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| | | | Joined: Oct 2016 Posts: 14 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2016 Posts: 14 | I have a 40 1 1/2 ton can I swap the internals from a later syncro 420 into my non syncro case. I want to keep the orig drivetrain with the torque tube setup.. Thanks in advance | | | | | Joined: Feb 2004 Posts: 29,875 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,875 | Nope, not even close- - - -the 420 is a much larger and more rugged transmission with bigger gears, bearings and case size.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Oct 2016 Posts: 14 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2016 Posts: 14 | I was under the impression that the early SM420 like mine are no syncro in any gear. The later SM420s were syncro'd in 2nd, 3rd and 4th. They're not the same basic trans, same case??? | | | | | Joined: Jul 2007 Posts: 444 Shop Shark | | Shop Shark Joined: Jul 2007 Posts: 444 | | | | | | Joined: Mar 2000 Posts: 1,184 Red dot, center of chest ... | | Red dot, center of chest ... Joined: Mar 2000 Posts: 1,184 | | | | | | Joined: Nov 2011 Posts: 1,611 'Bolter | | 'Bolter Joined: Nov 2011 Posts: 1,611 | I have a 40 1 1/2 ton can I swap the internals from a later syncro 420 into my non syncro case. I want to keep the orig drivetrain with the torque tube setup.. Thanks in advance If your 1940 transmission is original, none of the gears are synchronized. | | | | | Joined: Sep 2001 Posts: 29,270 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,270 | Here are 1940 transmission specs. Looks like no synchronous meshing gears in 1940 4-speeds. As shown in the Novak Transmission info, the SM420 was introduced in 1947 (probably for 1948 trucks). | | | | | Joined: Apr 2007 Posts: 1,038 'Bolter | | 'Bolter Joined: Apr 2007 Posts: 1,038 | Mcatt,
Your tranny is not a SM420. As others have said, there is nothing interchangeable with your tranny and the SM420. SM420s where used with torque tube driveline, but your torque tube would be too long and would require modification to fit a SM420. Alternatively, you could modify your driveline to open drive and use the SM420. Both choices are difficult and require professional skills.
Let me ask why do you want to change out the tranny? If it needs repair that is the easiest solution . If you don't like to double clutch, I would encourage you to practice more until you are good at it. Once you get this skill, you won't want to go back as it is part of the character of these older trucks.
Kent | | | | | Joined: Oct 2016 Posts: 14 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2016 Posts: 14 | Oh. I thought it was a sm420. And yep it's original what ever it is. The same bonehead that left the engine open to fill up with water and lock up let water get in the trans. The forward gears are rusted up as well asthe main shaft and won't budge. Thought upgrading would be easier and better than repairing. Found a sm420 from a 53 gmc but discovered it won't bolt to my bell housing. Have the entire gmc 1 1/2 ton chassis with open driveline and 2 speed axle. | | | | | Joined: Oct 2016 Posts: 14 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2016 Posts: 14 | Ok. Next dumb question. What tranny do I have. It's original. Has the little lockout lever and rod. NO syncros at all. Yeah I'de fix it if I can find the parts. The clutch clutch thing is cool with me. Not a racecar don't think it will ever really haul much. | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Entire GMC 1 1/2 ton cassis would have lots of useable parts-axle, driveline, trans. Bellhousing-GMC engines are slightly longer than Chevy's, easy to move radiator FWD an inch or so. GMC displacement is start of SN, I.e. 248xxxx, stamped on right side, behind distributor. Might be better than yours?
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Feb 2004 Posts: 29,875 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,875 | Check your private messages.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
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