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Jul 21st, 2025 | | | | Joined: Jul 2015 Posts: 62 'Bolter | | 'Bolter Joined: Jul 2015 Posts: 62 | After a day of Google and SB searches with no luck I'm stumped.I'm working with a 1953 SM420 Case #591665 open drive trans. It's a NOS but doesn't have the speedo assembly. I will assume the dealer would have made the call as to which gear combo would be best.
I have also scoured the oldcarmanualproject.com and found SM420 stuff but nothing with much reference to this.
I did find this for sale NOS but buying and trying is not my style. Any opinions are welcome.
Speedometer Drive Gear Unit Group No. 705
Part No. 608444 Stock No. G-85-35-96019 CHEVROLET
Composed of (1) 3660175 Drive gear / (1) 3660270 Gear fitting assembly
1951/54
Basketcase
| | | | | Joined: Jan 2001 Posts: 5,320 'Bolter | | 'Bolter Joined: Jan 2001 Posts: 5,320 | Speedo gear count is subject to diff gear ratio, and tire size.
You can use any gear count and correct any amount it is off with a speedo correction adapter. This connects to the outside of the transmission, and the cable then connects to it.
See the USA in your vintage Chevrolet! My Blog | | | | | Joined: Jul 2015 Posts: 62 'Bolter | | 'Bolter Joined: Jul 2015 Posts: 62 | Thanks Dave Yes I wasn't so much concerned with gear count just correct it with the adapter. I was concerned with ID of drive gear being different for different models Closed / Open drive trans - Yoke shaft size. Some of the 3 speed stuff looks identical. This one looks good. https://photos.google.com/photo/AF1QipMfqzWcGDeeFao4W2HL6-VeprmJg9aC6RE7tfFkIf the link works
1951/54
Basketcase
| | | | | Joined: Jan 2001 Posts: 5,320 'Bolter | | 'Bolter Joined: Jan 2001 Posts: 5,320 | Link does not work for me.
ID of drive gears are the same on "most" gears.
I have one yoke with a odd diameter where the gear fits, but as I say, most closed and open are the same.
See the USA in your vintage Chevrolet! My Blog | | | | | Joined: Jul 2015 Posts: 62 'Bolter | | 'Bolter Joined: Jul 2015 Posts: 62 |
1951/54
Basketcase
| | | | | Joined: Jan 2001 Posts: 5,320 'Bolter | | 'Bolter Joined: Jan 2001 Posts: 5,320 | That link shows what looks like a SM420 gear set. It has the right pitch on the driven gear.
I have many GM parts books, but I can't find that part number.
I think I will PM you with more info.
See the USA in your vintage Chevrolet! My Blog | | | | | Joined: Jul 2015 Posts: 62 'Bolter | | 'Bolter Joined: Jul 2015 Posts: 62 | Just an update on the search. Bought some more wall art off Ebay. The gear set sure is nice looking NOS but just as I expected not all SM420 yokes are created equal. The drive gear is about an 1/8" smaller than mine. But the driven gear is the right length. https://goo.gl/photos/dhAsKarrVdY23Srn8https://goo.gl/photos/JgAYpcuoybXV9fys8Back on the hunt. If I could just find a 14 tooth driven gear I could use the drive gear I have. Ken
1951/54
Basketcase
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