The Service Dept

THE SERVICE DEPT

JODY NICHOLAS

SEIZEVILLE

Recent/i' I had to bore my 250-cc Bultaco Matador 0.020 (n'er, plus 0. 003 for piston clearance. /1 guy at work, who c/aims he kiio ws ei'eri' thing about building ,notorc;'cles, said I was wrong in goIng 0. 003 more (or clearance. He said I should hare gone on/v 0. 0005 more. That seenis awful tight to ,ne, hut he claims that `S ho it' a/I the big hov.s set up their Buls. What do you think'

.James I~'rr

Rochester, N. Y.

Your "guy at work" has some burn dope and should be set straight. If only 0.0005 in. additional was left as clear ance when boring your machine, almost instant seizure would be the result.

As you protably know, the piston in your machine is made from an alumi num alloy, while the cylinder liner is a ferric compound or steel alloy. Alumi num expands more rapidly than iron or steel, which is the reason for piston clearance in the first place.

When design work on a piston! cylinder is done, the engineers have a pretty good idea about the differences in expansion rates, and therefore recoin mend a certain clearance. It is usually found during the factory's testing pro gram whether or not the clearance is sufficient.

With a two-cycle engine. piston-towall clearance is somewhat more critical than with a four-cycle engine because the transfer ports in the cylind~r act as `~valves" to control the incoming and outgoing fuel/air mixture. If the clear ance becomes excessive, a loss of power will result, and if there is not enough clearance, seizure is inevitable.

The correct clearance for your Mata dor is 0.003 to 0.003 5 if the machine is to he used in a non-racing situation, and 0.004 to 0.0045 if you're going to race it.

BRAKE FADE

I read with interest your road test report on the new 1971 Kawasaki Mac/i III, and was quite pleased to see that several key improvements have been made. However, your rating of the brakes is a real disappointment to me (i.e.,. fading after repeated high speed stops). I have experienced the same Jade with my `70 Mach III, and although I had been planning to bin' a `7/, I had been hoping that i/ic poor brakes were something peculiar to the `70 models, or perhaps even to niv hike. Then, I read in tile sa~ne issue your article on Don Vesco c Streamliner, and your descrip tion of how he had used the front end unit (the hydraulic disc) from a Honda 750 Four as his rear brake. I realize such a swap as that requires quite a bit of ingcnuiti', but I was wondering if it would be feasible, both economically and mechanically, to put a front end Honda disc on the front of the Kawasa ki. Any chance of doing it myself? (I don `t own any really sophisticated equipment of course.) Do you have any suggestions for tile rear end?

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BlakeR. Wiggs

Delta, British Columbia

It's really sad that a great many of today's motorcycle manufacturers haven't kept up with brake develop ment. As machines get larger and more powerful, so should the brakes, but this is not often enough the case.

It would be possible to install the Honda disc brake on your Kawasaki, but the cost' would be prohibitive, not to mention the fact that Honda spare parts are often not available.

With a lining area of 65 sq. in., the Kawasaki should stop better than it does. Actually, the first stop is quite good, but because of the brake lining material, fade sets in rapidly. For most people. the brakes on the Mach III are adequate, but some of us want more consistent stopping power.

lhe least expensive way to improve the braking on your machine is to send the complete wheel to a specialist and have him true the brake drum, install better linings, and then match the as sembly. Don Evans, of Evans Brake Service, 25485 E. Base Line, San Ber nardino, Calif. 924 1 0, is such a specialist and really understands motorcycle brakes.

ALL TIRED OUT

I own a `68 Triumph Bonneville with standard tires and it's getting about time to replace both front and rear skins. My bike handles well enoug/~, but I've heard a lot about the new idea in fitting fatter tires up front. I like "gassing it up "a lot on the mountain roads north of here and would consider paying premium dollar for the best possible choice in tire size and tread. Is the fat front tread the way to go? A nd will it change handling characteristics in any way to quicken or slow it?

Harry Griswold

Santa Monica, Calif.

Fatter front tires are currently being used by many of the AM A's top profes sional road racers and European produc tion machine racers. Tread patterns are changing as well. Only a couple of years ago, a rib-type front tire was considered essential for precise, high speed tracking and cornering, but times are changing.

Many production racing motorcycles are now using a larger tire with a zig-zag tread pattern very similar to the conven tional rear tire tread pattern. The main reason for going to a larger cross-section front tire is the increased contact patch with the ground and the consequent increase in traction, especially in corner ing when the hike is leaned over. Norton is now fitting a 4.10-19 rear tire to the front of its Commandos, and handling has improved somewhat because of it (see road test in March `7 1 issue of CW, page 46).

Of course, when a larger front tire is fitted, the front end of the motorcycle is raised somewhat, giving a slight change in the amount of steering trail. (Trail is defined as the distance, measured on the ground, between a line extended down the center of the steer ing head to a point in front of the axle, and a plumb line dropped through the center of the axle.) The increase in trail will result in slightly heavier steering at lower speeds, but it should make the machine more stable while "gassing it up" on the road.

There are many fine tires available these days which work very well. Prob ably the most popular is the Dunlop K70 series, available at all BSA and Triumph dealers. In addition, the Dun lop K87 (for the rear) and the Dunlop Gold Seal F6 (for the front), as fitted to the Honda CB7SO, make a good com bination. The Gold Seal F6 is a 3.25-19, however, just like the standard Bonne ville front tire. The new Dunlop K77. as fitted to the front of the 1971 Kawasaki Mach Ill, is also a fine tire for high speed road work.

There are several other manufactur ers producing special high speed tires for both wheels and we hope to do a comparison of them in the future.