1972 GMC TH400 Speedo gear - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jlampka · 10-21-2021, 08:40 PM

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?
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**Accelo** · 10-22-2021, 01:37 AM

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.

jlampka · 10-31-2021, 08:32 PM

Thank you accelo!

jlampka · 10-31-2021, 11:55 PM

Thank you accelo!

Rich72C10 · 11-01-2021, 11:23 AM

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

**va72longhorn** · 11-01-2021, 12:03 PM

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.

jlampka · 11-03-2021, 08:44 PM

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

**DPowers** · 11-04-2021, 07:30 PM

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

**72BBChuck** · 11-05-2021, 08:42 AM

Wow, very helpful info guys. Thanks! Chuck

10-21-2021, 08:40 PM	#1
jlampka Registered User Join Date: Sep 2013 Location: North Tonawanda, NY Posts: 237	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?

10-22-2021, 01:37 AM	#2
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	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. Last edited by Accelo; 11-04-2021 at 12:20 AM.

10-31-2021, 08:32 PM	#3
jlampka Registered User Join Date: Sep 2013 Location: North Tonawanda, NY Posts: 237	Re: 1972 GMC TH400 Speedo gear Thank you accelo!

10-31-2021, 11:55 PM	#4
jlampka Registered User Join Date: Sep 2013 Location: North Tonawanda, NY Posts: 237	Re: 1972 GMC TH400 Speedo gear Thank you accelo!

11-01-2021, 11:23 AM	#5
Rich72C10 Mr. Cheyenne Join Date: Jun 2020 Location: Kyle, Texas Posts: 1,687	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 __________________ Richard *'72 Chevy C10 Cheyenne Super WHT/MED BRONZE/WHT SBC 350 \| TH350 \| LWB \| AC \| Buddy Seats Upgrades:** Tilt \| Front Sway Bar \| Radio \| Sliding Window \| Bed Spare w/Lock Bed Rails \| Full LEDs \| Courtesy Lights \| Tachometer \| Glovebox Lock & Light \| Head Light Relays \| *Upgraded Fully Super Trim

11-01-2021, 12:03 PM	#6
va72longhorn Senior Member Join Date: Dec 2009 Location: Culpeper, VA Posts: 435	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.

11-03-2021, 08:44 PM	#7
jlampka Registered User Join Date: Sep 2013 Location: North Tonawanda, NY Posts: 237	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

11-04-2021, 07:30 PM	#8
DPowers One foot in front of the other Join Date: Aug 2008 Location: Parrottsville, TN Posts: 5,442	Re: 1972 GMC TH400 Speedo gear Check LA Gear. http://www.laspeedometergear.com/ __________________ DP's 66 Build http://67-72chevytrucks.com/vboard/s...d.php?t=361556

11-05-2021, 08:42 AM	#9
72BBChuck Senior Member Join Date: Jun 2007 Location: SW Washington State Posts: 216	Re: 1972 GMC TH400 Speedo gear Wow, very helpful info guys. Thanks! Chuck



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