Balance Plug

I have a Longbridge (integral manifold) head and am prepping it for a good hot tank cleaning. It has been sitting for ~30 years and the mice decided it would be a good storage area for all sorts of things. Everything is off/out of the head with the exception of one thing and I have no idea what it is or how to remove it.

Between the two inlet ports on the head is an additional cover. Behind it is the "balance plug". The manual and Moss catalog call it out as such but nowhere can I find any other reference to it, what it is, what it does, how to adjust it or how to remove it. It doesn't readily come out and I'd rather not destroy it as the Moss catalog says "NA".



Can anyone enlighten me on what this thing is and how to remove it?

Thanks much.

Mike T.

After a little investigation, I elected to leave it in place.

I know nothing about the Longbridge heads and wonder is there a passageway between the two inlet ports that the plug regulates/balances, much like a balance tube between inlet manifolds?

I think Michael is right. If you look inside a log manifold for a 6 port head, there is about a 5/8 to 3/4" hole in the center of the partition between the front & rear halves of the manifold. I assume this is a specific, 'tuned' size based on some factory testing? Like the way the tri-carbs started with one size balance tube and then enlarged it in later cars. I think it has to do with supplying a consistent vacuum signal to the SU carbs for smooth piston operation in those cases.
Dave

I was hoping that I could clarify that, with a picture through the bores before I sealed the casting holes, but I waited until the head was bolted on; all you can see are the head-studs as they bisect the inlet tract.

It's no wonder that DMH developed & implemented the Mille Miglia (detachable aluminum) manifold! I'm sure the original Morris design (shame on you!) caused DMH and Geoff some sleepless nights.

From the front



From the rear

Dave said: "I think Michael is right".

I am showing that to my wife!

Thanks for the replies. That makes sense it is a factory set item and why there are no references to it in the manual. I'll leave it as is for the trip to the hot tank. Happy motoring.

Miki T in Minnesota

Here's a log manifold with the four centre studs replaced with shorter studs to stop the interference to airflow. The studs are accessed via threaded plugs in the holes at top of the manifold.

Danny

That's certainly one way to do it!

Since that appears to be in an Austin (?) saloon, I suspect that the sanctioning body does not allow the switch to a Healey's head and separate manifold, as they're certainly plentiful and easy enough to acquire.

Always somebody trying to build a better mousetrap

Dandare said:

View attachment 52417View attachment 52418

Here's a log manifold with the four centre studs replaced with shorter studs to stop the interference to airflow. The studs are accessed via threaded plugs in the holes at top of the manifold.

Danny

Click to expand...

This is a Morris Isis (not mine) from New Zealand. This is just a cheap way to have a bit of a play...Healey stuff is not cheap to obtain in Australia /NZ especially as you need different carbs too. The H4 twins are pretty easy to acquire, this Isis had a single SU originally.

You may note that the Gallery or log as we like to call it is enlarged (big log) compared to what would be on the first 2.6L Healey.This one comes from the later 2.9L C series vehicles.



Danny

That's a beauty. Thanks for the lesson!