Does anyone know the purpose of the 1st & reverse friction ring in the SM420? It's a snap ring with teeth that goes inside the splined portion of 1st/reverse gear. GM part number was originally 591956 and it came with a neoprene strip 591955 Looks like all it does is apply friction to the main shaft to resist sliding the gear in and out of 1st, reverse, and 2nd.
I'm not sure how important it is, do you think i could just skip it?
Ok, I have at least a dozen or so Sm420 apart this last year from 48-67 models, never seen those parts inside a trans. However, it could be part of a TSB for transmission issues (post 1957)that have affected the second gear synchro retainer. I have seen two transmissions that have had their retainers dislodged after disassembly and inspection. My theory is the retainer can rotate when the synchro ring is pressed into the first gear friction material during a downshift, perhaps dislodging the retainer on extremely rare instances, extra wide retainer you show and the rubber friction material would stop ring rotation. I say use those parts, the rubber would probably just be laid into the retainer groove on the first/reverse gear then snap the retainer in after placing the ring in . Steve
Last edited by sstock; 11/03/20191:41 AM.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Did a little more digging, check this out: So you use those two parts if the groove on your first/reverse gear for the ring is 1/8", otherwise discard. As a side note the retainer is .062" if it is put into an .125 groove,one could see how it could possibly pop out, most R&R parts kits have the wider retainer to make it a nice snug fit, eliminating the need for the friction material and special retainer. GM must of gotten a few first/reverse gears with a different spec on the machining on the retainer groove. Who knows it is ancient history and all the guys fixing them are far and few.
Last edited by sstock; 11/03/20192:00 AM.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Thanks for the ideas!.... I might get myself a fatter snap ring. It's actually a different groove for the rubber & friction ring... I was too lazy to take a picture, but here's a red line on a random internet picture showing where it is. [img]https://i.imgur.com/J7jh2fp.png[/img]
The reason i ask is because my old friction ring is broken. I tried welding it, but it broke on the way back in... Mr. 54 3600 gave me some leads on where to buy a nos replacement.
It's definitely a wierd little part, seems like all it does is wear out the shift fork faster... maybe damp some vibration of the gear clearance on the shaft
Sorry, I missed that in your original post, all along I was thinking you were talking about the retainer for the synchro ring, the red arrow., my bad. Sorry for steering the ship down the wrong waterway.
Last edited by sstock; 11/03/20193:08 PM.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
No problem at all! I had no idea that the big snap ring came with issues. Any bits of history like the TSB you mentioned are super interesting.
That gear in your picture has two grooves instead of one like mine (and none in the random internet picture) Do you ever mess with the toothy friction rings in those grooves?
Nope, thinking they are a none wear item, they hold the gear on the main shaft nicely, I think only the later gear boxes early mid sixties had the extra groove and ring.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Maybe be a good way to rejuvenate my 2nd speed synchro energizer springs? I turned some little straws out of acetal plastic to clip onto the springs to make them push a little more on the synchro lugs
If you replace the spings and rivets, let me know how that goes, I threw away a second gear because of a broken spring.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
