I have been working through the mechanicals on my 42 WD20 and pulled the head off today to have a look. I had the bike running once prior to tearing into it to get it in mechanical shape. When the bike ran it was shooting fuel out the carb inlet so it was definitely running rich, but at least I knew it would run. It had the wrong carb on it as well.

Anyway, I pulled the head and decarbonized the head and the top of the barrel. The piston appears to be the original that it left the military with. It is 1mm oversized as marked on the rebuild tag. The top of the piston looks to have some grinding marks on it. There are some scores inside the piston sleeve. The exhaust valve has some pitting on it as does the head above the exhaust valve. The engine turns over freely.

Should I go ahead and rebuild the top end based on the attached photos or should I run as is? Thanks in advance for any and all advice. Big shipment of parts due to arrive from Draganfly this week. It's like waiting for Christmas morning to come.

Here are some before and after pictures. Also built a gearbox holder that has been a very nice way to move and work on the gearbox.

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email (option): bbrown3407atcharter[dot]net

Brian the head looks ok. I wouldn't worry too much about the pitting, although you could polish some of it out, but I doubt you'd notice the difference on the road. Your piston can't be the one it left the factory with, if it's 1mm O/S.
The scratches on the top look like someone else's de-coke treatment and the pock marks look like a small foreign object has been rattling around in there. The scores in the bore don't look good either. The only true way is to take the barrel to an engineer to get it miked up. Maybe it can be cleaned up with a hone and O/S rings fitted, but more likely it needs a re-bore. Ron

email (option): ronpier@talk21.com

Based only on the photos and without measurements I would say you should go for a rebore....The cylinder is 'scored' more than is desirable and it is obvious either a lump of carbon, a piece of piston ring or some other foreign body has been rattling about in there at some point...

Detailed measurements of the bore and piston/rings are really required though to determine the exact condition of both parts...

One problem is that BSA original pistons only go up to 1mm oversize so you have the largest one available from that source...As Ron has pointed out this engine doesn't have its original piston and has probably had at least 2 rebores in the past....

Pattern pistons of the period such as Hepolite or Heplex go up to +.060" so you might get lucky and find something in that size...Alternatively you are looking at a modern replacement such as GPN (Italian)...

It wouldn't really be worth fitting a new liner at this point I wouldn't have thought....

Regarding the exhaust valve and seat...If you intend to do some miles I would personally have a hardened valve seat fitted...(lead free seat). This will restore the valve seat to its correct position relative to the gasket face and being far harder than the barrel base material will give long service with very little wear....

The valve itself (exhaust) has a 'land' above the edge of the valve face...In other words, the angle of the valve seat and the top face of the valve do not meet in a 'pointed' edge... This land is there to enable the valve to withstand the very high operating temperatures without the edge failing...The valve actually runs 'red' at normal operating temperatures.

The land should measure no less than .030"-.035" wide after the valve seat has been refaced...

If in doubt replace the valve...which you may have to do this anyway if there is any wear present on the valve stem.

Also check the gasket face is flat, the valve guides and stems are within limits and that the valve springs do not require replacement...Ian

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

Ron and Ian,

Thank you both very much for the advice. Hadn't planned on doing much with the engine at this point, but might as well do it now. I know the piston is not the original one installed when it was new, but it was rebuilt by the military in 1953 and is marked as having an OS piston (1mm) installed. That piston has been in there for 61 years, so I guess it would be due for replacement.

A rebore is above my expertise level, so I will begin the search for a suitable rebuilder here in Southern California.

Thanks again for all the advice on this subject as well as all the other questions the forumers have. You both are a much appreciated resource!!

Thanks,

Brian

email (option): bbrown3407atcharter[dot]net

Hi Brian...Try putting on a post to contact 'L.A. Kevin'..as the name implies he is based in Los Angeles...He may be able to give you some contacts for spares/work shop help etc....Ian

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

Ian,

Thanks for the heads up regarding Kevin. We have been in contact since I purchased the bike and he has been a wealth of information. He even came over and helped me pull the clutch. We had a bear of a time getting it to break loose!!

Another benefit of restoring these historical items are the friends you meet along the way. I am enjoying the process.

Take care and thanks again,

Brian

email (option): bbrown3407atcharter[dot]net