Clearance specification

I understand being able to taker manufacturer specifications into consideration when it comes to various clearances; however, if you are performing a build that information sparsely exists online, or if you are trying to create a more powerful engine or just different then intended by manufacture where do you find or how do you determine what clearances you should try to achieve within the build project?

Clearances generally are determined by the materials and environment where there is relative (rotational or sliding) motion. You can calculate material expansion over the temperature range that will exist. So if you are changing materials -- then you need to be doing the calculations to determine the changes required to the clearances.

To understand if your calculations are right, you build (maybe several) engines and test, and tear them apart -- eventually you determine what you can and should do. This is exactly how full time engine builders develop their products.

Development is about trying things -- then measuring the results to determine if the change was better, worse or the same.

Where might this lead -- how about zero clearance engines -- they have to be pre-heated just to allow them to turn over. Want to fire up your Judd EV6 V8, well you better start the external water heater a couple of hours before, so you can have a 165 degF block temp before you stick the 3000 psi air starter in the gearbox.

You can get some real insight into this kind of development work from one of my favorite engine books - Beast: The Top Secret Ilmor-Penske Race Car that Shocked the World at the 1994 Indy 500. Available on Amazon

Here is part of the book on clearances while assembling an engine where they talk about heating the block to 350 degrees, then having seconds to install the cylinder liners...

Once the liners were in correctly, the next components were the camshaft bearings (four steel-encased needle-roller bearings) and then the camshaft itself. Each bearing was installed in an exact order with a tool designed specifically for that part. While the liners went in at room temperature, the bearings were placed in liquid nitrogen, decanted from a cryogenic vessel into a large insulated box, to super-cool them before being installed in the heated block.

The nitrogen process allowed the bearings to contract enough to fit in the exact location on the block. Warner emphasized the need to be very cautious with the dangerous liquid, especially in close proximity to a heated engine block. Safety was not taken lightly.

“When you pull it out of the nitrogen, you get a lot of vapor and it boils,” he said. “You wear gloves because the tools and the bearings are so cold. No need to get frostbite! If you hold it too long, condensation forms and it frosts over. That hinders you from sliding it into place.

“Instantly, the heat of the block hits the bearings and gets a bite on it and the frost begins to disappear off the tool,” said Warner, explaining the urgency to be accurate, as any misalignment would show up immediately.

“The whole point is to not use any force. You’d push it in easily so you don’t damage the bearing or damage the bore. “While [the block is] still hot, you put the cam in,” Warner said of the room-temperature camshaft that installed into the bearings.

“It really depended upon how confident and swift of hand and eye coordination [the builder was]. If you hesitate or are intimidated by the task, then you’re not the person to be doing that. Some people would chicken out and do it halfway and then put it back in the oven, but others could go boom and complete it in two and a half minutes.”

If an engine builder made a mistake in the process, the parts would be removed and the block would go back into the oven until it again reached the proper temperature. The final step was installing a brass and copper sealing ring onto the block above the liners to create a gas-tight seal in each cylinder when the engine is running. The tolerances are measured in 0.003 or 0.004 inch.

“This is why we never started an engine cold. It was always preheated,” Warner explained. “All the internal clearances are designed to be optimized at operating temperature. When it’s cold, tolerances shrink down so much. That’s why the engine is always quite hot when it starts.”