MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Vacuum Adaptor for MGB Manifold

Hi. I have a '71 BGT and want to attach a vacuum gauge to do some diagnostics. The 2 available places on the manifold are either -
A] at the ends (large thread - same as the crank case breather valve on early B's) or
B] just off centre.

I assume towards the centre will give a more accurate reading due to the flow characteristics of the manifold- ie, I am concerned that measurements taken right at the end of the manifold may be subject to turbulent flow which then may be interpreted as engine wear.

Is this a valid concern?

So, I would like to get an adaptor for 'B' but it is a 5/16 UNF-24 thread which seems *impossible* to find anywhere. There are lots of 1/2 or 1/4 BSP but I cannot even find a converter from the UNF to BSP size. MGOC lists the original part as unavailable.

Anyone with any relevant info please?

Thanks, in advance, Graham
Graham Moore

Which is the original part that you are referring to? Do you have a part number?
Dave O'Neill2

http://www.namrick.co.uk/acatalog/Home_U_N_F__HT_Hex_Head_Bolts_18.html

Google seems to show plenty of options.
Michael Beswick

Dave. The part numbers (ie 2 parts that would do the job) are (from MCOG parts WWW)

MGOC-SPARES > MGB > MGB FUEL SYSTEM > INLET (MGB)

70 - Adaptor - Overdrive vacuum switch AHH6962
80 - Adaptor - Vacuum advance pipe 12H357

Michael - I have checked out the link you refer to and can see bolts, nuts, and other stuff but I cannot see hose adaptors or convertors. Have you seen them on that site please? Graham
Graham Moore

There were originally ports for various purposes on various eras of inlet manifold, probably blanked off on your 71, use one of those positions, drilling and tapping the blanking plug. Or get something like the parts you mention, although the vacuum pipe adapter is 12H 3574. You would of course need the correct thread in the manifold for those, I think you stand a reasonable chance of the vacuum switch adapter fitting either of the smaller ports nearer the middle of the manifold. Goggling that shows the usual suspects, at not much more than pennies.

There is also the PCV valve adapter 12H1405, which I have used for an anti-runon valve, although that only fits one of the larger ports on the forward end of my 73 manifold.
Paul Hunt

Thanks. I have the PCV adaptor but I am getting unusual vacuum readings from it,quick flicks, suggesting worn valve guides. That may be the case but the adaptor is very long compared to the depth of the manifold, right at the end of it, and vertical. also, i am taking a 8mm hose down to an 5 and then 3 (just to get it to fit). As such i imagine there could be a lot of turbulence around the tip so i don't know if the fluctuations are due to the adaptor or wear in the head.
Graham Moore

The large swings, on the gauge, are caused by the vacuum line being too large in diameter. Usually, a small restricter is placed inside the line to smooth out the fluctuations. RAY
rjm RAY

"but the adaptor is very long compared to the depth of the manifold"

I did cut mine down as I felt it would restrict the air-flow into the manifold when my anti-runon valve opened and hence be less effective.

"Usually, a small restricter is placed inside the line to smooth out the fluctuations"

My RedeX gauge has a screw-clamp damper around the thicker part of the tubing near the gauge. This is adjusted to give more movement of the needle while setting-up timing and mixture, less when driving (it was marketed at the time as an economy aid). The other end has an adapter I made out of a bolt.

Paul Hunt

Paul, a very clever design. When I worked on a lot of motorcycles with four cylinder engines, I purchased a carburetor balancer with four separate mercury filled tubes. It came with several adapters to fit the various carburetors that were fitted at the time. It also came with small brass line restricters to dampen the movement of the mercury inside the tubes. It works very well, but is a bit cumbersome to use due to its size. I have a vacuum/boost gauge fitted to the intake manifold of the supercharger on my '67 B. It remains remarkably steady while in the vacuum zone and holds its position rock steady while showing boost. RAY
rjm RAY

This thread was discussed between 12/04/2014 and 14/04/2014

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now