Racing clutches from Quarter Master® are definitely a marvel of engineering. They are incredibly small and lightweight for the ultimate in low rotating mass, yet they can handle a ton of horsepower.
But no clutch-no matter how high quality-is going to last very long if you don’t set the throwout bearing clearance properly. A throwout bearing that is installed improperly will never let the clutch fully engage or fully disengage. Either way it’s going to cause the clutch to wear too fast.
The throwout bearing must ride at a specific depth on the input shaft
Fortunately, setting your throwout bearing properly isn’t hard. And you can do it with your engine either in your car or out of it as long as you can install the bellhousing.
If you haven’t done it already, the first step is to install the clutch and bellhousing as you normally would on the engine. If you are racing and use a midplate mount between the block and bellhousing, install that as well. Everything must be installed exactly as it will be on final assembly.
The throwout bearing must ride at a specific depth on the transmission’s input shaft. Quarter Master® sells a support rail for this purpose, but you don’t have to use it if you don’t want to. They actually provide plenty of washers with a throwout bearing so that you can accurately set the depth yourself. If you don’t use a support rail, the inside face of the throwout bearing will ride on the four bolts securing the transmission’s input bearing retainer. Remove these bolts and install the necessary number of washers so that the face of the bolt is 0.800 of an inch away from the mounting face of the transmission. If you have to use many washers, you may need to swap out for longer bolts to make sure you still have plenty of thread engagement. You also may not be able to get exactly 0.800 of an inch, but you should be able to get within a few thousandths of that target. The easiest and most accurate way to measure this distance is with a set of dial, or digital, calipers.
Accurate measurements are extremely important
With the throwout bearing properly set on your transmission, you can begin your measurements. Lay a straight edge across the center of the bellhousing opening and measure to the edge of one of the clutch fingers. You can get by with a tape measure in a pinch, but again, a set of calipers will help you improve your accuracy. And don’t be afraid to repeat the measurement several times until you get the same number again and again. Accuracy is important.
For our example, the distance comes out to 4.267 of an inch. This distance includes the width of the straight edge, but as long as you use the same straight edge for both the bellhousing and the transmission, that’s no problem.
Now make sure that the release bearing is fully compressed and slide it onto the transmission’s input shaft. Use that same straight edge you used before and lay it perpendicularly across the end of the release bearing to help you measure the distance to the transmission’s face. And just in case, measure both sides to ensure that they are equal. That way you know that the release bearing is properly aligned.
Continuing our example, the distance from the end of the release bearing to the end of the transmission-including the width of the straight edge-is 4.152″. So to find the release bearing clearance, subtract that from your bellhousing-to-clutch-finger measurement. The difference in this case is 0.115 of an inch.
Now admittedly, we got pretty lucky with our example because the 0.115 of an inch of clearance is right in the middle between the 0.100 to 0.125 of an inch of clearance that Quarter Master® recommends. But if you find you have too much clearance, the fix is easy.
All you have to do is push out the pistons and install some of the shims that Quarter Master® provides to take up the unnecessary clearance. Quarter Master® provides shims in a variety of thicknesses so you should be able to mix and match until you get exactly the thickness you need. Then all you need to do is reassemble the throwout bearing, reinstall the bearing on your transmission and repeat the measurement process until you get between 0.100 and 0.125 of an inch of clearance. If, on the other hand, you have too little clearance, you can remove a washer or two from each of the bolts securing the transmission’s input bearing retainer.
Once your thowout bearing clearance is correct, you are ready to continue with the installation of your Quarter Master® clutch as well as the rest of your driveline.
How To Set Correct Throwout Bearing Clearance 
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