QUESTION: What is it that prevents a modern piston design from being used in an older model four-stroke with the long full-skirted piston? I have a feeling the stroke has something to do with it and thus rod angularity and thrust. It would be nice to incorporate some of the newer tech into older model big-bore kits to reduce weight and friction. Jeff Swan Parkville, MO ANSWER: Quite a few adventuresome builders have taken this route, but the results have generally been poor. Here’s why. The bucket-like pistons of olden times had to be fairly heavy to provide enough heat-conductive metal to carry away the extra heat absorbed by the domed pistons of that time. Their domes could not be made as thin as modern pistons, which in turn could not be cooled without oil jets (think of those 112mm ashtrays in Ducati’s Panigale!). So the old-time pistons had to have thick domes to conduct heat outward to the walls, and the thickened region behind the rings then carried that heat down the considerable length of the skirt (tapering as it went) to spread that heat over a large skirt surface area and out to the cylinder wall through the oil film. In other words, those pistons were made that way in self-defense against heat. 
You are right that the mini-skirts of newer pistons would cut friction and weight. Also, the modern arrangement of using very short wristpins in pin bosses that are nothing but short “stalactites” hanging down from the flat, thin piston crown (less reciprocating weight means reduced bearing loads and losses) would do the same. Another point: Pistons run especially hot in air-cooled cylinders. During World War II, makers of large air-cooled radial piston engines for aircraft had instant problems with ring, piston, and cylinder scoring, plus detonation and piston failure when hostilities began. Why not before? Training aircraft are flown conservatively because who knew when Congress would cough up the bucks for replacements? But when war comes, throttles go to the wall. So a frantic effort to bring down piston temp began—from steel cylinders with integral-machined fine-pitch steel fins, to pressed-on machined-fin aluminum “muffs,” to caulked-in thin aluminum sheet-metal fins, set into shallow grooves on the outside of each forged-steel barrel (each B-29 cylinder had 54 of these!). But air-cooled bikes don’t cruise to the target on steady 70-percent power, and this is what made them practical in their time, always slowing for cops and corners. Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.Gadget Reviews: mamaktalk.com
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