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rear main leaks


Senior Member
newly rebuilt 1275 engine in 1964 Sprite

at idle fully warmed, it drips from the rear main every 5 seconds.

oil: 20w50
oil pressure @ 1000 RPM: 40lbs

Am I overwelming the rear main with too much oil and causing it to come out the back instead of beaing able to drain inward through the lip and holes back into the pan?

When I built the engine I did stretch the oil pressure bypass spring out a bit.... maybe a bit too much?

The engine does not have any sort of PCV system. the breather on the timing cover is just vented to atmosphere with a little filter on it.

Thoughts? What is the oil pressure supposed to be fully warmed at 1000 rpm?


Your main problem is probably the lack of vacuum connection to the breather. The carb is supposed to cause a light vacuum in the crank case, thus negative pressure and helping to retain the oil in the sump. With no vacuum, pressure builds up in the sump (piston seal leak & similar) and forces oil out past the rather crude scroll seal on the crankshaft.
There have been reams written on this subject in various forums and nobody seems to have come up with the definitive fix, but getting a vacuum to the breather might well help.


Great Pumpkin
Yes, I agree with Graham. The application of a small amount of vacuum can help this problem.

When my car was on the street (years ago), I made up an arrangement where I ported the crankcase vent into the intake manifold via a small, adjustable air valve (similar to a water facet). Obviously, I didn't want a full "leak" into the manifold: this would cause an excess "lean fuel" condition that would cause poor running. I used the valve so I could get *just* enough (after a bit of experimentation) to get a "controlled leak" that would keep a light vacuum in the crankcase. Once set up, the rear main leak was very minimal. You might have to re-set the carbs to compensate for slight leanness. This is similar to the idea of having a PCV valve, but with the benefits of adjustability.
I am currently building another street Spridget and will likely do the same thing again.

My race car has a custom fabricated spacer between one of the velocity stacks and carb. On a street car, I suppose this spacer would fit between the air filter and carb. This spacer has a port that is connected by a hose to the crankcase. This works fine too, but the race car never sits idling so this might not work so well at low speeds.

An old-style road draft tube might help too. But again, probably won't help during idling situations.

Also, you might still have excess crankcase pressure because your rings aren't fully broken in. If the car has just a few hours on it, you may want to run it a bit to ensure full break-in.


Senior Member
I'm not sure I have an excess crankcase pressure problem.

I took a plastic bag and covered the oil fill openning in the valve cover with it and didnt notice any movement.
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