Classic British Bike Magazine

BSA M20

A power loss, a burnt exhaust valve, and an easy fix

 

The rusty looking valve is the exhaust valve. Bad adjustment, or bad luck can burn the edge off of it, and then the compression has gone. It doesn't take much to put an engine out of service. Just a few thousandths of an inch here and there is enough.

The Problem

 

We've been running a 500cc BSA WM20 for about a decade. The "W", by the way, refers to a war department bike; meaning a rigid frame and girder forks. Maximum speed for this sidevalve single is around 55mph. But cruising at 45mph is more realistic.

 

One day, riding two-up through the sometimes beautiful county of Kent (but sadly, sometimes not), we ran out of puff up a relatively minor incline. The next hill was even harder, and so on. Fortunately, the final few miles were on level ground, and we managed to make it back to the Sump garages.

 

It was only then that we realised that the power loss had been going on for a little while longer than that; maybe a few weeks. Being a BSA M20, you don't have much power in the first place. So you can argue that the loss of a little isn't going to raise a very big flag. But on the other hand, you really do need all you can get if you want to even pretend you're keeping up with urban traffic.

 

The engine had covered around 4,000 miles since a full rebuild. There was a spark and there was petrol. But the compression was definitely down. So the next thing to do was to get the head off and check the bore and the valves.

 

 

Sump's own BSA M20 parked on an East London bridge. A typically good sidevalve slogger, but it's not indestructible. Eating an exhaust valve occasionally happens.

 

 

The Solution

 

Almost instantly, we could see that the exhaust valve was burnt. Sidevalves are hugely inefficient engines. But they were popular in their day because they were relatively cheap to manufacture and assemble. And being generally low compression engines, they would run on more or less anything volatile. But they do run hot. The air-fuel mixture takes a convoluted path between the carburettor and the exhaust port. The combustion chambers are rarely ideal. The spark plugs on sidevalves are often in a less than perfect position. And thermally speaking, they waste oodles of power that could be better used to spin the wheel.

 

 

A BSA M20 exhaust valve showing the extent of the damage. This one is definitely scrap - unless you can think of a new use for it.

 

 

If the exhaust valve lifter isn't set up correctly, it can raise the exhaust valve off its seat. Just a few thou will be enough to heat up the edge of the valve as the combustion gases try to slip past. Eventually that edge is gone, and so is compression. And no compression means no power.

 

Our valve lifter looked to be set up right, but that might not always have been the case- (and the BSA M20 valve lifter design isn't very good anyway).

 

 

 

A Terry valve lifter tool. This was sold on eBay in 2014 for around thirty quid. Buy one if you can. It will sell onward without too much trouble. But if you own a sidevalve for any length of time, you'll need it more than once. We're too modest to show you our own jury-rigged G-clamp effort...

 

 

The valve had to come out, and a new one needed to be fitted. But that meant using a valve spring compressor. However, none of the compressors we had in the tool box would work. The BSA M20 needs a different design. To speed things up, we ended up making a crude device out of scrap metal; essentially a G clamp with a fork to compress the valve spring in the valve chest. The tool took about an hour to make.

 

 

BSA M20 tappet chest. The exhaust valve is on the right. It's that spring that you've got to compress if you want to remove the collets. The little stub on the right is the valve lifter. That raises the tappet foot. Keep it adjusted if you'd rather ride than repair.

 

 

The valve was duly raised, the collets came out, and the pressure was released on the valve spring. Once the valve was out, it was even more obvious that it was useless. One edge was flattened slightly, and it was paper thin. But the valve seat was fine. All that was needed was to lap in the new valve.

 

A smear or two of valve grinding paste did the job very quickly. The residue of the paste was wiped away, and then barrel and valve chest was doubly checked. No more paste. Reassembly took no time, and we torqued down the head. This needs to be done in stages, and re-done after a few miles. Get a manual.

 

After that, the bike started on the second kick and burbled happily into life. That was some time ago, and it's still burbling.

 

Conclusion

 

This is a fairly easy job for the average home mechanic. You'll need Whitworth tools, but you should have those anyway if you're messing around with old British bikes. You'll need a valve lifter of some kind. You'll need a new valve (these are quite large with long stems, and they're comparatively cheap and available). And you might need a new head gasket unless you've got a solid copper one fitted, which we prefer (as opposed to a modern composite gasket). The solid copper gaskets can be annealed and re-used, and they're supposed to transfer heat better between the cylinder head and the barrel.

 

It should take you the best part of an afternoon, or maybe a full day if you're a headscratching kind of man. Or woman. But you needn't be daunted by this job. There's not much you can do wrong. You won't disturb the ignition timing. But you'll need to check the tappets (which is also a simple job).

 

Think you know better? Good, drop us a line at Sump and we'll include your thoughts/experience.

 

 

BSA M20 & M21 T-shirt:
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BSA M20 T-shirt

 

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