When Imperfect is Just Right

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Machinists love to make things fit perfectly. An example of this is the assembly of a Smith & Wesson K-frame revolver; when the side-plate is put in place and its fasteners done up, no trace of the joint can be felt with a fingernail. Perfect. Traditional old-time machinists literally lay awake at night, thinking of ways to fixture parts to improve accuracy, surface finish, and fit. Clever lads whose mind’s eyes saw farther and more accurately than their elders sometimes discovered that intelligence could get in the way of promotion.

When I started making a few simple things in the machine shop I discovered that a perfect fit can bring imperfect performance. Under OSHA’s required 140 Lumens/sq.ft., and with mike and telescoping gage in hand, a close thumb push fit between rear axle and bearing spacers looks and feels perfect. The clean, oiled axle glides into place with that creamy feel of focusing a camera lens. Everything in perfect alignment. The artist in full control of the medium.

But now those same parts come dashing onto pit lane at Daytona for the first gas stop, the jacks lift the bike, the air wrench whoops, and…the axle won’t come out. Several paralyzed “whaddo I do now?” seconds pass before some clear-headed person bangs it out of there with hammer and drift.

You won’t make that mistake again! Ease and speed of assembly are more important than close fit in this case. Trackside perfect and machine-shop perfect can be different. In order to work, parts must have ‘working clearance.’

In his combat classic “About Face”, the late David Hackworth describes himself and his unit seeing dozers in Dinh Tuong Province uncovering “…the decomposed body of an enemy soldier, complete with AK-47.

“I jumped down into the hole and pulled the AK out of the bog. ‘Watch this, guys’, I said.

“I pulled the bolt back and fired thirty rounds – the AK could have been cleaned that day rather than buried in glug for a year or so.” That automatic rifle had ‘working clearances’.

Randy Mamola had told me he'd get me a look at a factory Suzuki RG500 gearbox, and he was good as his word – I was led to a garage where the twelve gears and two shafts were put into my hands as an assembly. Everything was loose. Working clearance. In this case, I knew a little of the back story; in 1975 Suzuki had several gearbox problems with their new square-four disc-valve race bike. This looser assembly was the fix that resulted. And five years earlier, my Kawasaki H1-R had taught me that I must make sure that no machining marks could interfere with movement of either sliding gears or shift forks.

It’s fascinating to seek perfection in the machine shop, but it must not be allowed to interfere with function.