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Old 05-10-2017, 10:36 PM   #1
Judgeyoung
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T-400 speedo gear shaft too big

In fixing details on my recent 2-owner 72 GMC c2500 find, I pulled the speedo drive after the cable and speedo checked out good. Sure enough, the gear was twisted off, and either eaten up, or hidden in the tail shaft.

The drive gear appeared natural color, or maybe yellow which is usually 19 teeth. If my numbers are right, I need a 42 tooth for my all stock 3.54 rear and 235-85-16 tires. The remainder of the original shaft looked like it might have been white, or grey, but I am not sure.

Today my green 42 tooth gear came in and I pulled the drive. The new shaft will go in to where the spiral starts, but is too large to go farther. The old shaft mics around .312, as does the old piece. The new shaft is.331. I thought about reaming the hole with a valve guide reamer, but decided to slim the plastic shaft instead. I did, and it fit nicely, with the spiral remaining barely visible.

UNFORTUNATELY... when I went to re install, the drive would not go completely in. It appears that the bearing hole at the gear end of the shaft is either twisted off flush, or otherwise not opened to accept the end of the shaft.

Now, the questions:
1. Why did the first one shear?
2. Are my gear calculations in the ball park?
3. Why was the shaft too large?
4. What is up with the inner bearing hole?
5. If it it part of the old shaft, is there any way to remove it with the tail housing still on?

Thanks in advance for you help!
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Old 05-11-2017, 03:42 PM   #2
Judgeyoung
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Re: T-400 speedo gear shaft too big

What I "may" have learned is that the natural color drive gear can also be a 15 tooth, meaning that I need a 34 tooth driven gear. I got one of those today, and, miraculously, it is the correct diameter. Apparently, in order to make it harder to put the "larger number of teeth" gears into the "smaller number of teeth" housings, they made the shafts different sizes. It looks like I at least have the correct gear.

I also think that the bearing stub from the old gear may have sheared off in the boss inside the tailshaft, and while likely difficult because of not being able to see, I think that I can use a left hand bit, or other device, and carefully remove the stub from the housing. If not, I will have to remove the housing, which might be a good idea anyway since I still have not felt or retrieved the remainder of the old gear...

I know that trying to answer my own question is pretty lame, but maybe someone else with a similar issue will run across this and it might be of help.

Of course, I am certainly open to one of you experts putting me on the correct track!
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Old 05-11-2017, 05:10 PM   #3
knomadd
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Re: T-400 speedo gear shaft too big

Hopefully you've solved your problem. One thing to note, the case for the driven gears are only meant for a specific range. There is a different case for 34-39 teeth and 40-45 teeth. It appears, by TCI's website, there is also another case for 16-22 teeth driven gears. Here's their website http://www.tciauto.com/tc/speedometer-gear-calculator
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Last edited by knomadd; 05-12-2017 at 10:11 AM.
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Old 05-11-2017, 05:14 PM   #4
Judgeyoung
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Re: T-400 speedo gear shaft too big

Thanks KnoMadd, I suspect that the difference in housings is in the I.D. of the shaft hole for the gear, as I experienced. If I can get the "inside-the-housing" end of the sheared shaft out, and fish the old gear out, I should be good to go!

Thanks again for the reply.
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Old 05-11-2017, 05:23 PM   #5
randy500
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Re: T-400 speedo gear shaft too big

Don't hesitate to remove the tailshaft housing. the square o-ring that seals it to the main case is reusable and you should only loose a nominal amount of fluid. I usually jack up the rear to loose even less fluid.
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