The Stovebolt Site Forums The Shop Area Driveline Rebuilt sm420 3rd gear synchros

I had used hypoid gear oil in mine for years before I realized it was wrong. It never ground into 3rd but It took too long for the syncro to work and was slow shifting. Being an old 2-stick trucker I would take it out of gear without the clutch, wait for the RPMs to fall then drop it into gear with a dab at the clutch, just like I did on the non-synchro big trucks. This went on for so long that I forgot about it completely until it was time to change the gear oil. Then I read about the evils of modern gear oils and mended my ways. It now has SAE90 mineral oil in it and it shifts much better but I still use the old technique as it saves the synchros and one of the brass ones for the older SM420 is no longer available. I forget which one. GM changed the design in the mid fifties and the parts aren't interchangeable IIRC,