1972 GMC TH400 Speedo gear
 

My speedometer doesn't work so I disconnected the cable at the tranny and spun the cable using my cordless drill - the speedometer turned and held steady at 30 mph so now I turn my attention to the TH400 tranny gear. See photo - is there a spanner wrench or some tool to remove the face plate shown? any help s appreciated. Also - will most transmission shops have the gear?
Attachment 2140557

Re: 1972 GMC TH400 Speedo gear
 

No spanner needed.
Once you remove the screw and retainer you can put the cable back on to protect the threads and pull on the cable nut, with a pliers if you have to. It will come right out.
The three numbers (40-42-42) are the corresponding gears that go with the particular housing. So you should have one of the three gears in there. Other ratios require different housings.
There are two seals to keep this part from leaking. An "O" ring on the OD of the aluminum housing and a small seal that goes in the center hole and keeps the transmission oil from leaking past the gear.
90% of the time the plastic gear is worn and will require replacement if you want it leak free. Only hand tighten the cable. Tight will not keep if from leaking!
Yes most shops will carry the gear. Get a new one if you can and the two seals while your at it. The gears are also available on eBay.

Re: 1972 GMC TH400 Speedo gear
 

Thank you accelo!

Re: 1972 GMC TH400 Speedo gear
 

Thank you accelo!

Re: 1972 GMC TH400 Speedo gear
 

Ya, on my Th350 this gear (or I guess like it) had to be replaced to stop an annoying leak. The shop that was working on it had a new one in their "attic of old parts" stash! That and the shifter rod (whatever that is really called) was replaced - zero leaks from TH350 for thousands of miles!

They gave me the old parts, the gear shaft and shifter rod shaft were ever so lightly worn down. I guess it doesn't take much before the o-rings can't keep the fluid in! I so detest leaks LOL

Re: 1972 GMC TH400 Speedo gear
 

I may be mistaken ( I’m sure someone will be tell me if I am), but isn’t he going to need the exact gear that he’s taking out for a replacement, or at least the number off of it? I’d of sworn that I had to replace one with a different geared one on my blazer, 40 yrs ago, when the local sheriffs dept was kind enough to tell me that my speedometer didn’t match his radar gun.... in writing.

Re: 1972 GMC TH400 Speedo gear
 

yes I took my gear and counted 41 teeth - matched it up to the gear pulled out of the tranny. Thanks

Re: 1972 GMC TH400 Speedo gear
 

Check LA Gear.
http://www.laspeedometergear.com/

Re: 1972 GMC TH400 Speedo gear
 

Wow, very helpful info guys. Thanks! Chuck