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Vacuum advance mga

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Kilcunda, Victoria, Australia   AUS
I have 2 very good restored carbies for my MGA1600. However both have vacuum advance ports and I will need to block one off preferably with a threaded plug to keep options open. Does anyone know what the thread is. I appears to be a little over 1/4 inch diameter with about 26tpi. 1/4 bsf is too loose.

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Dandare Avatar
Dandare Danny H
Sydney, Australia   AUS
Should be 1/4 UNF I believe.

Danny

Kilcunda, Victoria, Australia   AUS
Slightly larger than 1/4inch. Thread is a bit coarser - about 26tpi
Ray

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10MGA Russell Goebel
Wynnum, Queensland, Australia   AUS
Hi Ray,

I had the same issue of blocking off the vacuum port when changing to Mallory Dizy (without vacuum advance). Tried everything, in the end the best fit was 7mm screw (bolt). I am sure the correct thread is NOT metric, but this was the best possible fit.

Russell

jonathan in mass jonathan eagle
weston, ma, USA   USA
Just went through this sort of. You can buy the fittings from moss. The vacuum cables they sell are designed to fit into a 1600 carb. The 1500 carb had a larger threaded hole. My goal was different than yours, as I just wanted to install the vacuum cable. I think someone had replaced mine with 1500 carbs. In any case both of these fittings are available. You could solder closed a piece of tube to get what you want.
Jonathan

Kilcunda, Victoria, Australia   AUS
Thanks -would not be surprised to find out it was metric. BA - British Association is metric based??? 0BA has a pitch of 1mm and each finer pitch is 0.9 times the previous. The bolt that holds the choke cam is 0BA and the vacuum advance thread looks similar, but the diameter is a little larger.
Ray

jonathan in mass jonathan eagle
weston, ma, USA   USA
I wouldn't mess with approximate threads. When you can just procure the correct pieces.

I would consider RTV. It might block the vacuum sufficiently and not damage the threads. Someday the future owner of the car will thank for it.
Jonathan

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10MGA Russell Goebel
Wynnum, Queensland, Australia   AUS
Hi Ray,

If it helps the 7mm screw fitted perfectly.....neither binding nor loose, just screwed in by hand then tightened, gently. Importantly, ensure the thread is short enough that it does not bottom out and damage the carby, instead tightening against the face of the vacuum take off.

Incidentally the thread is the same as the fitting on an Austin 1800 (Landcrab) carby overflow...if you can find one!

Russell

Dandare Avatar
Dandare Danny H
Sydney, Australia   AUS
I have a feeling that this thread changed size on the H types somewhere along the line. A 1/4 UNF set screw fits perfectly in one of a pair of H4s from my Wolseley.

Danny

Kilcunda, Victoria, Australia   AUS
I have ordered a vacuum port plug from SU Burlen in UK but am not completely confident it will be suitable after all the various pieces of information I have been given. I will message when it arrives.

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