ESSAY: The Devil is in the Details

In motorcycle racing, even the tiniest of problems can hand you failure.

blue TZ50 head nuts

When Kawasaki released its three-cylinder 500 H1R production racer at the beginning of 1970, it still contained some detail problems, one of which was occasional fracture of the 8mm studs that secured the cylinders and heads.

In the "Daytona Summary" printed after that year's Daytona 200, it was noted that this problem had affected 10 machines. It continued:

“Solution: Threads should be rolled in to eliminate sharp corners and improve stress lines. Carbon content and heat treatment should be changed to reduce brittleness. 8mm diameter should be adequate and need not be changed unless above solution does not solve the problem.”

The improved studs were soon provided to H1R owners and failures ceased.

These were not the first bolts I had encountered that were too brittle. The long engine-mounting bolts on my 1965 Yamaha TD-1B broke constantly (I would find the nut, with the threaded portion of the bolt still in it, dangling from its safety wire). So I made some soft bolts. Instead of breaking, they stretched!

In 1974 the revolutionary Yamaha TZ-750A arrived, giving us all a foretaste of the present era, in which chassis qualities and handling are central, pushing aside horsepower as the key to winning races. A couple of years ago in an Indianapolis hotel lobby I saw Kenny Roberts, walked over to him, and handed him one of the blue-anodized aluminum cylinder nuts (photo above) that had been original equipment on those '74s.

Without hesitation he said, “One of those goddamned things cost me the Daytona win that year.”

Riders never forget.

Giacomo Agostini won that race and Kenny was second, his bike overheating as a result of water loss caused by the head of one of those blue nuts popping off during the 200. Was the radius where the head joined the shank too sharp? Or was it too gradual, causing an occasional stress concentration when the radius pressed against the sharp edge of the copper washer under it? We didn't get copies of the report (you can be sure there was one)—just a set of 12 new clear-anodized replacement cylinder nuts that did not break. Yamaha can't have been too disappointed—Ago was also on a Yamaha.

In Atlanta practice, we broke one of the steel studs that those aluminum nuts screwed onto, and naturally it broke flush with the crankcase deck. With nothing to take hold of, the solution was one of the following:

Drill into the stud and use an EZ-Out to extract the very firmly seated remnant (hoping the “undrillably hard” EZ-Out doesn’t break in the process).

Or, drill an accurately centered hole into the stud, just big enough to leave nothing in the case but the spiral that used to be the stud’s actual threads. Wind out that stiff steel spiral, blow any chips out of the hole, and fit a new stud in time to build up a running engine for next practice.

Neither of these is without risk!

Yamaha soon supplied improved studs, and the TZ-750 went on to rack up nine consecutive Daytona 200 wins. The point of all this is that adequate funding and a stack of promising dyno sheets aren’t all you need for success. Every detail has to be right or some tiny problem hands you failure.