Tightening (or loosening) Clutch & Driver Nuts the Right Way
By John Copeland
One of the most critical elements of 2-Cycle engine performance is proper crankshaft alignment. If you do your own engine work you know how much time it takes to get the run-out within acceptable limits (usually .0002 at the most). And if you have an engine builder do your crank work, you know that it's not an inexpensive service. Just a little bit of crank mis-alignment can really kill top-end performance, and can lead to crankshaft breakage. Yet, for all that, too many karters get their engines ready to go and then promptly screw up all that hard crank alignment work when they tighten up the nut that holds the clutch or the driver on.
The problem is, once the engine is together, there is precious little to hold onto to tighten or loosen the nut on the driver or clutch. That starter nut on the other end of the crank is a tempting thing to use to hold the crank while you tighten or loosen the nut. DON'T DO IT! For those of you who don't know, the crankshaft is actually a 3 piece assembly. The crank pin that carries the bottom rod bearing is pressed between the two halves of the crank that stick out each side of the crankcase. Even though it's a very tight fit, holding the crank by one end and twisting on the other end is very likely to ruin the alignment of the two halves.
OK, so then how do you get that nut on tight enough when you mount the clutch or driver? Conversely, if you need to get that little stinker off when you need to? The answer is to hold the crank by the end that you're working on. How you do this depends on whether you're using an engine clutch or an axle clutch. In either case, all you'll need will be tools you already have in your toolbox, plus an odd bit or two.
For you axle clutchers, you need a pair of channelock pliers and either a 6 inch piece of old drive belt, or an old piece of chain the same length, depending on whether you're belt or chain driven. Just wrap the piece of belt or chain around the driver to protect it, and grab on with the channelocks. Be sure to set the pliers to the right grip range so you can get a solid grip, and so the plier's grip tightens as you apply pressure on the nut. Otherwise you won't be able to a good enough grip to get the nut on or off.
Too many engine clutch users seem determined to tighten or loosen the clutch nut with the clutch cover installed. Sorry, the only way to do this job properly is with the cover off. Every clutch manufacturer also offers a clutch wrench to hold the clutch "guts" solidly while you tighten or loosen the clutch nut. In a pinch, you can hold the clutch with your trusty channelocks, if you have a pair big enough. Just be sure to wrap something around the clutch where you're going to hold it, so you don't chew up the springs or weight levers. Overall, it's really best to invest the $10 to $15 for a clutch wrench and take the worry out of screwing up your high dollar clutch.
One final word: It's always a good idea to use a torque wrench to tighten the nut on the crank. Particularly on the Yamaha KT100SE engine, the notoriously soft crankshaft is relatively easy to bend out on the end. I've even seen more than one heavy-handed karter snap the threaded end off a Yamaha crank by tightening the nut too hard. If the keyway is in good shape, and the taper on the crank is smooth, and if the clutch or driver is lapped onto the crank (always a good idea), and with the proper key, correctly fitted, the specified torque of 22 ft/lbs is more than adequate to hold things in place. If any of the conditions above are not correctly prepared, it is highly likely that the clutch or driver will come loose, regardless of how tight you tighten the nut.
So there you have it. With tools you probably already have, and just a little extra care, you can safeguard that expensive crank rebuild, and probably enjoy a little better performance too. Take your time and do it right, it's worth it. See you next month.