SHIFT SELECTOR SHAFT 0-RING
The Guzzi 4-speed transmission has a small, removable casting which carries the shift selector shaft. Although the casting itself has a sealing gasket where it attaches to the transmission rear cover, the cast passage for that shaft does not have any provision for sealing against oil loss. Oil within the transmission splashes and flings to all of the spinning internal parts. The oil is NOT under any pressure to leak. The shift selector shaft is mounted above the level of oil within the transmission. Still, eventually some oil may leak between the casting bore and the shift selector shaft.
I fabricated a jig to hold the shift selector casting with the bore held concentric to my lathe. I acquired a special o-ring groove cutter. I create a groove in the casting which is 0.300" deep from the outside limit of the casting. I cut the groove 0.085" into the cast housing. This provides approximately 15% compression to ovalize the o-ring cross section and create a seal between the shaft and housing.
My jig allows for installation of the cast housing without tampering with or removing the shift return spring eccentric adjuster bolt.
You will need to acquire Guzzi part # 90-70-61-40 which is the same o-ring used for the same purpose within the 5-speed rear cover casting. You should grease the o-ring, the cast housing, and the shift selector shaft for easier installation. Inspect your shift selector shaft for any burrs or nicks that might catch and damage the softer o-ring material.
I can usually turn this task around and post it back out within 24 hours of receipt. You can pay by cash, personal check, or PayPal to firstname.lastname@example.org. $15 includes the machining work and return postage to you.
PS: I am NOT a professional machinist. I've 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. 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 tooling and machinery costs. I have successfully performed several dozen of these modifications.