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| | | | Joined: Mar 2007 Posts: 4,215 Moderator | | Moderator Joined: Mar 2007 Posts: 4,215 | I'm not sure if this practice is still done. My synchros aren't working so well,(SM420) thought maybe double clutching might help. | | | | | Joined: Jul 2013 Posts: 863 Shop Shark | | Shop Shark Joined: Jul 2013 Posts: 863 | Did you get that 90 weight gear lube? Did it help at all? If you replaced the GL5 with the 90 weight it make take a few trips to clean out the old lube from the synchos. As far as double clutching, I have done this and sometimes it helps. If you need new synchos try Drivetrain.com. They have all the gaskets and parts for the SM420.
"Pay attention to the details! It ALWAYS pays off."
1949 Chevrolet 3100 Series 1/2 ton Pickup
1964 Chevrolet C10 (Ol' Yella) (SOLD)
1958 Chevrolet Biscayne 2 door (SOLD)
1970 VW Beetle
| | | | | Joined: Mar 2005 Posts: 1,513 'Bolter | | 'Bolter Joined: Mar 2005 Posts: 1,513 | Where I live double clutching is a vital skill so as not to get rear ended as NOBODY comes to a complete stop or even has patience for anybody that does. 
1952 1300 Canadian 1/2 ton restomod You Tube | | | | | Joined: Mar 2008 Posts: 1,775 'Bolter | | 'Bolter Joined: Mar 2008 Posts: 1,775 | Some of the newer transmissions specify Synchomax or Synchromesh gear oil. It's just a GL4 gear oil. Supposedly the friction modifiers is GL5 oil are too agressive for the synchronizers.
Just something to think about. | | | | | Joined: May 2001 Posts: 1,904 'Bolter | | 'Bolter Joined: May 2001 Posts: 1,904 | Originally, I needed to double clutch into high gear of my SM420. It was kind of fun but I started to think about left-knee-longevity after having a little surgery on it.
I found that just pausing for an instant in neutral with the clutch pedal still depressed before completing the shift into high worked just as well.
1948 3/4-Ton 5-Window Flatbed Chevrolet 34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
| | | | | Joined: Mar 2005 Posts: 1,513 'Bolter | | 'Bolter Joined: Mar 2005 Posts: 1,513 | Double clutching to engage high gear must be only for the purpose of getting by with a worn synchronizer drum??? My stock 3 speed column shift transmission has a non syncro first gear normally requiring a complete stop and clutch pedal depressed until the gears stop moving before shifting. To drop into low while rolling I synchronize road speed to engine speed (estimating what the RPM would be if in gear) release the clutch in neutral, depress again and if they match it drops right into gear. Maybe I misunderstand the term as I thought this procedure was all about getting into low on a roll. 
1952 1300 Canadian 1/2 ton restomod You Tube | | | | | Joined: Mar 2002 Posts: 9,115 'Bolter | | 'Bolter Joined: Mar 2002 Posts: 9,115 | You should be able to do the downshifts with double clutching but it sounds like you really need new synchros. | | |
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