Leaking THM350
Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
Leaking THM350
My THM350's been leaking ever since I bought the car ('76 coupe). Not much, but enough that there's a wet patch on the garage floor after a few days. I've tracked the leak to the speedometer gear housing, where the cable attaches to the trans. It's usually just a very slow drip from the cable, so I assume the seal in the gear housing is bad and is allowing ATF to leak up the driven gear shaft and out of the trans.
I'd like to know what all parts I should consider replacing when I go to do this. I'm going to get a new driven gear housing, but should I also buy a new driven gear? I've read that the driven gear can sometimes wear at the seal and leak, necessitating replacement. Without having taken everything apart, I don't know if that's the case on mine.
What all parts should I consider ordering?
Thanks.
I'd like to know what all parts I should consider replacing when I go to do this. I'm going to get a new driven gear housing, but should I also buy a new driven gear? I've read that the driven gear can sometimes wear at the seal and leak, necessitating replacement. Without having taken everything apart, I don't know if that's the case on mine.
What all parts should I consider ordering?
Thanks.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
My THM350's been leaking ever since I bought the car ('76 coupe). Not much, but enough that there's a wet patch on the garage floor after a few days. I've tracked the leak to the speedometer gear housing, where the cable attaches to the trans. It's usually just a very slow drip from the cable, so I assume the seal in the gear housing is bad and is allowing ATF to leak up the driven gear shaft and out of the trans.
I'd like to know what all parts I should consider replacing when I go to do this. I'm going to get a new driven gear housing, but should I also buy a new driven gear? I've read that the driven gear can sometimes wear at the seal and leak, necessitating replacement. Without having taken everything apart, I don't know if that's the case on mine.
What all parts should I consider ordering?
Thanks.
I'd like to know what all parts I should consider replacing when I go to do this. I'm going to get a new driven gear housing, but should I also buy a new driven gear? I've read that the driven gear can sometimes wear at the seal and leak, necessitating replacement. Without having taken everything apart, I don't know if that's the case on mine.
What all parts should I consider ordering?
Thanks.
The housing is around $75 and the nylon gear is around $15 so you really should examine them before ordering any new parts. You'll need the large o-ring for the housing and the inner seal that bears against the driven gear.
Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
Are we talking about the same part? I'm referring to this part from CorvetteCentral: CORVETTE SPEEDOMETER GEAR FITTING. $15.
This is the part I believe is leaking, based on what I've read and video's I've watched, at at least the seals within it. The different driven gears are around $10, though admittedly I don't know which I need for a 3.08 rear end. I'll have to check my service manual tonight IF that's a part I should buy.
Thanks.
This is the part I believe is leaking, based on what I've read and video's I've watched, at at least the seals within it. The different driven gears are around $10, though admittedly I don't know which I need for a 3.08 rear end. I'll have to check my service manual tonight IF that's a part I should buy.
Thanks.
Last edited by Kris Tunetso; Jan 28, 2016 at 11:14 AM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
The seal on the driven gear is the usual suspect. No need to replace the whole housing.
On mine I put a small o ring inside the seal to make it grip the driven gear shaft tighter. Or you could replace the seal if you have a bearing machinery outlet near you.
The outer housing has an o ring that can be replaced if that is the issue.
Just be sure to lube it all back up before you re-install it or you'll be doing it again real soon.
On mine I put a small o ring inside the seal to make it grip the driven gear shaft tighter. Or you could replace the seal if you have a bearing machinery outlet near you.
The outer housing has an o ring that can be replaced if that is the issue.
Just be sure to lube it all back up before you re-install it or you'll be doing it again real soon.
Last edited by REELAV8R; Jan 28, 2016 at 11:33 AM.
Instructor
Joined: Nov 2014
Posts: 161
Likes: 17
From: Georgia
My advice. ..take out what you have, examine it closely,
you may not need new parts just seal and O-ring. My gear shaft had only minor scratches at the seal point. I used a Dremel with buffing wheel and rubbing compound to polish those out, use low speed and light pressure. I got the seal & o-ring at NAPA but seal was different from the original, get a GM if you can. The seal has what appears to be a snap ring holding it in place but it's just a pressure ring, easily comes out with a pick tool. Mark the housing before taking out , it is position sensitive to have the clamp fit back on flush. You will lose some tranny fluid so be prepared.
.
you may not need new parts just seal and O-ring. My gear shaft had only minor scratches at the seal point. I used a Dremel with buffing wheel and rubbing compound to polish those out, use low speed and light pressure. I got the seal & o-ring at NAPA but seal was different from the original, get a GM if you can. The seal has what appears to be a snap ring holding it in place but it's just a pressure ring, easily comes out with a pick tool. Mark the housing before taking out , it is position sensitive to have the clamp fit back on flush. You will lose some tranny fluid so be prepared.
.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
You will lose some tranny fluid so be prepared.
Instructor
Joined: Nov 2014
Posts: 161
Likes: 17
From: Georgia
Additionally to above, when removing the housing after you break the speedometer cable loose...leave it attached. ..will give you something to grip when you wiggle the housing out. Pics above were borrowed from another post.
Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
I thought to replace the entire driven gear housing just for sake of ease. Pop the old one out, pop the new one in, done. I'm willing to pay the extra for that ease.
I confess I'm not buying the parts from a corvette forum vendor. I need parts for my daily driver and am getting ready to put in an order from Rock Auto, so I thought I'd throw in the parts for this job while I was at it just to save a bit on shipping.
I confess I'm not buying the parts from a corvette forum vendor. I need parts for my daily driver and am getting ready to put in an order from Rock Auto, so I thought I'd throw in the parts for this job while I was at it just to save a bit on shipping.
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Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
For such an inexpensive part, I may as well replace the driven gear as well. That rules out the possibility of wear on the shaft causing a leak. So, I ran home on lunch and looked in my Assembly Manual, and for a 3.08 rear end it says the M20 trans uses a 19-tooth Natural driven gear, while the M38 trans uses a 21-tooth Red gear.
I don't know which I have, but all the RPO lists I've seen don't even mention an M20 or M38, only M21 and M40. I think, Think, that the M40 is a TH400.
This chart is straight out of my 76 AIM.
Ideally I'd have everything apart so I can see which gear I have, but I need to get an order placed today for the other parts I mentioned previously. Can someone smarter than me (doesn't take much) figure this out?
I don't know which I have, but all the RPO lists I've seen don't even mention an M20 or M38, only M21 and M40. I think, Think, that the M40 is a TH400.
This chart is straight out of my 76 AIM.
Ideally I'd have everything apart so I can see which gear I have, but I need to get an order placed today for the other parts I mentioned previously. Can someone smarter than me (doesn't take much) figure this out?
Last edited by Kris Tunetso; Jan 28, 2016 at 03:07 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Those look like manual transmissions. You have a TH350 automatic?
The driven gear is color coded. Look at the one you have and buy the same color. Each color has a different tooth count. Look here just below the pic of all the TH350 driven gears. Note how depending on which color you select changes the tooth count.
http://www.laspeedometergear.com/TH3...er%20Gears.htm
Get the same color/tooth count you have now and you should be good to go.
The driven gear is color coded. Look at the one you have and buy the same color. Each color has a different tooth count. Look here just below the pic of all the TH350 driven gears. Note how depending on which color you select changes the tooth count.
http://www.laspeedometergear.com/TH3...er%20Gears.htm
Get the same color/tooth count you have now and you should be good to go.
Last edited by REELAV8R; Jan 28, 2016 at 04:23 PM.
Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
That's the only chart in the manual: I'd think it covers all trans options for that year. 
Yes, I have a TH350.
I enjoy driving it right now as the weather is nice: if I put it up on stands it's not coming down till this is done. I'd like to keep driving until the parts arrive, so I was hoping I could learn which one to buy without needing to take the car apart.
Yes, I have a TH350.I enjoy driving it right now as the weather is nice: if I put it up on stands it's not coming down till this is done. I'd like to keep driving until the parts arrive, so I was hoping I could learn which one to buy without needing to take the car apart.
Last edited by Kris Tunetso; Jan 28, 2016 at 04:31 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
That's the only chart in the manual: I'd think it covers all trans options for that year. Yes, I have a TH350.
I enjoy driving it right now as the weather is nice: if I put it up on stands it's not coming down till this is done. I'd like to keep driving until the parts arrive, so I was hoping I could learn which one to buy without needing to take the car apart.
Yes, I have a TH350.I enjoy driving it right now as the weather is nice: if I put it up on stands it's not coming down till this is done. I'd like to keep driving until the parts arrive, so I was hoping I could learn which one to buy without needing to take the car apart.
You just need to jack it on one side to get enough room to get under the car. Don't forget to block it or use jack stands before you crawl under.
Last edited by REELAV8R; Jan 28, 2016 at 04:46 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
My stock 77 driven gear was 21 teeth and brownish red. That doesn't mean that yours is the same though.
When I got a new TH350 I had to get an 18 tooth gear to get the speedo to read accurately.
You can even get a whole set of those gears if you are unsure.
http://www.ebay.com/itm/18-19-20-21-...lVe7ny&vxp=mtr
also good price on that speedo housing on ebay.
http://www.ebay.com/itm/GM-Speedo-Bullet-Speedometer-Gear-Housing-New-Powerglide-TH350-350-Muncie/141872020322?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35218%26meid%3Dc5541e357e69426c9ed84f91786158d3%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D131535167654
When I got a new TH350 I had to get an 18 tooth gear to get the speedo to read accurately.
You can even get a whole set of those gears if you are unsure.
http://www.ebay.com/itm/18-19-20-21-...lVe7ny&vxp=mtr
also good price on that speedo housing on ebay.
http://www.ebay.com/itm/GM-Speedo-Bullet-Speedometer-Gear-Housing-New-Powerglide-TH350-350-Muncie/141872020322?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35218%26meid%3Dc5541e357e69426c9ed84f91786158d3%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D131535167654
Last edited by REELAV8R; Jan 28, 2016 at 05:03 PM.
Thread Starter
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
Well, I decided to just order the new housing. If it turns out the driven gear is worn and needs to be replaced, then when I order it I'll also buy the o-ring and seal to rebuild my gear housing and have it as a backup.
Thanks for the input everyone.
Thanks for the input everyone.