Thread size

I am having trouble identifying a thread size on a 1920 motor cycle carburettor.

It is not quite 22tpi on my Whit gauge and not quite 11mm on my metric. The o/d is .87".

Can anyone help ??? Thankyou.

Is it a British carb, or European?

BSF has a 22 tpi form, it may well be a special thread cut to a BSF form.

Can you see if it is a 55 deg or 60 deg thread form?

Or American ?

It is from a British bike. It is a very simple carb,someone suggested it might be a B&B.

It appears to be nickel plated brass.

11mm pitch - Now that is coarse!

Yes,it is at the top of the carb body,it is an external thread and I believe it should have a cap that screws onto it holding the Bowden cable and its adjuster.

I presume it is 11mm ,the gauge is marked Moore & Wright 801 M/M and the gauge leaf is marked 11.

.

Roger,

The gauge may be badly engraved [?]

... I think we can safely assume that the marking should read 1.1

**LINK**

MichaelG.

I get to sort out a lot of threads on old motorcycles. Usually when faced with things like you have I take my best check/guess and screwcut a bit of aluminium and try the part for fit.

Roger,

Michael is almost certainly right about the pitch since 22 TPI comes out to 1.155mm pitch. I am guessing that the measurement .of .87 is a nominal 7/8" major diameter. I would expect a British thread cut in the 1920's to have a TPI specified pitch rather than a metric. Does the thread appear to be coarser or finer than 22 TPI? I came up with an American Extra Fine specified 7/8 x 22 thread. but that is as close as I could come to something that is not just a special thread.

George

How long is the threaded length, because if it is only a few threads, you can get away with quite a lot of pitch error.
BobH

.

This page looks useful **LINK**

http://www.barnstormers.co.nz/147/brown-barlow-carburettors-bb/

No thread size mentioned for the top cap, but they seem to know their stuff and may be able to advise.

MichaelG.

You have to remember that threads that are approaching, or exceeding, 100 years old now have probably distorted over the years so may not fit a thread gauge as perfectly as a new thread. Best shot is to do as Chris Evans 6 said above and take your best measurements and make a dummy piece in the lathe and see how it fits. 22tpi sounds a good starting point.

It is not critical whether you use a 60 degree, as would have been used on American Extra Fine 7/8 x 22tpi, or a 55 degree screwcutting tool as most likely would be used on a British thread. The one will screw on to the other as evidenced by Whitworth and UNC threads of the same tpi, ditto UNF and BSF.

It's quite possible the carb manufacturer used their own mongrel thread for this as it's for a special part, not a standard fastener. Lots of it about on old motorbikes.

From practical experience over many years I have found that carburetor manufacturers do have their own standards.

For instance on an Amal pre-monoblock and TT carbs, the thread at the bottom to hold the jet block in is 9/16" x 20 - 20 thou.

Tap it out 9/19" cycle and you will get air leaks and it won't run right ever again. Without looking in the special taps drawer thee are others as well but can't recall them off the top of my head.

[ Still have the Amal tap and a 14.2mm spark plug thread tap so you can fit MOD screened plugs into your engine ]

Edited By John Stevenson on 05/05/2017 08:16:49

That's a rare'un John.