[cs_content][cs_section parallax=”false” style=”margin: 0px;padding: 0px;”][cs_row inner_container=”true” marginless_columns=”false” style=”margin: 0px auto;padding: 0px;”][cs_column fade=”false” fade_animation=”in” fade_animation_offset=”45px” fade_duration=”750″ type=”1/1″ style=”padding: 0px;”][x_custom_headline level=”h2″ looks_like=”h3″ accent=”false”]Rebuilding a Horstman Disk Clutch[/x_custom_headline][cs_text]Possibly one of the most overlooked parts of the engine is the clutch. Sure, we all adjust them to slip at the optimum RPM, but proper clutch maintenance is extremely important and can make the difference between winning at the track and having a long headache all day. Clutch maintenance involves more than setting the stall speed and oil level; checking air gap, spring heights and disk and floater wear is really where you learn how healthy the clutch is.

Assuming that most readers of this article are fairly new to overhauling their clutches, we will go through the process step by step.

1) Remove the third bearing support and clutch cover. Make sure you have a coffee can and some rags handy to catch all the clutch oil. Before you wipe everything up, look for metal shavings and other foreign matter in the oil and inside the cover. This could help diagnose a major unseen problem if there is one.

2) Next, get out the clutch holder wrench; this is a special made tool that is a must-have to hold the clutch. Take a torch to heat up the starter nut a little bit and break the clutch nut loose. Take the starter nut completely off. Now you can pull off the clutch with the clutch puller wrench. Thread it into the hole until it stops (you don’t need to go nuts tightening it) then use a wrench to drive the bolt into the end of the crank to pop the clutch off. Be careful when it comes off, it can go flying! Also be aware that there is a woodruff key on the shaft and it can fall off. Now the clutch is off and we can get to work.

3) If this is just a preventive maintenance check, all that is needed is to check the air gap, check the disks and floaters for wear, and check spring heights. To check air gap take out a set of feeler gauges and set it to .040″. Stick the feeler gauge between the top disk and the spring plate, if it just slips into the gap and can slowly be moved around then the air gap is right. If the gauge fits way too easily or there is some play between the feeler gauge and clutch gap, it will need a thicker floater or new disks if the clutch is maxed out on the floater size. To figure out what size of floater is needed, take the feeler gauge and keep making it thicker until it just slip-fits between the gap on the clutch. However much of a gap there is over .040″ is the thickness needed for the floater. As an example, lets say the clutch gap is .050″, and there is a .050″ and a .040″ floater in the clutch. Just take out the .040″ floater and put in a .050″ floater to make it a .040″ gap. Simple eh’! Floaters are made in .005″ increments ranging from .040″ to .060″. To change the disks or floater break the large 1 1/8″ nut loose on the front of the clutch, be aware that it is a LH Thread. Then the spring plate can be easily removed exposing the disks and floaters.

4) Inspect all the clutch disks- look for chipped up or jagged edges on the friction disk material. If large chunks are missing or the cross hatches are gone, or almost gone, it is time for new disks. The friction disks themselves need to be replaced if there is any amount of warping; having them blue or purple is not uncommon and won’t hurt the function of the clutch. If a new friction disk is installed, be sure to put a few laps on the clutch at about 500 rpm lower then desired stall speed for break-in.

5) A very important part of a clutch working properly is having good springs and ensuring that they are all set at the same height. If one spring has less tension than the others, or more tension than the others, then the clutch isn’t working in unison. This can result in clutch chatter or inconsistent stall speeds and lost lap times. Using either a dial caliper or a spring height gauge is the best way to make sure all the springs are correctly set. Make sure all of them are the same height so equal tension is placed on every spring and the spring plate. Then the spring plate will engage the disks the same every time and it will mesh the disk evenly. But keep in mind that when the clutch gets adjusted at the track it is VERY important to move each spring out or in the same amount or all this work is for nothing.

**Due to the differences in springs, weights and other factors; it is not possible to give an exact starting point for every combination. Make sure you measure your spring height and re-adjust your springs to that height. Even if you set your springs to the original height you will probably have to adjust the clutch a little bit to make sure your stall speed is correct. Replace the six pivot pins on the weights when you change worn out friction disks as a rule. Check for grooves and typical wear.

6) Now that we have our springs set correctly and the disks and floaters are taken care of, we need to reassemble the clutch. First, put on a friction disk, then the thickest floater, then the next disk, and so on. Next put on the spring plate. Make sure the triangle pattern on the pressure plate fits into the triangle shape on the hub! Install the conical washer with raised center up, and then install the 1 1/8″ nut with the step down to center conical washer. It is a left-hand thread, remember, torque it down holding the clutch wrench in a vice to 46 foot-pounds. Now you can put the drum back on the shaft- don’t forget the spacer between the inside of the crank and the drum. The drum spacer must be installed with the beveled edge towards the engine. Then install the woodruff key in the slot, make sure it is fitted properly with the top of the key just below the pilot portion of the crank. The easiest way to reinstall the clutch is to line up the woodruff key, the clutch disks and drum so everything fits together on the topside of the crank. This way gravity won’t make the key fall out and you can more easily see what you are doing. Make sure the key didn’t fall out of the groove by tuning the clutch slowly on the shaft; the PTO shaft will turn if the key is correctly in place. Add a drop of red loctite to the threads on the starter nut and thread it onto the end of the PTO shaft. Now get out the handy clutch wrench and a 5/16″ allen wrench. Tighten the starter nut extremely tight at 350 in lbs, because you do not want it to fall off. It is expensive and could hurt somebody if it flew off going 70 mph. The last part to install is the clutch cover. Place it on the drum and line up the holes. Be careful not to get the cover in a bind when tightening it down. Go in a star pattern until the bolts are all snug. Then go around and make sure they are all tight. Try spinning it on the shaft a couple times to make sure it spins freely.

7) Now that we have our clutch back on we are almost home. Pull the drain plug and fill the clutch with clutch oil to the 7:30 angle on the drain plug. Remember, the clutch is not a fully sealed unit and it will get rid of excess oil, but do not overfill it. Next, reinstall the third bearing upright. Make sure the third bearing itself spins free and is not bound up on the end of the crank. The clutch is on, the third bearing is tight, and you’re done![/cs_text][/cs_column][/cs_row][/cs_section][/cs_content]