FAST TRACK By: Michael Romeo
4-CYCLE DISK CLUTCH REBUILDING & MAINTENANCE
INTRODUCTION
Disk clutches are used in various racing applications. They are one of the best ways to put the power you are creating to the road, with the least amount of horsepower loss due to heat caused by slippage. At least this is what is supposed to happen. Many competitors do not properly maintain their clutches, or when they do, proper procedures are not followed. I will attempt to guide you through the process of maintaining your clutch, as well as properly tuning it for your various racing applications.
STARTING OUT - Step 1
First, we have to remove the clutch from your motor. Remove your bottom motor mount butterflys so you can take the chain off the clutch. Next you will need a ½" wrench and a screwdriver. Position the screwdriver against the flywheel of the engine. Do not put any pressure on the magnet, only on the flywheel itself. Next, use the ½" wrench to remove the bolt at the end of the PTO shaft. Your clutch should now slide off the shaft. Be sure not to lose the clutch key or any bolts as you will need them later.
Step 2
Now that you have the clutch in front of you on the work bench, remove the snap ring from the clutch. This is best done with a pair of snap ring pliers that you can purchase at your local auto parts store. Now that you have removed the snap ring, slide the drum from the clutch assembly. At this point, take the time to clean your drum with brake cleaner, and inspect it for wear. First, look at the sprocket (teeth) on the drum. You can really only inspect this while the clutch is off, so take the time now. Look to see that the teeth are not pointed, or as I like to call it "Ginsu Geared." Also make sure that the teeth are not slanted from heavy use. If you notice either of these, the drum (or at least the sprocket assembly) will need to be replaced. Also inspect the drum where the disks lock into it. If you notice grooves or excessive wear in this area, you will need to replace the drum. Lastly, spray some lubricant on your finger and rub it onto the inner bearing. Use the lubricant sparingly, as you do not want it to fling into the disks while racing.
Step 3
Now that you are finished with the drum, you are left with the heart of the clutch in front of you on the workbench. You need to check how much air gap you have. To do this, take a set of feeler gauges, and see how much distance there is between the friction disk and the top plate. New clutches usually come with about .035" air gap. The maximum air gap that can be used is .055:, while the minimum is .25". If your air gap is too high, this can be fixed by installing a thicker floater plate, which will decrease the amount of "play" between the disks. A high air gap could also be caused by the friction disks being excessively worn, but you won't know for sure until you completely disassemble the clutch. Just write down the air gap you recorded and save it until later.
Step 4
At this time you will need to "dig deeper" into the clutch to finish diagnosing any possible problems. Turn the clutch so that you are looking at the weights and springs. Now, remove the bolts around the inner most part of the clutch that bolt into the centre hub (their are about 8 of them, 5/32" allen head.) Once you have removed the bolts, you can proceed to take off the top plate of the clutch. Now the inner disks and floaters are exposed. Take the "top part" of the clutch and check to see what your springs are set at. Using a 1/8" allen wrench, turn each spring in clockwise, counting how many revolutions you have to turn each one. Write this down along with the air gap measurement you took earlier, as you will need it later. Now you can completely remove the springs from the assembly by turning them in a counter-clockwise direction. While you are turning the screws out, make sure the spring retainer bolts turn out "straight" and do not "wobble" while you are turning them. If they do, you will need to replace them. Now you can remove the weights and pins, checking for both wear on the pins and the weights being straight. Many times while racing, your seat or seat brace may come in contact with the clutch weights, causing them to become bent. If so, they need replacing.
Step 5
Next, inspect the top plate for uneven wear. If you notice the entire surface where the disk meets the top plate is not being used, chances are that the top plate is warped. You will need to replace this part. If you have even wear, light ly scuff the contact surface of the top plate to remove any "glazing" that may have occurred. Now, measure the thickness of the friction disks in your clutch. Using a vernier caliper, you should not read anything less than .115". If you do, you will need to replace the friction disks. Also, look for excessive wear on the friction disk lugs. If they are beginning to wear, replace them. Now, inspect the floater plate (not present on single disk models). Place the floater on a known flat surface and rock it back and forth, side to side. If it "wobbles," it will need to be replaced. If it is "true," then lightly scuff its surface using emery cloth to remove any glazing. Well, you are now left with the bottom plate and the hub. Inspect the bottom plate for even wear, much like you inspected the top plate. Again, if it is wearing evenly, simply scuff the working surface with an emery cloth to remove any glazing. Also, check the hub for any cracking, etc. at this time.
Step 6
Now, it's time for some math work (sorry guys, it's necessary!) If the air gap measurement that you took earlier was beyond the maximum, this step is crucial. If you put new friction disks in the clutch, measure (using vernier caliper) the difference in thickness between the old and new ones. Now, find the difference between the two numbers, then subtract it from the air gap you previously recorded. This new number is you new air gap! If the new air gap falls within the limits set out earlier, you're fine. If not, you will need to replace your current floater plate with a thicker one to take up some to the air gap.
Step 7
Now you're ready to begin reassembling the clutch. First take each part and spray it with brake cleaner to remove any dirt and oils which may be present. Now take the bottom plate and hub assembly and set it in front of you on the bench. Install the friction disk, then the floater, then the other friction disk. Now reinstall the top plate, making sure to put the weights and pins back into the assembly. Now you can reinstall the 8 allen bolts, tightening them to 75 inch pounds each. Yeah, it's beginning to look like a clutch again. Now, reinstall the 6 springs, and tighten each one all the way down until it stops. I hope you remembered to write down how may turns were on each spring bolt because you need to know that now. Turn each screw by that number. In doing this, you will be assured that your clutch engagement is about the same as it was before you took the clutch apart. Now you must ensure that all the springs are set at precisely the same height. If one spring has a little less tension or more tension than the others, the clutch will now work in unison. Using either a dial caliper or a spring height gauge, measure the height of one of the springs, then set the other 5 at the exact same height. Now, reinstall the drum/sprocket assembly, along with any washers that were used. Reinstall the outer snap ring using snap ring pliers, and you should now have a perfectly adjusted clutch in your hand. But now we have to put it back on the kart…….
Step 8
Slide the clutch unit back on the kart, either inboard or outboard, the same way you took it off. Be sure to reinstall the key at this time. Now, screw the retaining bolt and washer(s) assembly back onto the end of the crankshaft by hand. Take the starter handle and pull the cord out until the starter mechanism catches. Now take your ½" wrench and tighten the retaining bolt, while holding the starter handle. This keeps the motor from turning over. Now reinstall the chain, set your tension, and tighten the motor clamps and you're ready to race - almost.
Step 9
If you put in new friction disks, you will need to "break-in" your clutch at the track. This is done by taking 5 laps around the track, keeping the stress put on the clutch to a minimum. You do not want to excessively "slip" the clutch at this time. While you are on the track, look at your tachometer to see the engagement RPM. When you are done your 5 laps, you can now make any adjustments to the engagement that you see fit. To raise the engagement RPM, turn the spring adjusting screws clockwise (¼ of a turn will adjust the clutch by approximately 50 RPM.) To lower the engagement RPM, turn the spring adjusting screws counter clockwise. A ¼ turn will change the engagement by 50 RPM (approximately). When turning the spring adjusting screws, be sure to turn each one equally to achieve maximum performance. Once you have found the proper engagement RPM for your particular application, you are ready to race!! You are now getting the most out of you 4-cycle disk clutch. Thank you for reading, and I'll see you in "The Fast Track."