The Stovebolt Site Forums The Shop Area Driveline Speedometer gear stripped

The speedo gear in my 54 3100 was stripped and so far haven’t been able to find one short of buying another trans. The transmission gear appears to be there but condition was hard to see with a flashlight. It’s a Muncie 4 speed on early model 54.

Any ideas or help would be appreciated.

If you google "muncie 4 speed speedometer gear", I see a lot of possible sources. I'm sure you will need to add more description info, in order to find the perfect match.

Maybe this?
https://www.thegearbox.org/catalog/item/3864782/3697857.htm

Do you have a pic of it you can post? Going through my garage I came across a number of these type of parts. I may have what you are looking for.

Doug, Just a note that may help clear up the transmission and speedo gear you are talking about. From your earlier post on the speedo subject, your pictures show your transmission is an SM420. Calling it a "Muncie" makes folks think it is a later 4 speed like the info mentioned in cmayna's reply above. That won't get you the right speedo gear unfortunately.

Can you tell us what the tooth count is on the gear you have? I recently came across a source that had a few possibly correct speedo gears that may have solved your problem, but unfortunately I found this morning that they sold out. Give the tooth count on the one you have anyhow and folks can try and point you in the right direction.

Yes, per your earlier post Doug, it looks like a SM420. Doug, how many teeth is on the speedo gear that you are trying to replace?

Doug
I still have the one that the vendor Dan is talking about sent me the first time with the broken off piece of a cable stuck in it. I was able to get the piece out and get the gear freed up.
You are welcome to it. Vendor has not asked for it back. Only that I give him a good review on Ebay.
Check our my thread here in the Driveline. Your drive gear should be a 4 tooth and the driven gear a 12 tooth. The one I took out of my truck was a 6 on the drive, which I have since figured out is from a three speed tranny. The shop manual lists the gears and part numbers as I referenced in my thread.
Let me know. Dan was a great help.

I’ll pull it this afternoon or tomorrow. I have a situation at my shop that I need to take care of. I’ll take a picture of it and give gear count.

Thanks for all the responses.

Doug

Pulled it this evening. It shows 13 teeth. Only two are broken and with amount of gear bits I washed or retrieved with my magnet I’m thinking it is the inside gear that is torn up. I have no idea what that would involve.

Doug , Are you able to see into the speedo drive mount hole to count the teeth in the transmission/further check condition? In doing a little research this morning, the only 1954 Chevy trucks that I could find in the Master Parts Book that shows a 4 tooth drive gear matched to a 13 tooth driven gear (what's in your pic) were 3/4 Tons and 1 Tons with either a 4.57 or 5.14 rear axle ratio. Very possible someone stuck an incorrect 13 tooth gear in the transmission, or it could have been from a 3/4 ton originally???

As mentioned above, your gear set should be a 4 tooth drive gear matched to a 12 tooth driven gear same as "Crummy".

Getting to the drive gear requires disconnecting your torque tube and u-joint to get to the transmission output shaft u-joint yoke, unbolting that and pulling from the output shaft. Then pressing off the drive gear from the yoke before reversing the process to put it all back together. That is assuming you have the correct replacement gear set in hand.

If your drive gear on the yoke is still good, you should be able to make use of the driven gear mentioned by "Crummy" above to restore your speedometer function.

I have 13 teeth on the drive gear.. I will see if I can look into the hole and check the shaft gear.

The one you have out is the driven gear. A 13 tooth driven gear would be paired with either a 4 or 5 tooth drive gear on the transmission shaft. You probably should match what your drive gear has unless you've changed axle ratio or tire size. This page and the next in the parts manual may tell you what combination of drive and driven gear you have.

Doug,
It's not a bad job to get into the torque tube and pull the yoke. I did mine in about a hour. That was on the floor with jack stands. You never know what you might find in these old trucks when it comes to speedometer gear set.

Let me see if I have this right. I look up my tire size in the chart and the gears to use are to the left.

Thanks

Doug, Here's the procedure for selecting the correct speedo gear part number and tooth count. Start with the chart that says which transmission you have, 3-speed or 4 speed. Then go to your year and model, Tire size, axle gear ratio (ring and pinion tooth count), then the speedo gear part number, then to the chart of part numbers to find the tooth count of the speedo gears. Unfortunately, in the example I picked, they have an error, as the same speedo gears can't be used for different axle ratios. But that gives you the procedure.

The 1927-1957 Parts book LINK has more believable data. The same procedure will apply. Go to the pages after the one I linked to find the listing by 3 or 4 speed transmission.

[on edit] I didn't go back and see if you had a 3 speed or 4 speed.
Anyway, I tried to see which one the 1929-1957 parts book would say you needed, but that was a bit strange as well. It comes up with a 7-20 speedo gear set. So if that's what it had and someone put a 13 tooth driven gear in it, I'm sure it wouldn't last long as it wouldn't mesh well with a 20 tooth drive gear.

You could just go by your axle ratio, assuming you could find it in the charts and then go to the tooth count on the speedo gears. The tire size would make a difference, but unless you have radically different tire rolling diameter, it won't be a large change.

Here are a few pictures of the ones I have, one was in a older 420 trans that had sat outside that caused it to rust the other is from a 1969 chev that had a 350 in it from factory and the one that has the most teeth 20 I believe is in my 54 I took pictures beside the yoke so you can see the inside gear except for the 54 it is in the truck still.If your truck is origional it should not matter too much as long as you have a match set the gear ratios for the years the truck had the sm 420 did not vary by much for the 1/2 tons . I wonder if anyone has tried to grind the rivet holding the gear on the bullet and have someone make up a plastic gear to replace it .

Thanks Kevinski. Good info.
I looked in the hole as best I could and it looks like the gear on the axle is toast.

I also looked at the tires. I have P245/75R16 on it. I tried to find a match in the parts catalog but couldn’t find any. Perhaps someone else can translate it

One of those gears you have look pretty close to mine.

Doug

A 245/75r16 tire is very well matched
with 4-12 Speedo gear and 32-7 differential

245/75 r16 tire is nominally specified at 662 revolutions per mile according to generic tire size specifications
vs the 656.25 revolutions of the ideal diameter tire for the above Speedo and diff ratios

If you have these ratios Your speedometer will be reading very slightly high compared to actual speed.

The math:
1000 revolutions per mile is the speedo input
1000 x 12/4 speedo gears is a drive shaft speed of 3000 rpm

3000rpm into a 32/7 ratio diff is 3000/32x7 rotations per minute or rotations per mile at 60 mph of 656.25
-s

Any leads on where I could look for a 4-12 set

Thanks for the info.

Doug

Find a stock 3/4 ton with a sm420 and it should have the speedo drive gear you need
Just one wrench to remove it, might need pliers to undo the Speedo cable
And the inner worm style 4 tooth gear is the same for 3/4 and 1 tons with the same transmission
It’s surprisingly easy to pull out because it’s an open drive
Undo the universal joint cross and then the back cover has a few bolts and it’s all off
I recently replaced the rear transmissions seal on one of my 1 ton trucks and it was way easier than I thought it was gonna be. I think I did have to loosen the intermediate bearing holder to wiggle the drive shaft out of the way tho.

Also
Search eBay for the part number
Sometime people post nos parts in their original packaging by the part numbers with no idea of the application.
-s

But wait… what is your diff ratio again?
All that above stuff was for a stock 3/4 ton rear diff 32-7 teeth.
You have a 3100 1/2 ton?
Don’t go hunting those ratio gears yet
I was just commenting on the tire size and what it would match up well to

What exactly is in your truck for a rear diff?
-s

No idea what’s in the rear end. I’m guessing stock for a 1/2 ton truck.

Doug

Wait a minute. I had the old ring gear I replaced and it has 39 teeth.
Does that help?

I’m not familiar with the 1/2 tons but I can take a peak at the documentation later and see what ratios would work with common tire sizes for reasonable Speedo indications.
But it would be a good idea to check definitively what your rear and ratio is and report back
We will all sleep better that way
-s

Doug - Yes, the actual ring and pinion tooth count does help! If your 1954 3100 has a 39 tooth ring gear, the corresponding pinion gear should be a 10 tooth gear. Equals a 3.90 rear axle ratio. That combined with your known SM420 4 speed and 16” wheels should give us the clues needed to track down the correct speedo gear set.

More on this in the morning when brain cells and eyes are fresher.

Doug
Look up item number 26700753110 on Ebay. Has the parts you need. Plus a few more. Price is right. I paid 60$ for just the gears. Never hurts to have an extra yoke.

Good Luck
Jeff

Didn’t find a match Jeff

Could you send the description. That looks like an item number and I can’t figure out how to search for that.

Thanks
Doug

Doug,

I suspect what klhansen mentioned earlier is possibly what happened and why the speedo gears went bad.
An attempt to change the speedometer reading was done by only changing one of the speedometer gears at the transmission without realizing they must be changed as a matched set.
What you pulled out of the back of the transmission does not belong in a 3100.
The wrong set of combined gears lead to failure.


From several online parts manuals for a 1954 3100 truck:
speedo gear set 3709359 is a 7-20 teeth
rear axle that you confirmed is 3.90, 39-10 teeth
The ideal revolutions per mile tire is 732.6

The default factory tire size is 6.00-16 6ply which is correct for the above ratios.

Perhaps the easiest way to correct your situation is to find a good speedo gear set 3709359 7-20 ratio and change both to ensure no mismatch.
Then install 235/65 R16 to get the correct speedometer reading, this tire size will get you very close to a 6.00-16 rolling diameter.
If then the speedometer is still off, the speedometer accuracy should be bench tested.


If you want to have bigger wheels, then do NOT get the correct speedometer gears yet.
Find the tire size you want to go with and we can calculate which other factory speedometer gear set to hunt down.

-s

I’m going to stay with the tires I have since they are new and money is tight

Doug

okay to keep your larger tires and have a reasonable speedometer reading,
you need about a 13-5 transmission ratio gear set
the bad news is that there isn't one listed that is compatible with a sm420
the few gear sets of that ratio are for the automatic, the 3 speed and very old 4 speeds.

perhaps you need to find an adapter to bump up the speedometer reading by about 10% because your wheels get bigger and your truck is moving faster than normally indicated.

if you stick with the factory ratio in the transmission (hunt down the 3709359 part, 7-20 teeth your truck is supposed to have) then find 1565812 speedometer adapter
do not wast time on the other similar part number adapters, they are very different.


1565814 is used when you make the wheels smaller, and decreases speedometer indication by about 3% based on what i found, this is the wrong direction for you.
1565813 is used when you make the wheels larger and it increases speedometer indication by 1:1.133 however that's a bit too much, this one is listed in parts manuals when the wheels change by even 15%
1565812 however is used for about just your use case
- when going from 700-17 6/8ply to 750-17 8 ply ~9% change
- when going from 650-20 6/8ply to 750-20 8/10ply ~9% change

best of luck
-s

Thanks for the info. I’m off to do a google search now.
Doug

All - After getting caught up with the latest posts and information provided, I wanted to offer some updates on the gear set identified by 2ManyTrucks. Not trying to be a wise guy or anything, just identifying a minor error that needs correcting so Doug has the correct info to search for what he needs.

The GM# 3709359 7-20 tooth gear set is listed for the Passenger Car and Truck 3 Speed transmissions on page 398 of the 1929-57 Chevy Master Parts and Accessories Catalog (1st snapshot pic below) and will not work for Doug in his SM420 4 speed unfortunately.

GM# 3707598, also a 7-20 tooth speedometer gear set, is listed on page 400 of the same Catalog (2nd snapshot below) and will work with Doug's SM420. The good part of this info is that same gear set was used in the 1954-55.1 (1st Series) 1/2 ton's with 4 speeds AND (drum roll please) 1955.2-59 1/2 ton's equipped with 4 speeds and 3.90 rear gears plus also selected 1960-62 C-10's with 3.90 ratios (depending on tire options). Pictures 3 and 4 are from a 1933-62 Chevy Master Parts and Accessories Catalog that show those applications.

So the gear set Doug needs can be found in a wider group of source SM420 transmissions if all that can be found is used. Haven't had any luck yet finding anything offered on the web for this GM# 3707598 set .

I saw one on eBay for 41-48 Chevy 3 speed. 14-5 gears. Any idea if that would work or not.

Doug

Gdads51 is right, i had both numbers in my note pad and copied the wrong part number to my forum post.
3707598 is the 7-20 gear set to look for and it should be fairly easy to source used because of how many vehicles it was used in, perhaps even finding a nos should be possible with a bit of time.

the part numbers i posted are from the 1929-1954 master parts catalog, so there could be many newer part numbers to look for if you dig through parts books and do cross referencing in later years.
those speedometer adapters proved difficult to find any information on, i was only able to find the specific ratio of one of them, the other two had to be inferred from their application listing in that part manual.
-s

edit:
found the speedo adapter listed ratios and stampings, it was in the parts manual before the speedo gear sets (the copy online wasn't searchable but browsing it the old fashion way i got lucky.
speedometer adapters group 4.329 - 1565812 ratio 17:16 ( stamped 1563871 ) - will increase speedometer reading by 6.25%
it won't fully compensate for T-Dougs intended tire size, his speedometer will still read about 3% too low with this adapter however it's likely still the best option for his situation.
but now you have a new number to search as many parts on line will probably be listed by the stamped number on the part itself because the packaging could be long lost.

T_Doug,
you need to look for gear sets for a 4 speed sm420 tranmission,

GM listed a different part numbers for the same ratios between the two transmissions suggesting they couldn't just use the same part between 3 speeds and 4 speeds.
non of the gear sets for the 3 speed overlap to 4 speed sm420 applications in the parts manual that i could find
that suggests they physically wouldn't fit
-s

I did a search of those part numbers and came up empty.. lol

Doug

Doug, Don’t give up just yet! I spoke with fellow Bolter KCMongo who said he has a whole bunch of used SM420 transmissions and there may be a gear set just waiting to be found. I’m going to go take a look tomorrow. Will report back on results soon.

The adapter is on eBay now
Search for the stamping number not the part number
It’s like $70 or so

Edit: eBay item 326011932047
-s

Doug.
The first one I found got sold yesterday.
Search in ebay motors " 1948-1954 Chevrolet Truck 4 Speed Rear Bearing Retainer Open Drive 1950 1953. Listed by Vintage Parts Kings. Has the speedometer gears set with it. Price is a little high.

Jeff - If you look at the pictures close up, the tooth count is not the 7 tooth drive gear and 20 tooth driven gear that Doug needs from what is visible. They look like the 5 tooth x 13 tooth gear set similar to your recent acquisition unfortunately.

I'm about to head out to KCMongo's shop to see what he may have amongst his goodies. Wish me luck!

Status update on search for for Doug’s correct speedo gears: Sadly no luck with my search through 14 different SM420 transmissions at KCMongo’s place.

Found lots of serviceable used 5 tooth drive x 13 tooth driven gear sets in case those might help someone in the future.

Doug - sounds like it time to put an ad up in the Swap Meet.

Thanks for taking the time to search for me. I ended up buying the one Jeff referred to. If I have to I’ll get a converter to bring it close.

Thanks
Doug