The Stovebolt Site Forums The Shop Area Engine Shop SM 420 Speedo Drives?? How many axle ratios ??

My 59 SM420 was originally hooked to a NAPCO 4wd unit on a 3/4 ton. I am using it with my 59 1/2 ton with stock 235. It has a stock 1958 4:11 rear axle.

The 59 SM420 has been totally rebuilt and is really sweet for country rides and even the once and a while 50-55 mph highway ride but I have tried 3 different speedo drive gears in the side of the gearbox; a 11,12 and 13 tooth and none give me right speeds and NONE go over 35mph.

I noticed the teeth are different shape on the 11 and 12 than the 13. The finer teeth on the 13 are closer to correct than the squared teeth on the others. Were the speedo drive gears different for different years? How many ratios could they have had? The speedo is new and working fine......Tried 2 different cables also and they do not change the accuracy.... Is there a drive with more than 13 teeth? Thanks

If you're trying to make the speedo read higher you need a smaller speedo driven gear. The fewer teeth it has, the faster it spins around.

Seems like the fewer teeth drives have too much of a squared off larger tooth surface. The drive inside the gearbox feels like it is more of a fine worm type drive. Perhaps the drive gears are specific to batch years of specific SM420's or trucks. I could not even get the 11 tooth to register a movement in the speedo and when i pulled it back out it appears the teeth are getting chewed on a bit due to a non mesh.. Just seems like it is not a clean drive fit.

Unlike the plastic gears in the later GM transmissions, where you could get away with just changing the driven gear, the drive and driven gears in an SM420 have to be changed as a set. There would have been a set for a 4:10 one for the 4:57, and a set for the 5:14. If you change only the driven gear you will chew up the drive gear and good luck finding another.
Denny Graham
Sandwich, IL

Hmmmmm, it almost sounds like you may have a gear for a different tranny.

Check these guys out. Call the tel. no. on the web site and talk to one of the guys there, they know their stuff. Good folks to do business with too.

Thanks guys......Denny that makes sense and I always thought that if you figured out which of the 3 ratios the gearbox had, you could match it by fitting the correct speedo drive. I thought they are 11,12 and 13 tooth to match the ratios.......but now I am wondering if that is too simple. Looks like the speedometer drive gears are perhaps a bit different depending on what vintage gearbox they came out of.

Those are not the only axle ratios involved either. This transmission was used '48 to '67 in 1/2-ton to 2-ton and possibly larger trucks. The probable rear axle ratio list would be pretty long...

3.90
4.11
4.57
5.14
5.43
6.17
7.20

And I'm sure this is not an exhaustive list.

If your speedo works and the drive gear and driven gear work in the trans, the easiest way is to correct it is with an adapter. Speedo shops can make them. Find your per cent of error either by using a gps or using the odometer and comparing it to mile markers on the interstate. The 54 GMC parts book lists 75 different adapters. These guys have speedo adapters. http://www.scottspeedometer.com/

Have you checked the parts manual, http://chevy.oldcarmanualproject.com/1929_57chevyparts/index.htm
group 4.337

There is some information if you look a few pages back. Mostly part numbers, but it does list the driven gear tooth count for each ring and pinion gears set.

Joe

Thanks guys. Joe, I could not determine what group 4.337 was telling me when I looked. I do not think those tooth count published numbers relate to the driven gear that the cable goes into unless I am missing something.......

I would like to be able to find a reference for the tooth count on the driven gear(cable affixes to) to the various ratios. But the real issue for me since my SM420 is from a 59 NAPCO 3/4 ton frame, is how to determine what the INSIDE of the gearbox has in it for the other half of the driven/drive combo....

An adapter would be fine, but as Denny mentioned, the driven gear should match in a mechanical mesh so as not to be tearing either part up in the crashbox. I never thought to check that drive gear before installing the gearbox back in the truck. Live and learn....I will get it figured out even if it is trial and error.

The parts manual really wasn't much help, but thats where I always go first.

Can you see the driven gear? maybe mark one tooth with white paint and count as you rotate?

The newer transmissions use a round adapter for cable gear, you can rotate it around so a range of gears will fit one adapter. If yours is a fixed piece, then you and Denny are correct, and only one gear fits each driven gear.

Good luck in your hunt.

Joe