Did they ever make a different size input shaft for the SM420?
someone asked me about the size on one I am selling, said he needs the bigger one, that was used on semi trucks!
I think he is yanking my chain as I have never heard of anything other than the 1 1/8 10 spline input shaft but, I wanted to ask, as I, of course, don't know everything!
See the detailed description from Novak here: SM420
Nowhere in there does it describe a larger diameter "input" shaft. There is a note about a different spline/diameter "output" shaft and internals different in a rare number of these trannys.
I also did further checks on the web and found no other reference to an oversize SM420 input shaft.
I would say the answer would be a definitive "nah".
Dan
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
Thee is an oversized bell housing, flywheel, and clutch assembly that's used on some of the big block gas V8's in dump trucks, but I think they're set up for heavy duty Clark or Spicer 5 speeds. I don't recall ever seeing the big input shaft being used on a 420. 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!
yes, I have read Novak's site, and actually have 5 extra SM420s laying around, I have never seen anything on the internet referencing a larger input shaft either.
GMC lists 4 different input shafts for the SM 420 I don't know what the difference is. I own several 350 GMC with a SM420 as well as one ton and 3/4 ton pickups.
In looking through the Hollander, it appears that 1948-1959 SM 420 is the same, but it was different when it was used behind V6 GMC and V8 Chev in 1960-1962. It also looks like it applies to 1958-1959 Chev with a 348. These would be very rare because most trucks this size wold have had a 5 speed.
I trust the GMC master parts book and Hollander than I trust Novak's information.
Understood about there being a bunch of different part numbers for SM420 input shafts, as well as some of those numbers noted above that have been superseded by newer numbers for the same parts.
I could not find any reference to the "2387442" number, but did find an additional number "3787254" that seems to have been used from '62-'67.
Quite a rabbit hole to go down to chase all that info down. What I did find and saved some snap shots for posting later if anyone is interested, is there are 2 different input shaft part numbers listed for SM420s of various years/series that had the added description of "(1 1/4" shaft dia.)" tacked on the ends of the part descriptions. Primarily noted in the Chevy books from Oct '62 and May '79. But it doesn't indicate what this "1-1/4" measurement applies to.
Is it the actual OA spline shaft diameter, or possibly the diameter associated with mating up to the main gear shaft and bearings? I couldn't tell and further checks to try and see if their was a clutch disc with a corresponding 1-1/4" spline shaft opening (instead of the more common 1-1/8" shaft) left me scratching my head too.
I am going to let anyone else interested take a stab at this.
Dan
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
interesting, he did say these are in 1962 C60 trucks, and sent me a pic and asked if I could not see the difference! (Kind of hard via a pic, and not not having the two side by side)
Paintshop edit. I can't see much difference in the splines but the shaft tip is different. To get a better idea, clean shafts and measure several points with calipers.
They are not mine, if they were, most definitely, I would clean and actually measure them! In one of the posts above , it looks like they made an 1 1/4 10 spline, so, that may be the one that the shaft tip looks bigger on.
I replaced my 1950s SM420 with a 1967 model. The new one simply bolted right in so the input shaft diameter, number of splines and pilot bushing diameter were all the same. However, the input shaft is different internally. The old one used a spanner nut as a bearing retainer and the new one uses a snap ring. Also, the new one was machined for a proper oil seal. So, the part numbers would be different.
