The Stovebolt Site Forums The Shop Area Driveline Bent Speedo Gear Mystery

I could use a little insight here guys. I have a '54 SM420 tranny in my '52 3100. My speedo stopped working a couple of days ago. I unhooked the cable at the tranny and spun the inner cable and the speedo needle jumps as it should. I took the gear housing out of the side of the tranny. The gear only spins about halfway around and binds. It appears the shaft is bent. My first question is- What would cause the shaft to bend like that? I don't want to replace the gear only to have the same thing happen again. My next question is- Where can I find the gears for the SM420? Any help is greatly appreciated.

I know it is hard to find the half ton gear for the speedometer drive.... The 3/4 ton is easier, but makes the speedometer read wrong.....

There is a bit of a recess that the driven gear runs in. It is very common for a broken bolt that holds the U' joint together to get jammed up with the gear in this area.
I would start by inserting a magnet in the hole the driven gear came out of. You may also want to pull the bell back to inspect and clean thoroughly.

I did the rearend gear conversion from 3.90 to 3.55 which already has the speedo reading about 10 MPH low. Any idea how which way the 3/4 ton affects the reading?

I'll give the magnet a try tomorrow. Wouldn't a broken bolt in the u-joint cause some vibration in the drive shaft?

Thanks 'lightning and Dave for the suggestions.

Not at first, but eventually it will. It would be nice to catch it before major damage is done.

Absolutely! I'll try the magnet later today. Thanks Dave.

Well, after being out of town for four days I finally got the chance to try the magnet as Dave suggested. That's the good news. For the bad news, check the link for the pic. I pulled out all sorts of scraps. I didn't get a bolt but did get a split washer along with enough other metal to know there's major damage. That being the case, I know I'll need drive and driven gears for sure. I should find out the extent of the damage later today. I'll get back with more info. Thanks again for the suggestions.

http://i42.photobucket.com/albums/e323/abbynormal44/SpeedoWreck_zps6c1707f1.jpg

That is what I was afraid of all to common.

Before I tear everything apart, how difficult is it to replace the drive gear (I've never done it before)? Is it just a matter of droppiung the drive shaft by disconnecting the u-joint and pulling the yoke or is it more involved than that? Also, I swapped the ring and pinion gears from the original 3.90 to 3.55. Since then, the speedo registered about 10 MPH lower than the actual speed. Can I make up for that in the speedo gear change?

The driven gear is pressed on the front yoke.
These 4 speed speedo gears are getting hard to get, so being you have changed out the diff gears I would buy any speedo gear set you can find and then use a speedo correction adapter to compensate.

I just found a NOS gear set on Ebay for $80 (including shipping). It's the same part number referenced in the parts & accessories manual- 3709359. Since they're so hard to find, I'm thinking I'd better snatch them up.

Sorry, should have stated the set is the same set mentioned in the manual for the '54 4-speeds.

1954 1/2 ton had a 3.9 diff(4.11 before 54) so the gear set is a little different tooth count than the earlier ones, but you already said you have a 54 trans. The NOS set should make your speedo read the same as before. It any case a speedo correction adapter will be needed if you want the speedo exact.

Dave, I have a '54 SM420 and a '54 RE in my '52 3100. I know 3.90 is what I had before installing the 3.55 set. I'm not too worried about the speed being 10 MPH off right now knowing I can add an adapter. I figured I was lucky to find a NOS speedo gear set.

OK. Now i'm pretty confused.I removed the U-Joint yoke expecting the drive gear to be in pieces accounting for the pieces I found (see link below for pics). It isn't. It's dinged up some put intact (see pics). There were no missing bolts from the U-Joint trunion bearing retaining clips. The bolt holding the yoke onto the main shaft was in place and tight. The main shaft teeth are in good shape (see pics). So my question is, where the heck did the pieces come from? As you can see in the pic with the broken pieces, there is also a split (lock) washer there. I checked the drained oil and also removed the side plate from the tranny and found no more pieces. I can buy another gear set and install them, but that doesn't solve the mystery of where the broken parts came from. HELP!!!!!

http://s42.photobucket.com/user/abb...1&_suid=1409613008337046232981696039

OK. It just dawned on me that I had the u-joint and yoke replaced several montha ago when I had a new clutch and pressure plate installed. The old u-joint was completely trashed and had worn out the yoke. Now I'm wondering if my mechanic didn't leave parts of the old trunion bearings in there somewhere. That could also account for the lock washer. Unless anyone can see something in the pics that I'm overlooking, I think that may be the root cause.

Looking closely at the larger pieces I found and comparing them to a trunion bearing, they are an exact match to the size and the radius. I think my suspicions are correct.

It does look like some parts were left over inside during the last job.
It is sooooo common for broken pieces to be left behind in the recess around the driven gear.

I feel more at ease now with simply replacing the drive and driven gears. Thanks for all of the feedback Dave. I'll let you know how it goes.

Today, I received the gear set I ordered and immediately kicked myself in the butt. I took the PO of my '52 3100 at his word when he told me the tranny was a '54. After all, I new the rear end was a '54 after having done the 3.55 conversion. Here's the butt kicking part. The tranny is a '51. AFTER looking at the gear set I decided to check the code stamped on the tranny case. B191 (February '51). The gear set was the same from 48-53 but changed in '54. Aaargh! Time to resume the search. There is a junkyard near here that has an old 3100 that still had the SM420 in it the last time I was there. I guess I'll be going there this weekend.

Did you change to the 1954 3.90 Ring & Pinion or to a 3.55 Ring & Pinion?

The speedometer gear set changed in 1954 because the rear-end changed from 4.11 (47-53) to 3.90 (1954/55st).

If you have a replacement 3.55 R&P, your speedometer will still be off with a 1954 3100 speedometer gear set.

Yes, I changed to a '54 3.55 R&P gear. The speedo was reading about 10 MPH lower than the actual speed (I don't have the original tire size either). I know I can compensate for variations like that with an adapter after getting a Speedo gear set.

The 3.55 R&P gears conversion is not a change to a 1954 rear end. The 3.55 rear end was not standard in an 1954 1/2 ton truck.

My 1954 3100 trucks have been converted from the original 3.90 rear end to the 3.55 rear end.

I also have tires on original 16" wheels that are about 10% larger diameter than the original tires.

To compensate for the speedometer error from the 3.55 and the tires, I use an adapter from The Speedometer Shop. My speedometer/odometer now has less than a 1% error.

http://speedometershop.com/rep-pag.htm

I think that Jim Carter also sells an adapter at a good price.

I should have said I have a '54 rearend that I changed from a 3.90 to the 3.55 from Patricks. I've searched al over the web and junkyards/swap meets within a 50 mile radius of my address- nothing. I tried to rehab the original gears today. I cleaned up the scars in the teeth with a Dremel and straightened the shaft on the driven gear to the point that it spins freely. Got everything put back together tonight. Will try it out tomorrow.