One of the last jobs I need to do to get my restoration project complete is the tank. It was only after I sorted out all the boxes of bits that I bought in a moment of madness that I found that I had an M21 engine and B31 plunger frame.

That hasn't actually been too much of a problem but I now see that I also have an M21 tank. This seems very similar to the B31 but has the front mounting lugs fitted higher and it won't therefore fit at the front.

I've already had the dents removed, holes around the seams welded up and a coat of primer applied so now I have to decide whether to have the lugs cut off and repositioned before finding someone to paint it - or maybe look to switch to a B31 tank.

I could make up a couple of plates to attach to the lugs and then bolt through the frame but it might not look too pretty.

So, before I start munging it, I thought I'd see if anyone might be interested in a p/ex for a B31 tank in similar condition - or have any other thoughts?

Regards,
Bernie

email (option): bernie.a.jones@gmail.com

Hi Bernie,
I'd be looking for a m20 plunger frame with engine and gearbox mounts and swap everything over. Plunger components are the same, wheels and guards the same. you may have to change the oil tank and the battery mount. Then sell the B31 frame

email (option): spacemonkeym@gmail.com

Funnily enough, when I first found I had bits of two bikes I did think of looking for the parts to complete each and have two bikes instead of one!

I may do that one day but for now I think it will have to take to the road as a composite as I'm keen to get riding after two years of rebuild!

Cheers,
Bernie

email (option): bernie.a.jones@gmail.com

Hi Bernie...The tunnel through the center of the M Series tank differs from the B Series tank.(and so does the frame)..That's why the brackets are in a different place. The (civvy) M series tank also has a 'joining pipe' underneath, at the front, to connect the two halves of the tank. Military M Series tanks didn't have this pipe but had twin taps.
The B Series tank, due to the different tunnel form has only one tap but doesn't require the joining pipe, both halves of the tank coming together at the back.
Generally it is more difficult to find B Series tanks and good ones usually fetch more (I have been looking for a good one for a while).
If you decide to alter your M Series tank it is worth cutting off and blanking the joining pipe fittings underneath and adding the second tap boss as on the military tanks.
Removing the civvy M Series tank is a pain (and messy) as the tank has to be drained first and when you disconnect the joining pipe (which you have to do to remove the tank) any residue in the tank and what is left in the pipe p****s out everywhere...
The only neat way to do the alteration is to cut the tank lugs off, make some new ones the required shape and weld them in place so that they appear to have been made like that....Ian

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

Keep in mind the rear mounting holes position too, when repositioning the front brackets.