Hi
This B31 engine that I putting into my WM20 has run uo against a problem, and I was wondering if any of you guys had a solution or can help. The carbourettor (a concentric) fouls the R/H petrol tap. The carb body is right beneath the tank hole for the tap.
As I see it, I can plug that hole off and just us the L/H tap, or try to get a heat proof spacer to move the carb further away from the cylinder. Anybody know where I can a spacer.
It is 1956 B31 engine going into a 1942 WM20 frame
Thanks

email (option): Gasboy@btinternet.com

Hi Peter..This problem occurs because only the WD and AA tanks have twin taps. B31/33 tanks have one tap as do M20/21/33 tanks. However, because of the different tunnel layout on the two models the M Series tanks have a 'balance' pipe fitted at the front to join the two halves of the tank together and allow petrol to get from one side to the other.
So, if you opt to blank off the second tap bear in mind it will effectively reduce tank capacity.
For that reason it is probably best to go for a carb spacer and keep the second tap...Ian

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

What tank did you use to clear the head?

Good point Douglas...If Peter hasn't altered the bottom of the tank I'm guessing he has a post war replacement tank for a WDM20. I have seen these with the recessed base. It is posible later AA tanks also shared this feature...or Peter has come up with a variation I have never heard of ...Ian

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

..or an early tank ?

Hi Rik, Yes, that's another candidate I hadn't thought of ...Ian

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

My tank has the cutaway underneath to clear the head, and the cut away at the back R/H side for a vokes filter.

What is an AA tank?

A tank off an Automobile Association M20. They were fitted with a sidecar for the mechanics tools etc.

email (option): horror@blueyonder.co.uk

would a 1952 b31 plunger tank fit they have simlar mounting bolts and are bashed in underneath for a better word to take the taller ohv engine i think i read on this forum you may need to alter the front lugs on the tank to raise it a small amount

You need to lower the tank, about 1/2 inch.

Still won't fit.I've never heard of a cutaway tank having provision for ohv motor? Have you test fitted it yet? Be prepared for disappointment.

I think you will find that Roger was refering to a plunger tank.

None of the B Series tanks are a 'straight fit' to the M20 without modification of the mounting brackets as although they are the same basic pressing the tunnel and brackets are made for the (different) B Series frame.
AA tanks were of the same basic pattern as the military tanks, but without knee grip rubber screws and with a smaller filler cap which hinges in a different direction to military types.
Standard M33 tanks have the recess but have only one tap, a 'joining pipe', provisions for tank badges (different for different years) and a later style filler cap.
Wartime WDM20 tanks have flat bases which will not fit with an ohv motor as there is no clearance for the rocker boxes/head. The exception is the early large tank which does have this feature.
Post war replacement tanks are the same as wartime types in their overall features but some, at least, have the recess in the bottom for an ohv engine..this may come from the use of the M33 basic pressing for M20s.
There were also various military contracts for foriegn goverments postwar which resulted in some variations to tank styles...however, to my knowledge these are not fully documented.....Ian

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

I am going to plug the right hand side tapping and just use the left hand side. If I get short of petrol on a ride I will lay the bike over to get the petrol to slosh over. That is the easiest option for me.

But it doesnt identify my tank, which would be interesting to do.
If it was a post war replacement I guess it wouldnt have the back cutaway for the vokes filter.

So which tank has the cut away to accomodate an OHV engine, and the cutaway at the back for a vokes filter, a 3/8 tap each side, a brass filler cap and fits a 1942 WM20 frame?
Could it be a rare or special tank.?

And thanks to everbody for the usual replies and help

email (option): Gasboy@btinternet.com

A photo of the tank would be really helpful for an ID, otherwise it's just guesswork. There are photographs of the early tank (that for some reason BSA owners refer to as a 'balloon' tank - funniest bloody balloon that I've ever seen !) with the Vokes cutaway.

The other possibility is that someone built a post-war ohv military 'replica' of something that never existed and of course it would've had to have a Vokes and canvas grips 'cause everybody knows all army bikes had those'

Your tank doesn't conform to any standard wartime tank pattern. It is possible 'cut off' tanks (with the recess) were still supplied post war as spares. All Vokes equipped M20s were from the last two wartime contracts, so the newest M20s going into the post war period were those that were Vokes equipped. I am unsure if, or for how long, the filter was retained into the post war period.
UK forces maintained a post war presence in various locations where a Vokes filter could have been deemed a requirement so it is entirely possible it remained on late production machines which had it fitted..As Rik says a photo of the tank..and particularly the recess might help to identify its origins..As a point of interest filler caps made completely of brass usually emanate from India, where anything is possible... ...Ian

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