How Do Piston-Cooling Oil Jets Work?

Hall-of-fame engine-builder Tom Sifton would be proud

Around 1940, manufacturers of air-cooled aircraft engines were having trouble with pistons running too hot. They were scoring their cylinders, there were seizures, there were breakups. So engineers experimented with the idea of supplying extra oil from an oversize pump and directing jets of oil up underneath the pistons to cool the underside of the domes.

At the time of those experiments, the investigator wrote, “Too bad we can’t solve the oil-control issues that are caused by squirting so much oil around inside the engine.” They overcame that problem with improved oil-control rings.

When famous Harley-Davidson tuner Tom Sifton was working with the late Joe Leonard, he decided to attack one of the worst problems of the flat-head KR racing engine, which was piston seizure in distorted cylinder bores. He thought to himself, “If it works for aircraft engines, it might work for me.” So ol’ Tom included piston-cooling oil jets in the race engines that he built for Leonard.

Such oil jets have been used for a long time in Formula 1, they are in use in MotoGP, and they are now showing up in production engines. In this Honda CBR600RR, for example, a tube runs across the engine. It has a hole for each cylinder, from which a jet of oil shoots up to hit the underside of the piston dome, splash across to the other side, and fall back into the crankcase.

So this production engine is employing methods of piston cooling that were experimental in 1940, that in 1946 became an integral part of both military and civil aircraft engines, and which—through Tom Sifton, a very imaginative and gifted engine builder—became a part of motorcycling in general.