Reconditioning the +4 Clutch Release

Recondition clutch release parts

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+4s and early +8s have a magnesium bellhousing joining the engine and Moss transmission.  To release the clutch an aluminum collar slides fore and aft inside this bellhousing. A carbon ring is tightly fitted to the forward end of the collar and, when the collar is moved forward, it bears on the clutch pressure plate. To move the collar, linkage from the pedal assembly pushes a stud that is screwed into it.

The stud protrudes through a slot in the bottom of the bellhousing. When the clutch pedal is depressed and the linkage pushes the collar against the rotating pressure plate the stud bears against the side of the slot and eventually wears a notch that can significantly increase the amount of foot pressure required. There are several common methods for repairing this fault. I have tried three.

Welding: The first and most obvious solution is to use weld to fill the worn area and then recut the slot. Not many people have experience welding magnesium and this application is particularly difficult because the porous metal is saturated with oil. I tried experienced professional welders and the result was OK but not great due to weld porosity. Also, welding does not do anything to improve the  longevity of the system.

Wear Plate: The wear is only on one side of the slot. Adding a metal plate to that side of the slot provides a bearing surface for stud to slide on. Any material harder than magnesium will do but I’ve had good results .020″ thick, 316 stainless steel. the slot is enlarged a bit more than .02″ with a file. A hack-saw blade holder is used to cut slits at each end of the filed recess. A wear plate is cut from stainless shim stock and sized to be a snug fit between the slits. The slits retain the plate fore and aft and prevent it from moving into the slot.  There is a lump on the bellhousing, next to the slot, that needs to be ground flush. A hole is drilled and tapped for an #8-32 machine screw to retain the wear plate. This is a relatively easy solution that will give good service for a very long time.

The following pictures show the process for installing the wear plate on 5674.

Sleeve: You can also sleeve the bellhousing and cut a new slot for the stud. This is the most expensive solution and is appropriate when the wear is not isolated to the slot where the stud travels but is also in the bellhousing tube causing the aluminum collar to be a sloppy fit. The sleeve is usually one used for engine cylinders and is fairly easy to source. Installation is another matter entirely. The bellhousing must be bored to the correct diameter and the sleeve pressed in. Boring is a specialized machine shop operation and because of the size of the bellhousing not all shops are capable of doing it.

The following pictures are of a bellhousing (on another car) that was repaired by Morgan Spares using a sleeve.

The carbon in the collar can also be worn or broken. To remove the old carbon first grip the back end of the collar with  clamping pliers (Vise Grips) and heat the aluminum at the carbon end with a propane torch.  Lightly tap the collar against something while heating it. After a short time the carbon will drop out. Note: carbon is brittle, it will break if it falls on something hard so have a towel or something soft under it. The best repair for a worn or broken carbon is a new replacement. At the time of this writing, new carbons were on back order from the factory and were not available through any of the usual suppliers. Depending on the extent of the damage or wear an acceptable alternative to a new carbon is to reface and shim the old one. Chuck the collar in a lathe and with a carbide insert in the tool holder skim the worn face. The most I have ever removed is .100″ (less than 1/8″). After skimming remove the carbon from the collar, make an aluminum spacer equal in thickness to the amount skimmed and reassemble.

Putting the carbon in the collar is the reverse of removing it. Put the collar on the bench with the carbon recess facing up. Place the carbon on top. Heat the collar with the torch and, when the collar has expanded sufficiently,  gently tap the carbon into the recess.