Showing results for 
Search instead for 
Did you mean: 

Piston Slap: Proper ventilation of the Midget's crankcase?

Tony writes:


I have a 1976 MG Midget in which the original SU carburetors and manifold have been replaced with a progressive two-barrel Weber DGEV and its associated manifold. With this arrangement the PCV valve and plumbing to the intake manifolds no longer exists, so the valve cover vent port is now plumbed to an enclosed catch can that vents via a hose to atmosphere below the engine (like back in the ’50s). Will the lack of manifold vacuum to draw off of the valve cover cause over-pressurization of the rear seal and such? I’ve noted considerable oil consumption and oil leaks (as in every classic British car).


Read the full article on

View Entire Topic

I would recommend a leakdown check. That will tell you the condition of the rings and valve seats. Obviously, if you see more than a ten percent loss past the rings or valves, some work is in order. What a leakdown won't tell you is the condition of the valve guides and stem seals. To check the guides, remove  the intake manifold and visually check the bottom of the valve guide in the port for oil. Wet is not good. Do the same on the exhaust; proceed accordingly.  Venting the crankcase is a must! Remember, every time a piston goes down, it's moving air beneath it, and compression losses past the rings tend to accumulate in the lower part of the engine. Enough of that accumulation combined with those superior (!) British crankshaft seals and other gaskets results in oil going where physics tell it go. You mentioned that you have upgraded the aspiration to side-draft carburetors, so you might consider equipping the engine with a small vacuum pump to evacuate the crankcase. The discharge can be plumbed to a vented catch can which can then be emptied either back into the engine if you wish or into your used-oil storage container in your garage. I recommend the latter. If you decide that a vacuum pump will work for you, you will want to seal up the factory side cover vent to gain the most out of your new system. Another benefit of the vacuum pump is a slight bump in horsepower due to reduced pumping losses, but it's doubtful that you would feel it.

Intermediate Driver