“What I Learned Today” With Jeff Smith — Not All Leaks Are External

Editor’s Note: For the next few months, we’ll be sharing weekly lessons that are hard-earned from nearly four decades of engine building experiences, exasperations, and general mayhem that either we’ve experienced personally or have been associated with through friends’ miscues. The title intones that most of these errors and screw-ups could have been easily avoided had we been paying attention to the details. So rather than suffer a similar fate, we offer up these lessons learned — the hard way.

A friend recently rebuilt a Gen-VI big-block but it suffered from extremely low — 10 psi — oil pressure at idle. At first, he thought it might be an errant gauge, but after trying three different gauges, it was clear low oil pressure was genuinely the problem. Unfortunately, the engine was already in the car, as he didn’t pressure lube the engine beforehand. We disassembled the front accessory drive and removed the timing chain cover hoping that we’d discover a missing front oil passage plug – but they were all in place.

This is the oil galley passage in a Gen-VI block. Note, there are three oil galley plugs in the main galley (arrows). If one or more of these plugs are omitted, the engine will have very little oil pressure.

After a discussion with our friend and machinist Don Barrington of Barrington Machine in Santa Clarita, California, he mentioned that Gen-VI engines use three pipe plugs in the lifter valley. This particular block had been fully machined and all the pipe plugs had been removed for cleaning. Upon inspection, our friend discovered that one of the pipe plugs in the lifter valley was left out. This created a large internal leak that caused the low oil pressure. He added the pipe plug and the engine now had normal oil pressure.

A common upgrade for performance engines is to use a front pipe plug with a 0.030-inch hole drilled in one of the plugs to lube the timing chain. Be Careful — we’ve fallen victim to a pipe plug with one of these holes finding its way to a rear galley plug, where it created a massive leak as soon as we pressure lubed the engine. This is a very easy mistake to make if you are not paying close attention.

The arrow points to the oil plug that should be present on all small-blocks to ensure oil pressure to the main journals and the rest of the engine. If the block has been thoroughly cleaned, this plug will likely be missing and will need to be replaced.

Finally, on a small-block Chevy, there’s a pressed-in pipe plug underneath the rear main cap that must be in place to prevent a massive internal oil leak. If this plug is left out, there will be no oil pressure. Along with that one, also make sure to install the oil plug below the left (driver side) rear deck surface. If this plug is left out, you will have a massive external oil leak that will be very obvious the first time oil pressure is created. We were shot in the face with oil upon discovering that little oversight.

All of these potential issues can be identified before the engine is installed if the builder merely runs through a pre-lubing process using a dummy distributor with its shaft driven with a half-inch electric drill motor. It’s far better to identify a lubrication problem before the engine is installed in the car.

This is a small-block Chevy with pressed-in cups for the oil galleries. Note that the two outboard cups have been drilled with a 0.030-inch oil feed hole in each to lube the timing chain.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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