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Bullet Two Disc Clutch for Kart Racing Engins

Download PDF Bullet Two Disc Clutch Instructions

This clutch is similar to the original 1978 Horstman disc clutch. SMC offers the following changes for 2007:

  1. Re-engineered drive hub to increase stiffness, reduce weight and reduce the number of parts
  2. Modern disc material with a higher coefficient of friction and greater temperature range
  3. Re-engineered springs and levers to minimize chatter
  4. Easy, inexpensive track-side repairs by the racer without grinders, special fixtures and proprietary knowledge

Tuning - Before mounting this clutch on the engine’s crankshaft, each of the six brass levers must be loaded with the appropriate number of brass washers and each of the six springs must be adjusted to the appropriate length accord to the type of engine.

b2 table 1

The data listed in Table 1 are estimates or starting points. It is your responsibility to fine-tune the clutch to achieve maximum performance. Guidelines are as follows:

  1. Heavy levers work better than light levers. Use as many brass weights as possible without exceeding the springs’ capacity to set engagement rpm. A big advantage of the SMC Bullet clutch over clutches of similar design is its ability to accommodate heavy weights without causing clutch chatter.
  2. In theory, a clutch should be adjusted to engage the engine at the rpm that produces the greatest torque. In practice, clutches of this type will cause the engine to “bog” anywhere from 200 to 600 rpms and the engagement rpm should be increased slightly. Use caution to prevent over-slipping and the consequential damage to clutch discs and drive plates.

Maintenance - Proper care will extend the life of the clutch and maximize performance.

• SMC clutch discs are made of aircraft brake friction materials, therefore brake cleaner in combination with compressed air is recommend for removing dirt, oil, dust etc.

• The drive plates and pressure plate are very hard and can be aggressively cleaned with coarse sand paper without affecting their thicknesses. In contrast, the clutch discs should not be sanded because they are soft and “broken in” or “bedded” to match the tiny deflections that occur in the drive plates during racing. No minimum thickness dimensions are specified. Clutch discs and drive plates should be replaced when a noticeable drop in clutch performance occurs. Do not use re-ground drive plates or pressure plates.

• There is no air gap specification. The SMC Bullet clutch was designed to eliminate air gap sensitivity.

• The sprocket bearing should be cleaned with a solvent (such as brake cleaner), dried thoroughly and lubricated with a couple of drops of motor oil.

• Moisture in the atmosphere, calcium or mud from the race track, power washing, rain etc. will cause steel parts to rust.
Corrosion will cause clutch performance to diminish. Attention immediately after the race day is finished might be necessary to protect the clutch from corrosion. Clutches stored in damp basements, garages or trailers during winter months will corrode and require more extensive maintenance when spring arrives.

A 300 pound kart with a stock Briggs & Stratton flat head engine, on a dirt oval running two practice sessions and three heats per day will typically go 20 race days before a Rebuild Kit is required. That is assuming proper maintenance and tuning.

Recommended spare parts: SMC Bullet Rebuild Kit P/N 4869, Six Compression Springs P/N 4847.