Ups and Downs of Taking Things Apart

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Kevin Cameron (Robert Martin/)

More than a week ago I was brought a Yamaha TD1-B engine with a backstory. Its original owner had taken it to Daytona in 1965 as an AMA Novice, ridden the first practice as far as where the chicane is today (it was added in 1973), and decided motorbike racing wasn’t for him. The bike sat unused since then.

To inspect the crankshaft I had to split the cases, which requires removing 20-odd 6mm Phillips-head screws. Not hard: Loosen them with a hammer-driver, then spin them out. The hammer-driver’s set of internal ramps turn a hammer blow into a powerful loosening torque, combined with a strong downward force on the driver, preventing its Phillips bit from “camming out,” or being driven up and out of engagement by the loosening torque. That worked on all but one screw, which was down inside a hole deep enough that the hammer-driver couldn’t reach it. Naturally I tried pushing down on my No. 3 Phillips with everything including my chin, but when I applied torque, up rose the bit.

This is the action that has so often converted Phillips heads into something that looks like it just re-entered the earth’s atmosphere, a shapeless blob with no means of turning it. Air wrenches allow even faster and more comprehensive damage to Phillips heads.

How could I keep the bit engaged rather than rising up? The hydraulic press! I’ve spent many happy hours at this press, which so wonderfully multiplies human strength. I could put the whole engine in the press, with the butt end of the Phillips screwdriver under the press ram to keep it from camming out. With just a couple hundred pounds of force holding the driver in place, the torque to loosen was surprisingly little. The same result could be had by wedging the engine and screwdriver under a bench and using a 4-foot-long two-by-four to apply the same force.

Anyway, with all the screws out, and with a measure of trepidation, I set about splitting the crankcase halves. For this vertically split engine there is a special tool, consisting of two long 8mm bolts to screw into the crankcase and centered on the projecting crankshaft end, a bridge connecting the two, and a hole through the center of the bridge for a large screw.

The tapered end of that screw bore on the taper hole at the end of the crank. At the turn of the screw, the cases separated easily and 30 turns later I had the ignition-side crankcase off. I never had it so easy! It was like I was living in those ideal pages of the service book!

It was just as easy to push the crank out of the drive-side case half. I was delighted. We have all had the experience of starting a job which should be as easy as it always looks in the service book (with brand-new parts, naturally), only to discover that the cases are glued together with something fiendish, or that the passage of decades has given the crank ends an unbreakable grip on their bearings. Settle in for hours of frustration.

The next engine to be dealt with (I needed parts from it) looked as though it had belonged to five different owners, each of whom had it apart multiple times, earnestly trying and failing to do everything right. I started in, and the impact driver loosened most of the screws routinely. One cylinder was frozen in place by piston rings rusted to the cylinder wall.

Next came the problem case and cover screws with damaged Phillips heads. Sometimes the impact driver mashes its way in just enough engagement to loosen the screw; that took care of all but three. There was no room around the screw heads to get hold of them with Vise-Grip pliers, but there was access by which I could saw a screw slot, using a hacksaw. Aw, shucks; the resulting slots weren’t quite wide enough to accept the wide blade of my 3/8-inch socket slotted screw socket.

This is why it’s foolish to throw odd tools away. I rummaged through a box of seldom-used files. Ah, here’s one that is the right thickness and has file edges. A few strokes widened the saw slots enough to fit the slotted screw blade. With that on the end of the trusty Vessel impact driver, the problem screws were loosened.

I had a backup plan not available to the casual handyman: the milling machine. Brute force! Just set the whole engine on the bed of the Bridgeport, clamp it down, and mill the heads off of the problem screws! Or if you happened to have any head porting equipment (die grinder, flexible shaft machine), you don’t need the mill—just chuck up a ball-end carbide cutter, don the safety glasses to ward off tiny steel arrows, and carve the heads off those screws. There’s always a way. Just keep thinking and the next experiment will come to you.

But even with all eight screws removed, the clutch cover wouldn’t budge. Tapping on it with a soft hammer gave that solid sound that says “No way!” Some manufacturers thoughtfully provide little bosses on the cover that you can tap against in a cover-removing direction with a brass drift and hammer. Not here—it was neatly flush all the way around.

Then I saw it. There was a drain plug low on the case. If I unscrewed it a turn or two, its exposed edge would give me purchase against which the brass drift could be set. Three taps and the case was loose. The last assembler had glued both faces of the gasket so the gasket was torn in half by my work.

The case was loose, but not free. The reason this engine had been in a junk pile was revealed by a 15-degree bend in the shift shaft, projecting through the case. That’s a solid steel shaft, 13mm in diameter, and it takes a major impact to bend it. A crash. I wondered what the rest of the bike had looked like on that day. And its rider.

Straightening the shaft would require a perfect “un-crash,” reversing reality to leave the shaft perfectly straight. Not likely.

Maybe there’s another way. Some engines, such as Kawasaki’s H1R of 1970 for example, had a projecting shift shaft on both sides, so that both English and metric riders could have the shift pedal where they wanted it. But this YM1 305 Yamaha engine provided shifting on the left only. That means the shift shaft terminates inside the engine. Rather than trying to pull the case off over the bent shift shaft, maybe I could push the shift shaft outward from the inside?

Eight more Phillips heads released the gearbox sump plate, revealing the shift mechanism. Bright light revealed a circlip holding the internal shift arm onto the bent shaft. Pop, a twist of a small screwdriver released the clip. Now with a long drift pin I was able to tap the shaft outward. Success! The case cover lifted off with the bent shift shaft still in place.

Time for dinner.

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