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Thread: Vacuum advance 6020 carby bodies.
Posted 21 July 2017 at 07:08:03 UK time
R Watson, Victoria, Australia

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.

Posted 21 July 2017 at 15:58:38 UK time
Mike Moore, Cheshire, United Kingdom

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

Posted 21 July 2017 at 18:14:47 UK time
D Rawlins, Alaska, USA

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.


Posted 23 July 2017 at 01:54:57 UK time
Mike Ellsmore, Victoria, Australia

BA thread?

Posted 23 July 2017 at 08:42:30 UK time
Paul Dean , Scotland, pmz.dean@gmail.com

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.


Posted 23 July 2017 at 10:15:24 UK time
D Rawlins, Alaska, USA

> 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.



Posted 23 July 2017 at 18:51:10 UK time
Dominic Clancy, Switzerland

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

Posted 24 July 2017 at 23:56:25 UK time
Mike Ellsmore, Victoria, Australia


Posted 02 August 2017 at 08:44:05 UK time
Mike Ellsmore, Victoria, Australia

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.


Posted 03 August 2017 at 08:47:34 UK time
Chris at Octarine Services, Essex, United Kingdom, chris@octarine-services.co.uk

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

Posted 03 August 2017 at 10:57:23 UK time
Mike Ellsmore, Victoria, Australia

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.

Posted 03 August 2017 at 16:08:35 UK time
barneymg, USA at large, barneymg@mgaguru.com

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

Posted 13 August 2017 at 07:59:28 UK time
R Watson, Victoria, Australia

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.

Posted 13 August 2017 at 18:17:20 UK time
DW DuBois, USA

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

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