Your engine tolerances are too tight

October 1, 2013 | By Jim Richardson


My 1956 Chevrolet 150 has a freshly rebuilt 350 cid V-8 with an Edelbrock 650 cfm carb. My problem is with hot starts. The motor turns over fine when cold but real slow when hot. Sometimes it won’t turn over at all and I have to let it cool down. It always starts eventually but it’s very embarrassing.

To date I have installed a high torque starter with a heat shield warding off heat from the headers. I have put a oneinch spacer under the carb. I have also installed an electric fuel pump to ensure fuel delivery. I don’t think it’s flooded; don’t smell fuel. Some folks think it’s vapor lock; I don’t really know; don’t understand it. Can you advise?


Your problem is not with the fuel system. The reason the engine is slow to turn over or won’t turn over at all when it is hot is most likely because the tolerances between the pistons and cylinder bores are too tight. The engine block is made of cast iron, and the pistons are made of aluminum. Aluminum expands more as it heats up than iron does. The tolerances in your engine are OK when everything is cool, but when it warms up, the pistons expand more than the block and that causes more resistance and friction for the pistons and rings. If the clearances between the pistons and the bores are off by as little as one thousandth of an inch beyond the specified range, this sort of thing happens.

On the other side of the coin, if the pistons are too loose by even a thousandth or two, you will be faced with piston slap when the engine is cold. Regardless of whether these tolerances are too tight or too loose, you have a problem. Too loose is actually not as serious as clearances that are too tight. A little piston slap at start-up time is not fatal in the short run, though it will probably eventually get worse and cause more noise and more cylinder bore wear. But tolerances that are too tight can cause rapid wear and overheating.

When you have any engine re-machined, bring your replacement pistons to the machinist so he can fit them precisely. If you just bore the block to the correct nominal bore, the pistons may or may not fit, even though they too are supposedly machined to the correct nominal bore.

Over the years I have also seen new crankshafts that were supposedly properly machined that were too fat and had to be turned down slightly to get the crank bearings to fit properly, and I have often seen pistons that — though they were nominally the correct bore — were too big or too small.

Some old-time mechanics would just drive a car as it is with your problems hoping that the engine will “wear itself in” but I don’t recommend that because it probably won’t wear in to the proper clearances, and you could very likely damage the engine. Unfortunately, the right way to deal with the problem is to refit the pistons (and bearings, if required) and that means pulling the engine, disassembling it, and re-machining where necessary to get the correct clearances.

Assuming the problem is piston clearance, you may be able to fix it at home using a cylinder hone hooked up to a 3⁄4" variable speed household drill. You would have to pull the heads and pan, and then take out the rods and pistons, but you could leave the engine in the car. Of course you would have to use rags stuffed below the bores, and a lot of solvent to clear out all the swarf from the hone.