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MG MGA - Vacuum advance 6020 carby bodies.

I have two nice restored carbies (su6020 bodies) for my mga1600 but both have the vacuum advance ports. I will need to block off one with a screw in plug but the thread has me beaten. Can anyone tell me what it is please.
R Watson

It would be logical for it to be BSP thread but 1/8" BSP is too big. If it is 1/16" BSP it would be hard enough to get any fittings over here let alone Australia.

How about e-mailing Burlen Fuel Services (present owner of SU) because if it's an exotic thread you will probably not find a fitting anywhere else.

A cheaper,quicker solution would be to clean out the hole and fill with Araldite or similar.................Mike
Mike Moore

I can't remember right now if the tubing was flared or used a compression fitting. If the latter, use the standard fittings, but substitute a short piece of brass rod in place of the tube that is normally there.

-Del
D Rawlins

BA thread?
Mike Ellsmore

I have used a variant of Del's solution. Just use the correct pipe and end then chop pipe after a few inches and squash it hard with pliers.

Paul
Paul Dean

> BA thread?

Not sure. John Twist made a video a few years ago detailing 4 different thread forms on an MGA carb body, but I don't think he talked about the threads for the vacuum advance.

https://www.youtube.com/watch?v=A8oo2LoPPA8

-Del
D Rawlins

I believe it is too coarse to be a BA thread. The carbs are more Whitworth threads than UNF or BA.

Dominic Clancy

BSF ?
Mike Ellsmore

I thought I would resolve this with a email to Burlen. They came back and said it is 5/16 BSF 26 tpi only problem is 5/16 BSF is 22 tpi and 1/4 BSF is 26 tpi! I have asked them to clarify and also advise what the thread is on the other end of the nipple that screws into the carb body - see image attached.

Mike Ellsmore

Why not just link the two ports to a Y piece and take the vacuum pulses from both carbs?
Chris at Octarine Services

Just to clarify what the actual thread is on the H4 SU bodies for the vacuum advance connection.
Burlen have come back to me and advised that the thread in the body is 5/16" BSF (22 tpi). For the concours buffs the original nipple is 5/16 BSI (British Standard Iso) 26 tpi on the other end - this is an unusual thread stemming from BSC (British Standard Cycle) where threads over 1/4" are all 26 tpi - as it happens 8mm X 1mm pitch is almost identical - assume this is where the Iso comes from. Someone else can research what the nut and pipe details are for the original vacuum advance connection to this nipple.
Mike
Mike Ellsmore

That Twist video is showing an MGA front carb throttle body, so it does not have the vacuum port.
barneymg

Mike, I have one of those nipples and it does not screw into the body - much too wide, My vacuum advance unit goes directly into the body with a hollow screw and a small "acorn" fitting around the brass tubing. Burlen have indicated they have a vacuum blocking plug which I have ordered, but with all the confusion about threads, I am not hopeful. I will message once it arrives.
Ray
R Watson

My bet would be a very small BSW (Whitworth). There are several small BSW screws used in the T series carburetors. Cheers - Dave
DW DuBois

Mike, your information appears to be correct but there must have been a change in the thread at some stage. I have about five 6020 bodies and the union you pictured is too big for all of them. Burlen have sent me plug ABF110 which is also 5/16 BSF and too big. The union would take a female brass pipe fitting. I have located a spare male fitting that I can block with solder that does fit. Thread pitch is 1mm as near as I can tell. Problem solved but I am still curious. I have included an image of the union, plug and male fitting.
Ray


R Watson

Thanks for posting... That's a compression fitting around a piece of tubing, so it shouldn't be too difficult to get a piece of solid brass or copper rod the size of the tube, and install it with a new compression fitting, no soldering required. Or squash the tube like Paul suggested.

-Del
D Rawlins

I have found the correct thread with help from Joe Curto. It is 7mm x 1mm and appears to fit perfectly. Not a common metric size and a bit difficult to obtain.
R Watson

This thread was discussed between 21/07/2017 and 09/09/2017

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