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TH125C rebuild by batboy
Started on: 10-02-2021 09:46 AM
Replies: 12 (226 views)
Last post by: batboy on 10-12-2021 11:07 PM
batboy
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Report this Post10-02-2021 09:46 AM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
I began working again on my V8 Fiero that was parked for 11 years. I’ve recently dropped the cradle to change engines (out with the old and in with the new). I decided to freshen up the TH125C since it’s out of the car. I know this transmission has definite limitations. But, I’m stubborn and plan to keep it. Besides, I've had two Fieros with this trans and never had a problem with the TH125C. I'm a little nervous since this is the first transaxle I've had apart. I've worked on a TH350 once and a Muncie 4 speed manual a couple of times (long ago).

I have the following parts stockpiled for this project: (1) TH125C master rebuild kit (CT Powertrain Products), (2) Superior shift correction kit (K125, basically same as TransGo SK125 ), (3) Teckpac detent (TV) cable (K65950), (4) pump parts (pump shaft, rotor, vanes, and rings), (5) Borg-Warner Hy-Vo chain (A84700A).

QUESTIONS: I’ve heard for better performance you can add one more clutch disc to the clutch pack. What else can I do to beef it up? Is it worth the price to get Kolene clutch discs?
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Dennis LaGrua
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Report this Post10-02-2021 10:47 AM Click Here to See the Profile for Dennis LaGruaClick Here to Email Dennis LaGruaSend a Private Message to Dennis LaGruaEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by batboy:

I began working again on my V8 Fiero that was parked for 11 years. I’ve recently dropped the cradle to change engines (out with the old and in with the new). I decided to freshen up the TH125C since it’s out of the car. I know this transmission has definite limitations. But, I’m stubborn and plan to keep it. Besides, I've had two Fieros with this trans and never had a problem with the TH125C. I'm a little nervous since this is the first transaxle I've had apart. I've worked on a TH350 once and a Muncie 4 speed manual a couple of times (long ago).

I have the following parts stockpiled for this project: (1) TH125C master rebuild kit (CT Powertrain Products), (2) Superior shift correction kit (K125, basically same as TransGo SK125 ), (3) Teckpac detent (TV) cable (K65950), (4) pump parts (pump shaft, rotor, vanes, and rings), (5) Borg-Warner Hy-Vo chain (A84700A).

QUESTIONS: I’ve heard for better performance you can add one more clutch disc to the clutch pack. What else can I do to beef it up? Is it worth the price to get Kolene clutch discs?


You can do good things with the TH125H but you will still be limited to three speeds. With the right mods this trans can take a lot more power than both the Getrag and Muncie. The parts you mention should provide you with a strong rebuild. As for adding another disc to the second gear clutch pack , thinner discs are available that can add strength and keep the clutch pack at the same overall length. There was a guy ( Pat Ciarcia) way back when, that ran a V8 drag race prepped Fiero. He use a built TH125H without any problems . That car ran 10's in the 1/4 mile. While I prefer the 4 speed autos I have a built TH125 in my turbo Fiero. It shifts with firmness and is solid as a rock.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

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batboy
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Report this Post10-02-2021 10:27 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
UPDATE: I took the side pan and bottom pan off. Everything looks clean and sludge-free. The trans fluid was a little dark, but was still red. It does not smell burnt. There's a number hand painted on the case. I suspect this tranny was rebuilt before I bought it. Disassembly is slow, because I'm taking photos and putting parts/fasteners into zip lock baggies and labeling with a Sharpie. So far, nothing looks damaged.

Dennis, you mentioned thinner discs. I have to admit I don't know much about that stuff. Maybe I should not get too fancy on my first rebuild (keep it simple).

Here is what it says on my rebuild kit: Complete Raybestos friction clutch kit.
◦ (5) forward frictions at 0.069" thick for 1985-on transmission
◦ (5) direct frictions at 0.069" thick for 1985-on transmission.
◦ (5) low/reverse frictions at 0.090" thick for 1985-on transmission.

QUESTION: Is there a seal where the dip stick tube goes in? It looks like the previous owner used a piece of rubber hose and some RTV.

EDIT: Disregard my question, I found a new dipstick tube seal in my rebuild kit. I'm back on track.

[This message has been edited by batboy (edited 10-03-2021).]

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Mark_j
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Report this Post10-03-2021 10:17 PM Click Here to See the Profile for Mark_jSend a Private Message to Mark_jEdit/Delete MessageReply w/QuoteDirect Link to This Post
Be interesting to see pictures of the rework process. I've done manual gear boxes in the past, but never had the the opportunity to mess much with automatics.

On thing I would suggest since you are going to put some additional power through it. Make absolutely sure your planetary gears are in good shape. I had the unfortunate experience of having the planetary in a 700R4 shatter when there was no reason for it to do so. This, after I'd had the tranny reworked and the shop that did the work and transmission upgrades for the 700R4 assured me that "everything was fine" before I swapped the 307 that came in the truck to an essentially bone stock LS9 350. Irritating thing was it let go after a leisurely trip to eastern washington while coming up to a stop light about 2 miles from home. Had only put like 25K miles on it and had never abused it.

YMMV
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batboy
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Report this Post10-03-2021 11:38 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
I got the transmission mostly disassembled. I haven't examined everything yet, but so far it looks pretty good. There's evidence suggesting this trans was likely rebuilt by a shop before I bought car.

The drive chain has 3/4" of slack and it's supposed to be replaced if it reaches 7/8". I already bought a new chain, which was a good call on my part since the original chain is marginal. The band is looking a little toasty, but I have a new one in the rebuild kit along with various seals, o-rings, and gaskets. The rebuild kit has all new clutch discs too. I still need to measure clearances before I put it back together.

If you go to YouTube and look for TH125 rebuild, there are a couple of decent vids showing how to do this. If I can do it, then anyone can do it.

No questions at this time.

EDIT: Photos were requested. Here's where I'm at right now, trying to get a snap ring off (first photo). The second photo shows some of the parts that came out of the trans. The lower left set of gears (planetary gears and sun gear?) seem a little loose. I'm trying to figure out how to measure the clearance.



[This message has been edited by batboy (edited 10-04-2021).]

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Dennis LaGrua
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Report this Post10-04-2021 12:00 PM Click Here to See the Profile for Dennis LaGruaClick Here to Email Dennis LaGruaSend a Private Message to Dennis LaGruaEdit/Delete MessageReply w/QuoteDirect Link to This Post
I would highly recommend that you purchase the transmission rebuild manual. IMO the trickey part is replacing the check balls in all the correct positions and getting the cover plate on to sit correctly. Don't forget to use trans gel to hold all the parts together while reassembling the trans. A friend of mine uses vaseline for this and says that it works but I've never tried it. You don't want any springs popping out of the cover plate or check balls out of the valve body during reassembly. A trans parts house should be able to set you up with thinner clutch discs and plates..

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

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batboy
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Report this Post10-04-2021 11:18 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
Yes, I should have a transmission rebuild manual. I do have a detailed exploded view with the parts being actual photos. I also did a fair amount of research. The true test will be whether or not it works.

For the direct drum the instructions say to use 4-5 frictions and 4-5 steels. The trans had 4 frictions and 5 steels when I disassembled it. There is also a thick steel (direct pressure plate) that goes on top of the pack. I could not fit 5 clutches in and still use the thick steel. So, I just put the same number of discs back in that came out. At least it has new Raybestos frictions (USA) instead of half wore out clutches.

Regarding the 5 check balls in the valve body and 1 check ball in the auxiliary valve body, I had a stroke of brilliance during the disassembly and snapped photos showing the check ball locations.

I hope to have this transmission back together in a couple more days. Oh yeah, almost forgot, I better read the shift correction kit instructions before I go much farther.
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Joseph Upson
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Report this Post10-05-2021 06:12 PM Click Here to See the Profile for Joseph UpsonClick Here to Email Joseph UpsonSend a Private Message to Joseph UpsonEdit/Delete MessageReply w/QuoteDirect Link to This Post
The 125 is a piece of cake to build. Just keep the parts clean, especially if you disassemble the valve body. I rebuilt at least two of mine which worked very well, with the shift improver kit installed. You would be wise to talk to a builder familiar with the transmission for ideas on improving strength. Adding additional clutches where possible as suggested would be wise, it's not an added complication. I used ATSG manuals for rebuilding and they provided info on updates to make to improve performance.

I could not get 3rd gear to hold behind a turbocharged 3.4L for very long with just a shift kit. 1st and 2nd were fine, but 3rd would start to slip so you need to find out how to counter that threat. Increasing pump pressure as much as safely possible would be helpful and looking into gear ratio options might be a good idea. There are many, I posted a few pages from a trans manual with detailed info on this here about 20 yrs ago. The info might still be available on the forum.

[This message has been edited by Joseph Upson (edited 10-05-2021).]

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batboy
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Report this Post10-05-2021 07:59 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
Just keep parts clean, check. I've definitely been doing that.

I spent half of the day replacing seals and o-rings. The other half was spent prepping gasket mating surfaces and more cleaning.

Wish I could change gear ratios, I have 35/35 sprockets and would love to get 33/37 sprockets, but I can't find any nowadays. Most of the gear sets you see on eBay are for the TH125 not the TH125C (thicker chain and lock up torque converter).

Not sure I would call this rebuild "a piece of cake" but, if I can do it, anyone can.
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Report this Post10-06-2021 06:13 PM Click Here to See the Profile for Joseph UpsonClick Here to Email Joseph UpsonSend a Private Message to Joseph UpsonEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by batboy:

Just keep parts clean, check. I've definitely been doing that.

I spent half of the day replacing seals and o-rings. The other half was spent prepping gasket mating surfaces and more cleaning.

Wish I could change gear ratios, I have 35/35 sprockets and would love to get 33/37 sprockets, but I can't find any nowadays. Most of the gear sets you see on eBay are for the TH125 not the TH125C (thicker chain and lock up torque converter).

Not sure I would call this rebuild "a piece of cake" but, if I can do it, anyone can.


I call it a piece of cake because when I successfully rebuilt mine I had as much experience rebuilding automatic transmissions as you, none. Call it confidence I guess. I found the link to the info I posted and to get the combo you want in gear ratios you want car-part.com and a search for the right core is a good start. The link contains ID codes and cars that the gear ratios were used in. 4 cylinder cars usually received the highest stall torque converters, that being said, I mounted a 4 cyl Fiero tranny and converter to a V6 swapped in its place and the acceleration was impressive for what it was as the V6 with 3:33 final drive usually received a lower stall.

https://www.fiero.nl/forum/...060811-2-070109.html
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Report this Post10-06-2021 09:19 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
Ok, I'm stuck trying to install the shift correction kit. The instructions say to remove the pressure regulator from the valve body, but doesn't tell you how.

I have the Superior brand shift kit, maybe the TransGo has better instructions? Anyone have those? Google didn't help me.

I found out another thing about my trans, it's not the original one, it's a 1987 or later trans and the Fiero is a 1985.

EDIT: Nevermind, I got it... had to remove a roll pin.

[This message has been edited by batboy (edited 10-06-2021).]

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Report this Post10-07-2021 08:30 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
UPDATE: Today, I got the bottom pan and side pan sanded and painted. I finished the shift correction kit installation. I took the clutch packs back apart. I found out you're supposed to soak the friction discs beforehand and I didn't. While they were apart I found out you can use thinner steels, so I was able to add another friction to the direct and forward drums. I still need to replace the torque converter seal, but it's going back together.

[This message has been edited by batboy (edited 10-07-2021).]

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Report this Post10-12-2021 11:07 PM Click Here to See the Profile for batboyClick Here to Email batboySend a Private Message to batboyEdit/Delete MessageReply w/QuoteDirect Link to This Post
UPDATE: I have the transmission reassembled. I took last weekend off and went camping. That was part of the delay. This trans rebuild was harder than I thought it would be and took longer than I expected. But, it's done now and I feel confident that it's ready. I replaced the drive chain, intermediate band, sprag, clutch discs, steels, seals, gaskets, pump parts, TV cable, etc. I installed a shift correction kit and put an extra friction disc in the direct clutch pack. Next, I'll mate the engine and trans together on the cradle and get it ready to reinstall.



The old chain had 3/4" of slack and the new chain has 1/4" of slack.

[This message has been edited by batboy (edited 10-12-2021).]

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