Welcome to our resource for MG Car Information.



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 MGA - crankcase venting

Hey all, The motor on the 1600 is freshly rebuilt and has 204 miles on it. To my dismay, it has leaked about a quart of oil. When it was apart, the shop used Barney's site and put the new rear main seal "kit" in place. They held up the process so that I could see first hand that it was done.

When I put the &^%$* air cleaners on today, I noticed that there is a positive, slight, pressure coming from the valve cover hose that connects to the front cleaner.

Could this indicate that the crankcase is "pressurized" and the oil is being forced by the new seal? If so, is there a fix? Thanks again, Tom, "in frustration".
Thomas McNamara

In addition to the valve cover vent, there is (supposed to be) a draft tube extending downward from the the front tappet cover to below the oil pan bolt rail. That makes two 1/2 inch diameter vents from the crankcase, so it should not be possible to have any measurable pressure in the crankcase. Blow into the valve cover vent to be sure it flows freely out from the draft tube.

For the rear seal to work the drain slot in the front side of the engine rear plate has to be plugged.

Where does the oil leak out?
Barney Gaylord

Hope your repair shop used a Viton seal, also although mine is a Twin Cam, I added a breather to the front tappet plate similiar to the Le Mans cars.
Rgds Trevor Burnett
t burnett

The leak is from the cotter pin hole, just like the TC and TD. As a test, I will loosen the valve cover cap on a test drive. It appears to seal the valve cover? Ideas? Tom
Thomas McNamara

You don't have a valve cover vent or a draft pipe?
Barney Gaylord

Hey Barney, yes there is a pipe running from the side cover to just below the sump bolts. At idle, there was a small amount of "positive" pressure going into the front carb air cleaner? I dread having to dissassembLe the carbs and manifolds to check the side down tube.

Will blowing into the tube prove anything? I noticed that the tube end is free of oil inside. Does this indicate anything? Tom
Thomas McNamara


You still have answered only half of Barney's questions.
You have the pipe from the side cover down to sump level.
But do you have the pipe from the valve cover to the air cleaner?
Barney suggests blowing air into the valve cover pipe (with the cap on).
This does not require removing the air cleaners nor the SU's.

M F Anderson

Crankcase vent pipe.

David Lake

This thread was discussed between 15/04/2013 and 16/04/2013

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.