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Advice Please

Hi folks, I've just opened up my engine (WM20) to establish what condition the piston, barrel and rings are in, having never looked inside such an old engine before.
Initially, it seems the rings are undamaged, and the barrel is very smooth. The piston "was" doing it's job well before I opened the thing up, but I'm unsure if the appearance is what you would say was "normal" for such a creature...???

I was also looking for any numbers on the piston crown, but the black crud may well be obscuring things, so, my questions are thus...

1: Is this a normal appearance for a piston?
2: How best should the black crud be removed (or do I need to think about replacing?)
3: Where are any numbers situated?
4: In the absence of any numbers, does this mean it's a standard piston/barrel?

For reference, the spark plug was a nice chocolate brown when it was checked.

Thanks guys.




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Re: Advice Please

Hello Ian,

Black crud on the crown of a side valve engine is pretty much the norm, most of the time, in my experience. It's especially likely if the bike has been used for short runs when the mixture may be rich from the choke and the engine hasn't had a chance to settle into a long run to burn off the crud. If you gently remove it with a piece of wood or very, very carefully with a screwdriver then you'll find some numbers on the piston crown.

As far as I can see the piston doesn't have any obviously nasty scrape marks along the side (sign of seizure) or any severe oil blow by stains, so I'd be tempted to say it needs no extra attention. Oil blow by is the dark stained area that's just below the gudgeon pin and below the rings on each side. I've certainly seen worse than yours. Others may have a different view. As "The Motorcyclist's Workshop" (1941 printed) says, you have a nice 'well-seasoned piston'. All you need to do is check the little end and big end for up and down movement. There should be very little of that! 5 thou is sometimes mentioned as sufficient. Again, different owners may have different ideas about that.

Was there any reason you took the barrel off? If it ran well before then it may not need any fettling. It might be advisable to check the valves and their seats.

I hope this helps,


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Re: Advice Please

['There should be very little of that! 5 thou is sometimes mentioned as sufficient.'] ...

There should be NO perceptible up and down play in the big end bearing...If there is it will require attention. However, there will be 'side to side' end float of the conrod and some 'tilt' along its length will be evident, particularly if the engine has done some miles...

Consequently take great care when checking the big end that you are only applying a vertical up and down force...

The small end should turn freely and the piston should fall easily from side to side under its own weight...If the small end shows any sign of tightness it will need to be eased out...That is pretty unlikely as you have already run the engine for a while...Whilst moving freely as described above there should be minimal up and down play in the bush...In other words it should be barely perceptible...Again take care that you are only applying testing force in a vertical plane and not tilting the piston...

The piston itself looks fine...If there are any deeper scratches in the skirt these should be eased with a smooth file but only to ensure there are no high spots adjacent to the scratch..Do not use emery for this job...

Some limited 'blow by' is normal in these engines, causing the brown marks on the piston skirt. Likewise the carbon build up on the piston crown

A light hone and a new set of rings might be beneficial whilst the engine is apart...Check the ring gaps to determine whether that is the case....Ian

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Re: Advice Please

Thanks for the helfpul advice guys. The staining is something I wasnt too sure about. My main reasons for opening her up were to check the condition of piston, barrel and rings as she had started leaking oil from the valve lifter. On looking though, i saw the gaskets were all shot, so thats one job to do. There is no noticeable movement on either the big or little ends and the piston flops front to back freely.
Having not seen any numbers on the crown yet (not started crud removal) should I expect to see numbers even on standard fit pistons?
Thanks again for the advice
Best wishes

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