Many years ago, we built a mild-street 350 small-block Chevy that from the first running, wasn’t just consuming oil, but burning an unusual amount of it. On a road trip in our Chevelle, the engine consumed a quart of oil about every 350 miles! What was amazing was that the engine was not leaking and the exhaust was not tinged with blue smoke.
At first, we assumed poor valve guide seals so we invested in quality Viton blue rubber seals and carefully reassembled the top end, but that didn’t help. This required further diagnostics until we removed the intake manifold because of a slight leak at the rear china wall. With the intake ports open to visual inspection, we discovered several small oil droplets in the runners of all the intake ports. At first, we blamed the intake gaskets, but after two changes of gaskets with no improvement in the oil use, this forced us to look deeper.
That’s when we learned from our friends at Fel-Pro that we had a mismatch in the angle of the intake manifold to the port angle of the cylinder heads. Essentially the gaskets were adequately squeezed at the top of the port, but the bottom was not loaded nearly as heavily. This allowed intake manifold vacuum to pull oil from the lifter valley through the gasket and into the intake port.
Originally, we tested this by drilling small holes in the intake gaskets and using lead shot to measure the gasket crush of the manifold against the head. While this test works well, Hughes Engines in Washington, Illinois, sells an inexpensive kit of 0.100-inch diameter wax string that you can use to check intake manifold crush against the cylinder heads. Place the string alongside the intake ports as shown in the accompanying photo. If the string is crushed more on the top of the intake port than the bottom, then the manifold needs to be machined until the bottom crushes slightly more than the top. Any competent machine shop can mill the intake manifold sealing flanges to accomplish this task.
In our case, we milled the intake and the oil usage problem was eliminated. The engine went from consuming oil at a rate of a quart every 350 miles to one quart every 2,500 miles.