SMC 4-Cycle Engine Clutch

The SMC 4-Cycle Engine Clutch is a lightweight disc clutch designed specifically for kart racing. This model is for use with Stock and Restricted five horsepower engines that engage in the range of 2500 to 4500 RPMs and have a maximum rotational speed of 7000 RPMs.

The SMC 4-Cycle Engine Clutch exhibits a smooth yet firm engagement that efficiently transfers more of the engine's torque to the road. The clutch will generate much less heat, the engine will turn fewer revolutions per lap and the kart will accelerate faster.


This clutch is set for an engagement speed of approximately 4000 RPM on a stock motor. If this clutch is to be used on a restricted motor, read "Spring Preload" paragraphs on the following page.

1. Mount the clutch on the engine crankshaft. It can turn in either direction but the preferred method is to run inboard i.e. clutch sprocket next to the engine.

2. Insert a key between clutch and crankshaft. Install the crankshaft washer and flat head socket cap screw and torque to 20 foot pounds (240 inch pounds).

3. Align the axle sprocket with the clutch sprocket.

4. Drive for only one or two laps and note the tachometer reading coming out of the corners.


Adjust this clutch to engage the engine at its peak torque RPM. Peak torque RPM can be determined with a dynamometer or with a stop watch and acceleration tests.

"Over slipping" occurs when the clutch engages at an RPM higher than peak torque RPM. Acceleration is slow but more importantly it generates a large quantity of heat that accelerates lining wear, warps drive plates and tempers hardened metal parts.

"Under slipping" occurs when the clutch engages at an RPM lower than peak torque RPM. Acceleration is slow but unlike over slipping, no clutch damage occurs. Changing the engagement RPM is a simple process of changing the air gap.

Air Gap - The sum of the clearances between drive plates and clutch discs or "air gap" affects the engagement RPM. Increasing the air gap will increase engagement RPMs. Decreasing the air gap will decrease engagement RPMs.

A .002 inch change in air gap causes a 100 RPM change in the engagement speed.

For example: if the tachometer reading during the break-in lap was 3800 RPMs and the engine's peak torque occurs at 4000 RPMs the clutch is under slipping by 200 RPMs. It is necessary to increase the air gap by .004 inch.

1. Remove the outside drive hub retaining ring with snap ring pliers. Note that the retaining ring has a convex side with smooth edges and a concave side with sharp edges. The convex side with smooth edges should always face the drive plates.

2. Remove three drive plates, two clutch discs, and the pressure plate assembly (balls, springs and pressure plate). Do not disassemble the pressure plate.

3. Remove the shims and measure their thicknesses with a micrometer.

A. Substitute thicker shims to reduce air gap. (fewer RPMs)

B. Substitute thinner shims to increase air gap. (more RPMs)

4. Reassemble the clutch. The retaining rings are subject to very high stresses. If either retaining ring is distorted, replace it. Double check the orientation of both retaining rings. The convex side with smooth edges should face the adjacent plate. A reversed retaining ring will affect the air gap.
Make sure each retaining ring is fully seated its groove.


1. Disassemble the clutch and clean all parts with brake cleaner and compressed air.

2. Clutch discs may be lightly sanded to restore surface finish. Minimum disc thickness is .080 inch. Glazed or warped discs should be replaced regardless of thickness.

3. Drive plates may be sanded or ground to restore surface finish.

4. The drive plate adjacent to the balls will have 12 wear spots. Polish these 12 areas with 1000 grit sandpaper.

5. Inspect the surface of each of the 12 balls. If the black coating has worn off, replace the balls.

6. Remove nicks and corrosion on the drive hub with a file or sandpaper.

7. Check spring preloads for uniformity and magnitude.

8. Lubricate the bearing with a high quality grease. One end of the inner race has a radiused edge on its inside diameter that conforms to a similar radius on the crankshaft. Reassemble the inner race with its radiused edge toward the outside of the clutch.

9. If the clutch is exposed to rain or high humidity, disassemble it immediately and dry all parts to prevent rust.

Lockup is a condition where the clutch disengages at a lower RPM than the engagement RPM. In extreme cases the clutch does not disengage at any RPM and heavy braking causes the engine to stall. If the clutch is locked up while the engine is turning less than peak torque RPMs, subsequent accelerations will be very slow.

Lockup is caused by friction between drive plates and drive hub or friction between drive plate and balls. Review items 4, 5, and 6 above.


Spring Preload - This step is optional but highly recommended for optimum performance. Each of the 12 springs is adjusted to apply an appropriate preload to the balls.

1. Remove the pressure plate assembly and attach it to a flat surface with a bolt and washer through the hexagonal hole.

2A. For engines that develop peak torque at 4000 RPMs or more, the springs should be bent toward the balls to induce a small preload. Position a dial indicator perpendicular to one spring at its flat, tapered section opposite the ball. Note the number on the dial indicator. Remove the ball and again note the number on the dial indicator. The difference between these two numbers is the spring preload. For engines that develop peak torque around 4000 RPMs the suggested preload is .005 inch. For engines that develop peak torque around 4200 RPMs the suggested preload is .025 inch.

2B. For engines that develop peak torque at 3800 RPMs or less, bend the springs so that a gap exists between the ball and the spring. Use a thickness gage to measure the gap. For engines that develop peak torque around 3500 RPMs a dimension of .010 inch is suggested and for engines with peak torque
at 2800 RPMs use .020 inch. Handle this pressure plate assembly carefully because the balls will fall out if the pressure plate is inverted.

3. The amount of spring preload is not critical, but variations in preload among springs is important. Try to match all 12 springs within .003 inch or less.



 Owners Manuals