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Posted By: KEVINSKI Sm420 - Mon Jun 13 2022 12:52 AM
Sm420 in 54 gmc ,found at the bottom ,any idea what caused this tranny was working okay . 3rd picture is from another tranny showing the parts that were sheared off.

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Posted By: Dusty53 Re: Sm420 - Mon Jun 13 2022 03:04 PM
Kevinski , I'm curious as well. That looks like the energizing lugs off of the 2nd speed gear synchronizer cone. Hopefully one of our experts will chime in and I can learn something too.

GM Group 4.383 , part number 591566
Posted By: BC59 Re: Sm420 - Mon Jun 13 2022 07:46 PM
Originally Posted by Dusty53
Kevinski , I'm curious as well. That looks like the energizing lugs off of the 2nd speed gear synchronizer cone.

GM Group 4.383 , part number 591566


That's what it looks like.
Don't know about your area but around here SM420 units are easy to find.
Posted By: Dusty53 Re: Sm420 - Tue Jun 14 2022 12:33 AM
FYI, if your thinking of rebuilding / repairing there are two new / nos 2nd speed synchronizer cones on Ebay, not over-priced. Search for GM# 591566 on Ebay. Good luck and hoping to hear what could have caused this type of damage.
Posted By: KEVINSKI Re: Sm420 - Tue Jun 14 2022 02:02 AM
Not a problem had a extra one from a 67 pickup with the back up switch that I saved from the mid 80,s .I am going to use that one and was just curious as to what happened as the lugs are quite thick.I was using the truck up to a few years ago and it had a few issues but it still worked okay ,I think after pushing the clutch in it would make a noise then if I put it in neutral let the clutch out and pushed it back in and into gear the noise would stop (not a grinding just a rolling noise like something was still spinning).
Posted By: Dusty53 Re: Sm420 - Thu Jun 16 2022 03:29 AM
Agreed on the thick lugs, its a mystery to me for sure.
Posted By: 1Ton_tommy Re: Sm420 - Sat Jun 25 2022 04:58 AM
Save the good parts. Some parts are no longer available new, like main drive gears, I think. I learned this from buying parts from Novac 4x4 in Utah. Also some 2nd gear syncros for the earlier boxes, I think. We used to think these SM420s were indestructible but the last of them were made almost 60 years ago. Fortunately there are still lots of good used parts and complete boxes around.
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