I’ve got a 355ci small block Chevy that I rebuilt about two years ago re-using the old cast pistons. I put new rings and bearings in it along with a new oil pump. About three months ago, the engine started making a knocking sound when it’s cold. After about three or four minutes, the noise goes away but lately I’ve noticed that it takes longer for the noise to stop.

My friends are split between two different ideas about the problem. The first is a rod knock from a bearing that’s going away. The others blame this on piston slap. Do you have any advice on what I should do?

T.M.

The unfortunate answer is that either way, you are looking at repairing or rebuilding the engine. While both camps are potentially correct, both will also recommend rebuilding the engine.

If you really want to know ahead of time, there is one relatively inexpensive way to determine with some clarity where the problem lies. All you need to do is take a used oil sample from the fully-warmed up engine and submit it to one of several companies we will list below as sources for this information. (The cost of a used oil sample is only around $25 to $35 and you can have the results usually within five working days.)

If the used oil sample comes back with a high aluminum count in parts per million (ppm), this would indicate that the pistons would be to blame. This might be matched with a high iron ppm count as the piston is also likely abusing the cylinder walls. Of course, if you used aluminum engine bearings (often called bi-metal bearings) then a bad rod bearing would also come back with a high aluminum count.

However, if you used tri-metal engine bearings that are made up of lead, tin, and copper then a high metals count with any of those materials in the used oil sample would indicate a problem with the bearings instead of the pistons.

Thoughts on the Impending Rebuild

Frankly, you should plan on replacing all the pistons and bearings in the engine regardless of the cause of the noise. Many enthusiasts mistakenly think they can re-use the old pistons and just use new rings and hone the cylinder. The problem with that approach is likely a much wider piston-to-wall clearance that could be causing the piston slap.

This often can occur with 2618 alloy forged pistons that require a wider cold piston-to-wall clearance because this alloy grows more than forged pistons that use a 4032 alloy. The 4032 pistons can generally be fitted with a much tighter piston-to-wall clearance compared to the 2618 pistons.

It sounds like you will probably be faced with boring the block and use cast replacement pistons, which is fine for a mild street engine. Cast pistons are not as strong as forged but they can be obtained at a budget price.

For example, Silv-O-Lite offers a set of 5/64 inch ring, four eyebrow cast pistons in either 0.030- or 0.040-over sizes for less than $175 for a set of eight at the time of this publish date. (The part number for a 0.030 inch oversize piston is UEM-1470-030.) That is a really good price but of course you will still need rings, bearings, machine work, a new oil pump, and gaskets assuming the engine doesn’t require other major parts.

V8 engine on a stand
These are typical four eyebrow replacement cast pistons from Silv-O-Lite. These pistons are very affordable and can be used in a mild street engine with excellent results as long as the engine is properly machined and assembled. (Image/Jeff Smith)

We’d highly recommend that you bore and torque plate hone the cylinders at the very least and deck the block surface to ensure a quality head gasket seal. Another wise idea would be to have the crankshaft turned or at least polished and then to measure everything for proper bearing clearances using a micrometer and dial bore gauge instead of Plastigage.

If you don’t have micrometers or a dial bore gauge, find someone who does and who knows how to use them. If you have to, pay them to measure and help you set all your clearances. This will pay off with reliable engine performance over the long run. That wax material called Plastigage is notorious for inaccuracies. Few know that this stuff also has a shelf life and becomes brittle after a time which can affect its accuracy.

This effort will cost more than you think to do correctly but the money is well spent if the engine is assembled correctly. Also make sure if you are using a flat tappet cam that all the lifters spin when the cam is turned. A lifter should spin roughly quarter-turn for every four turns on the camshaft.

If a lifter does not turn, it will eventually fail.

Have fun with your rebuild! It’s worth the effort to keep your machine running!

Used Oil Analysis Sources

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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.