Speedometer gear?

[m3talc0re]

Does anyone know exactly which speedometer gear I need for my 83 gmc Sierra with a TH350 auto trans?

My speedometer doesn't work. Pulling the dash out and spinning the little, um, spinner on the guage seems fine. The little dongle doesn't spin when the truck is in motion. So I replaced the cable. It still doesn't spin. When I told the old one off, the end that went into the transmission seemed stripped. It wasn't square like it should have been. I'm guessing the gear is stripped out. Would the housing cause issues?

[hatzie]

I assume 2WD. 

Plastic Driven gears for the Chevrolet & GMC TH350/TH375 with 7/8" driven gear speedometer housing hole.
There are at least six driven gears that can be identified by color. Be sure to get seals along with the gear.

• 17 tooth speedometer gear 3987917 PURPLE
• 18 tooth speedometer gear 3987918 BROWN
• 19 tooth speedometer gear 3987919 NATURAL
• 20 tooth speedometer gear 3987920 BLUE
• 21 tooth speedometer gear 3987921 RED
• 22 tooth speedometer gear 3987922 GRAY

http://www.classictransmissionsolutions.com/th350-turbo-350/



BOP TH350, TH700R4, & NP208 with the large housing and large gears have at least twelve possible combinations.

[m3talc0re]

Which tooth count though? This had me lost looking online

[hatzie]

Which tooth count though? This had me lost looking online

Pull the little bullet and look at the gear.  As long as you don't have different axle gears or different size tires get the same color gear.  It'll work just fine.

[m3talc0re]

Yeah...

[VileZambonie]

Looks like Hatzie gave you exactly what you needed there. You should get a housing too.

[m3talc0re]

I didn't know it just came out that easy or if it had other things going on. I didn't want to pull it out and have all the trans fluid pour out or anything of that sort, lol. Why replace it though, the housing seems fine, just a little dirty on the outside. The teeth on that gear though are completely fubar'd.

[bd]

That's an aluminum support bushing.  The hole for the gear axle needs to be circular.  Wiggle the driven gear from side-to-side.  If the gear wiggles further than ~0.006", or if it moves further in one direction, replace the bushing.  Next, remove the gear and inspect the axle hole.  If the hole isn't perfectly circular, replace the bushing.  Otherwise, the speedometer needle will exhibit an annoying bounce.

[hatzie]

I didn't know it just came out that easy or if it had other things going on. I didn't want to pull it out and have all the trans fluid pour out or anything of that sort, lol.
Why replace it though, the housing seems fine, just a little dirty on the outside. The teeth on that gear though are completely fubar'd.

Dang.  That driven gear is chewed right up. 
The original gear housings were made of pot metal and they tend to wear on the plastic gear shaft.  Replacement steel housings are cheap and you can buy the gear with the housing pretty cheap.

With a driven gear that looks like that... what does the plastic drive gear on the end of the mainshaft look like? 
I'd pull the tail housing off and take a look at the drive gear.  Just make sure to clean the crud and dirt off the top of the tail housing to transmission seam before you pull it off.
The drive gear isn't much more difficult to replace than the driven gear.  It'd be a darn shame to order a driven gear and chew it up instantly with a beat to death drive gear.

GM only lists two drive gears in the catalog for the 1983 2WD TH350. 
GM 6261782 9 Teeth Green Drive Gear.
GM 6261785 10 Teeth Purple Drive Gear.
GM 6261781  Drive gear spring clip.

Given the GM 3987921 RED 21 tooth driven gear and assuming it and the transmission are original to the truck...  In 1983 the truck could've come stock with P205/75R15 tires, 2.73:1 Rear axle, and the GM 6261785 PURPLE 10 tooth drive gear or P205/75R15 tires, 3.08:1 Rear axle, and the GM 6261782 GREEN 9 tooth drive gear
All this info is in the parts books.

[m3talc0re]

I'll have to give that a look. I didn't even think about another plastic gear in there.. Why would they use plastics like this? XD
I think the stock tire size is P235/75R15 btw, guessing that's a typo?

[hatzie]

I'll have to give that a look. I didn't even think about another plastic gear in there.. Why would they use plastics like this? XD
I think the stock tire size is P235/75R15 btw, guessing that's a typo?

They're a fuse.  Plastics are easy and cheap to manufacture and replace.

Stock base model tires were pretty small.  My 1985 came to me with the bone stock 205/75R15 tires. I installed 235/75R15 tires on it and changed the speedo driven gear to match.

If you have 235/75R15 tires I'd change to the BLUE 20 tooth driven gear.  Q on the below table...

This is the online version of the PDF parts book page 1043 that Ian Harding setup from my scans.  You can download offline PDF from the link in my signature if you want.
http://gmpartswiki.com/getpage?pageid=59803

[m3talc0re]

When I pulled that gear housing out, should fluid have come out?

[hatzie]

When I pulled that gear housing out, should fluid have come out?

Likely.  It wouldn't be shocking to see some ATF come out.

[m3talc0re]

I don't think any came out.. There was some on the gear and housing, but none really dripped or poured out.

[hatzie]

I don't think any came out.. There was some on the gear and housing, but none really dripped or poured out.

I wouldn't worry about that.  You can check the ATF level and top off after you button everything up.