Tech Talk

The clutch, as used on a Ro­tax en­gine in a mil­i­tary ma­chine, is a bit ba­sic in op­er­a­tion. It works... but can cer­tainly be im­proved.

Classic Dirtbike - - Contents - Words and pics: Tim Britton

Old bikes are nice, they’re fun, they’re ‘our’ thing or you’d not be read­ing this… they can be im­proved… we have a look at a Ro­tax clutch.

When look­ing to make any im­prove­ments, you will need a few bits. As stan­dard, the mil­i­tary Ro­tax en­gine Can-am has a fairly sim­ple method of ac­tu­at­ing the clutch. In­side the case there is a ramp cast and this has three pid­dling lit­tle ball bear­ings in it. A pressed steel fab­ri­ca­tion rides up on the bear­ings as the lever is pulled. It works well enough on the road and even on a trail ride, but in the cut and thrust of an en­duro, things get a lit­tle tougher.

Last year’s Hot Trod Vin­duro was the fi­nal straw for my clutch hand, as I re­alised by the end of the Sun­day all I was man­ag­ing to do was take up the slack in the ca­ble rather than lift the clutch. The Can-am gear­box is a ro­bust thing but even so it goes against the grain to abuse me­chan­i­cal parts in such a way for too long. Some­thing had to be done.

One of the ad­van­tages of the Ro­tax en­gine is the num­ber of com­pa­nies who have used a vari­a­tion of it to build their own ma­chines, rather like Vil­liers en­gines of years ago, so there is a wealth of in­for­ma­tion around, plus a few spe­cial parts here and there.

Ask­ing around, gen­eral opin­ion was the Aprilia ver­sion of the ac­tu­a­tor was the way for­ward and once the bits were on the bench it was ob­vi­ous this was a more meaty method of work­ing the clutch.

The Aprilia kit re­quires the orig­i­nal ramp to be ma­chined flat so the new base plate can sit squarely. Such work is be­yond my work­shop ca­pa­bil­i­ties so a ma­chine shop was sought and a large milling ma­chine made short work of the ramp and soon ev­ery­thing fit­ted into place. A lit­tle light re­liev­ing on the cen­tral hole to make sure noth­ing was stick­ing and I looked for­ward to a sim­pler life for my clutch hand… un­til it dawned the Aprilia clutch ca­ble en­ters the case at a dif­fer­ent point to the Can-am and the op­er­at­ing arm is slightly out of line with the hole in the case.

Ac­tu­ally the Aprilia uses a dif­fer­ent case which would have been an op­tion but this would have meant con­vert­ing to pre-mix and I wanted to re­tain the auto-lube fa­cil­ity of the orig­i­nal. Also the Aprilia style clutch case is rather rarer than the Can-am one. Luck­ily, my ma­chin­ist said it’s not a prob­lem to al­ter the ac­tu­at­ing arm and all would be well, but af­ter that it was my job to sort out the face spring.

Drain the oil out – be care­ful not to undo the kick- start spring in­stead of the drain plug. If you do, the en­gine needs to be stripped to re­assem­ble it. There are pre­cise lengths to Can-am­case screws, get them wrong and the screw will punch a hole in the in­ner cast­ing, which is bad. This bit of card­board with a ba­sic case out­line will en­sure the screws are in the right or­der. Be­hold, the clutch. It’s worth clean­ing the plates too when you’re this far in to the pri­mary side. They can get sticky so a few mo­ments with some clutch cleaner is worth the ef­fort. The orig­i­nal clutch ac­tu­a­tor can be seen here, it’s held in place by a spring and the arm re­turn is helped by a coil spring. In the or­di­nary world it works, is a bit heavy but okay, in an en­duro with con­stant us­age it gets tire­some re­ally quickly. These pid­dling lit­tle bear­ings are not re­ally up to the job, even with con­stant lubri­ca­tion they can stick in their re­cesses and then all the face cam can do is slide rather than roll. These 10mm­di­am­e­ter balls used on the Aprilia ver­sion are­much bet­ter and will al­low a lighter move­ment as the ac­tu­at­ing arm is longer too. The end where the nip­ple sits will need a tweak to hold the ca­ble the other way though. This bad boy soon shifted the orig­i­nal ramp and al­lowed the case to ac­cept the back­ing plate from the Aprilia kit. Once the ma­chin­ist had fin­ished it was a nice press-fit into the case. Note the tapped boss still in place so the clutch ad­juster has the stan­dard amount of thread to screw into for max­i­mum strength. This did­mean the rear of the back­ing plate needed re­liev­ing a lit­tle so it could sit flat in­side the new re­cess. And here it is, sit­ting in place. It was at this point the re­al­i­sa­tion dawned that the part of the arm which holds the ca­ble nip­ple wouldn’t be in line, or at least not as nicely in line as it could be. So, a lit­tle mod­i­fi­ca­tion af­ter a few sec­onds’ thought, by the ta­lented ma­chin­ist and welder who did the fancy stuff, while I stood and nod­ded sagely. Here it is in place. Yes it works, only slight cloud is the face spring, if any­one knows of a spring stock­ist with a selec­tion of small springs then please let me know.

Above: Sim­ply re­move the kick- start, gear lever, all the case screws, footrest and brake cross- over shaft, then the case slips off eas­ily… with a bit of wig­gling.

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