an exhaust valve every 2500 miles ('2-3000 miles of hard use') it would use 12 exhaust valves in that time..say 8 if it wasn't ridden hard ALL the time.
There were 126000 M20s manufactured during the war...so theoretically they would have consumed something over 1,000,000 exhaust valves...hardly a success story! ...Ian

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

I wonder if the awful truth was that the Army wanted BSA M20s simply because they were disposable and no one expected them to outlive their exhaust valves?

email (option): dannydefazio@sumpmagazine.com

Your a Cad Sir!..What a ludicrous suggestion! ...and if it was true the Army miscalculated by 67 years...and counting ...Ian

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

At last I've got this bloody computer to work!
Hi all, the exhaust valve story wasn't from me, I can usually get up to 8000 miles between regrinds. I don't think Ginger heard/ took note of all that was said. Still a good article though. Its a pity the bike featured wasn't the best example, I'm sure a fully loaded mid to late war example wold have been better.
Cheers, Mick.

email (option): mick@motorbikemike.org.uk

Motorcycling is full of 'folklore' regarding the characteristics and 'faults' of many bikes.
These are often loosly based exagerations relating to features of a bike..'My Panther used to kick back so hard it threw me over the handlebars and out of the garage window'..or 'That M21 was a great bike, it would pull in top gear up hill, down dale with a double adult sidecar, the wife and two kids on board'.
or..rash statements about the 'faults', not based on a thorough knowledge or understanding of the technical facts and often second hand information..'Ariel Square fours sieze the rear cylinders', 'Sidevalves burn valves and blow head gaskets', BSA A10s have a weak timing side bush'..etc. etc.
There is usually something that forms the basis of these stories..singles do kick back if over advanced or incorrectly timed and they do pull well at low revs, albeit slowly. Ariel Square fours do run hot and wear the rear cylinders more quickly than the front, you can easily blow a sidevalve head gasket or burn a valve if the bike is not set up correctly and A10 timing side bushes will fail if fed dirty or insufficient oil...
A good example is the Triumph pre unit frame..just about anybody will tell you they are 'flexible'..but why are they? You won't find many who can answer that...it is better to consider these stories in detail to deduce the reasons for them and their degree of accuracy rather than to blindly attribute them to 'design faults' and endlessly repeat them... particularly so in the case of bike magazines who seek to guide or inform others..Ian

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