Reverse engineering

Could it be an SU carb? They often lost the top cap which provided damping to the piston by means of a small air hole on the early models.

Russell

No you can't. You need a lathe.

Being pedantic, SU carbs damped the piston by means of a two part brass piston ( loose sleeve on a very undersized piston ) carried on a rod held by the top cap, and in the oil within the hollow stem (and guide) of the large Aluminium alloy piston that carried the needle on its lower end. The air vent was there purely to allow the air above the Piston Guide to move in or out as it moved to and fro.

I think that later Stromberg constant vacuum carburettor, (which used a rubber diaphragm instead of the metallic Piston), used a similar system for damping out the individual cylinder pulses, and to provide enrichment during acceleration).

To produce a replacement for the missing Setscrew / Bolt / Stud / Fastener, you will need to cut a male thread; either with a die, or to screwcut it. Whichever way you go about it, you need to ensure that the resulting male thread is the correct size and pitch, for the tapping that you have. Once you have determined what the thread is, you should be able calculate the O D of the male thread, based on: Core diameter, (which you will have measured,) plus twice [thread depth minus slight truncation], (to provide clearance, and prevent root/crest interference) before cutting the thread.

As Hopper says, to carry out these operations properly, you have to use a lathe. Cannot see how a Dividing head comes into it!

Howard

That's right Howard but you are thinking of later models. The earlier models (c. 1920s and 1930s) just had a plain brass cap with a small hole.  They also lacked the spring return for the piston, relying on gravity.

Russell

Edited By Russell Eberhardt on 19/08/2018 19:27:38

If it is an SU carb this might be what Roger is looking for.

http://sucarb.co.uk/dampers-caps-oil/brass-caps/vented-round-knurled-edge-brass-damper-cap.html

Russell

Abject apologies,I was cutting gears on my horizontal mill (with dividing head) and threads on my lathe (without dividing head) when I wrote my original post I didn't know my one from the other.

The orevious photos show the carburettor as is. I have checked all of the obvious catalogues and web pages but cannot find this carb. It is described in the brochue as an "automatic carburettor" !!! There are no markings anywhere,float chamber,carb body .jet, etc etc. The thread as seen in the photo is 1/4" long and has an exterior diameter of 7/8".

If your thread gauges won't measure that thread, you might want to try metric gauges, or using a tap as a gauge to find one that matches. If all else fails, measure number of teeth in 1/4" with a vernier and magnifying glass and multiply by four to get TPI. Set your lathe gearbox or changegears to that TPI and cut a sample thread and compare. Once you get pitch right, its a basic internal screwcutting job. Size of the hole up the middle is the OD of the male thread, minus the thread depth X2, plus 5 thou for clearance.

Herewith an original drawing of the "Automatic Carburettor " taken from a 1920s edition of The Motor Cycle.

I have also found an old Whitworth thread gauge which indicates the size as 24 3/16 7/32

Does this mean the thread size is 3/16 Whit ?

It isn't 3/16 W.W. That is 3/16 overall diameter. The thread is the same pitch and form as 3/16 W.W. but 7/8 diameter. The same pitch of thread was used on both 3/16 and 7/32 W.W.

Note that 7/8 BS cycle thread was also 24 tpi but had an angle of 60° rather than 55°.  Are you sure it's not that?  I think it's more likely.

Hope that helps

Russell

Edited By Russell Eberhardt on 31/08/2018 15:03:57

Thankyou for that ,Russell. Without a BSF gauge is there any way of checking the thread, What I was intending was to cut an internal 24tpi Whitworth on scrap tube and offer it up to the carb body.

.

Roger,

If you can take a nice sharp close-up photo of the thread, I'm happy to try measuring it.

MichaelG.

.

Here is one I prepared earlier :

Edited By Michael Gilligan on 31/08/2018 18:41:57

Roger, I'm sure your antique part will not care what thread angle you use.

Tony

If you have a 3/16 (or 7/32) BSW bolt, you could offer that as a thread gauge. That should tell you whether the thread is Whit form or Cycle . The light around the thread should show any mismatch between the Carburettor thread and your BSW "gauge" .

Once you know the thread form and pitch, you can then bore the mating piece to

(7/8 - (2 x thread depth)) and then cut an internal thread to produce a 7/8 x 24 tpi thread that should screw onto your carburettor. You may need to devise some form of sealing between the threads (I am loathe to suggest PTFE tape!), possibly a low strength anaerobic?

Howard

Ah! Got rid of that stupid emoji!

Edited By Howard Lewis on 31/08/2018 20:29:30

Without benefit of Michael's careful measurements from a photo, the thread could be either 24tpi or metric 1.0. The difference is small - 24tpi is M1.056 and 25.4tpi is M1.000

I 'd make a test piece as Roger suggests.

As my lathe is metric I'd try M1.0 pitch cut into a bore 21.0mm internal diameter. The thread needs to be about 0.61mm deep.

If I owned an Imperial Lathe, I'd try 24tpi. The thread depth will be much the same, say 25 thou, and the bore 0.827 inch diameter.

Decide between 55 and 60 degrees by offering a metric or imperial thread cutter up to the carb, and use which ever is closest.

Offer the test piece up to the carb and see how well it fits. I agree with Tony - I think any combination of metric, imperial, 55 or 60 degrees should be close enough to screw together. Even with the wrong pitch and thread angle, you should be able to get a good tight fit by removing metal slowly as the thread approaches the theoretical 0.6mm deep and stopping as soon as the carb screws in.

The main challenge is making sure the joint doesn't leak petrol. My guess is the original was fitted with a washer.

Dave

PS Grateful if someone else would confirm the dimensions. Maths and I aren't best friends...

Edited By SillyOldDuffer on 31/08/2018 21:16:22