THROWOUT BORE RESTORATION
The clutch throwout system consists of a long rod, Inner Body, radial bearing sandwich, Outer Body, and O-Ring. The latter of these parts slide in a bore of the cast aluminum rear cover for the transmission. The Outer Body includes an o-ring to seal the bore and keep gear oil inside. With age and extensive use, the sliding steel Outer Body can wear or erode the inner surface of the bore. The parts are working in soft aluminum. Lips and ridges develop in the bore and these tear at the surface of the o-ring causing leakage. Replacing the o-ring provides only temporary relief until the roughened bore again damages the o-ring. The bore damage becomes especially severe if the bearing disintegrates and the steel shards serve to quickly eat the softer aluminum bore.
The very early 4-speed transmissions used a 20mm bore and compatible internal sliding parts. The latter 4-speeds and all 5-speeds use a 22mm bore and compatible internal sliding parts.
To cure this leakage and restore the rear cover, I have obtained stainless steel tubing stock with 25mm OD in both 20mm ID and 22mm ID. I have created a jig or fixture to mount the rear cover into my lathe with the bore on center so that I can precision over-bore the throwout opening and install a stainless steel insert sleeve to restore original dimensions. I can restore the rear cover on either the 20mm version or the 22mm version. If requested, I can upgrade the 20mm version to 22mm to allow for better supply of throwout bearings. If so, you would need to source the necessary larger Inner and Outer bodies, bearing, and O-Ring,
External view of rear cover with 20mm ID insert. Note that the OD bore removes a small amount of material from the edges of the throwout mounting perch.
A particularly severe case of bore damage. That groove should not be there.
I provide this service to the Guzzi community without any profit. You cover the postage both ways. The seamless, stainless steel tubing comes from the UK and I have to buy it in bulk length to then cut and chamfer each insert. There were many hours invested in designing and building the holding fixture. In addition to the bi-directional postage, I would like to have $25 to cover my materials and tooling costs. Add the return postage cost to that. Even after all the setup and experimentation work, there is probably two hours of shop time to chamfer, hone, and install the insert. I have to work slowly as this is a large mass turning in my lathe and its center of mass is offset from the bore center. You will need to remove the three ball bearings which reside in the rear cover. Those add too much additional mass to the offset rotation in my lathe. If removed carefully, they can be reinstalled if they are in good condition.
PS: I am NOT a professional machinist. I have never had any formal machining or metallurgy training. I do not own the most accurate or sophisticated machine tools. I make no warranties about the quality of workmanship, tooling, or materials. I might even damage your rare parts. If you want 'professional' quality and assurance then go pay a professional fee for one-off machine work. I'm a hobby machinist willing to provide some unique services to the Guzzi community. I make no profit from these services. I only charge simple fees to help amortize the necessary material, tooling, and machinery costs. I have successfully performed several dozen of these modifications.