A little different beast - TH425

[fog]

I'm in progress of building a drag Fiero with a TH425 and GN powerplant, and have hit a brick wall with the trans.

The basic internals are the same as 400, so right drum, 34 sprag(installed other way, since 425 rotates backwards), decent steels and frictions, metal clad seals etc. is available and will be used.

But firstly, there is no manual available witch lists the bolt torque and clearances. Especially to the part of the trans that is different from 400?

Secondly, I would really like a transbrake, but there is none available. Is there any "old racer diy transbrake" trick?
The transbrake era had to start with some backyard mechanic with some brake pipe and inline solenoid?

Other parts are covered. Sealing kit and TH400 power parts will do the job.

Taking the trans to a shop is not an option, since I'm too far from the States and local shops dunno anything about those boxes.

Thanks in advance.

 

[WIKEDV6]

Dude, we need to talk, I have a stage II motor getting ready at the shop to go into a Fiero with the TH 425. I haven't thought about the T brake yet. I have questions and concerns about the trans axels.

Prasad

 

[fog]

Dude, we need to talk, I have a stage II motor getting ready at the shop to go into a Fiero with the TH 425. I haven't thought about the T brake yet. I have questions and concerns about the trans axels.

Prasad

I'm building the "reverse rotation" style, so the diff is flipped. Plan is this:

Pass side: short stub, eldo CV, custom length 33 spline axle, eldo CV, minivan suspension (has 33 spline wheel bearing and can be modified to 4-1/2 bolt circle, has 5x115). Also big vented brakes for holding on the line.

Driver's side: short stub, machined minivan bearing bolted to it, long stub connected to it, eldo cv, custom length 33 spline axle, eldo CV, minivan suspension.

The driver's side long stub hits the #5 rod, so in works is a 2" spacer between the bellhousing-engine and flexplate-crank.
That will move the axle below the #2 main bearing and plenty of clearance.


But my original questions still needs some answer, 'cause I need to start ordering parts.

 

[WIKEDV6]

Man, you're in Europe? Holy cow! I talked to my tranny guy and he said he is not aware of any TB for the TH425. He stated that the chain may snap if you launch of the brake. You may want to PM Don Wong on this board and pick his brain.

Thanks for the info, I am sure I will be talking to you later!!!

Prasad

 

[fog]

Man, you're in Europe? Holy cow! I talked to my tranny guy and he said he is not aware of any TB for the TH425. He stated that the chain may snap if you launch of the brake. You may want to PM Don Wong on this board and pick his brain.

Thanks for the info, I am sure I will be talking to you later!!!

Prasad

Yes, Europe so no local help is around.
I would not worry about the chain. Since it is before all the transmission mambo-jambo it will get the power that engine makes, and the transbrake fact is the same as holding the car with brakes. The chain is standing still in both circumstances and starts to spin when the car is let loose.
The chain has proven to handle about 800ft/lbs, but that can be cured...

I had holding issues the whole season(I raced a 3800 turbo Fiero this season). The front just walked away with locked wheels on the line. I dont even want to think what kind of brakes is needed at the back to hold the GN monster while spooling up.

 

[WIKEDV6]

I see what you're saying about pushing the front wheels through.

Wow!!! How much does it cost to modify the chain setup like that?

Prasad

 

[fog]

I see what you're saying about pushing the front wheels through.

Wow!!! How much does it cost to modify the chain setup like that?

Prasad

Dunno. I have not talked with those guys that did that(alltough Finland is only 50 miles north over the gulf from Estonia). But I sure will when I snap the first chain.

But as I can see, it is just longer bolts, sleeves, some plates and additional chain plate(the one that is behind the chain) and additional sprockets. So just some DIY and patience.

 

[gsgns4me]

But firstly, there is no manual available witch lists the bolt torque and clearances. Especially to the part of the trans that is different from 400?

Try to find a late '70's to early 80's Cadillac Service Manual that covers Eldorado's (maybe Seville, also?) on e-Bay, Amazon, etc.

If you can decide what year to use, you may still be able to get one from Helm, Inc., also. They're the guys who provided the manuals for GM.

 

[yullose]

I might have an old 70's Chilton manual out in the shop that covers that stuff...

I remember reading through it when I started tinkering with TH400's a long time ago... and specifically recall seeing mention of the sprag being installed backwards on the FWD cars.

I'll look for it tomorrow if I think of it.

 

[Otto J]

If you can installed a second caliper and and adjustable proportioning valve with the proper torque convertor you can hold 15-20#s no problem

 

[fog]

My trans parts will arrive this week, so I'm starting to tear the beast apart.

I have found a solution that could be adapted. I just need to investigate where to drill the taps.

Something like that could be adapted I think:

Maybe someone will chime in on how to find the right passages.

The valve body looks like this:
With cover
without cover

 

[KL Mallender]

You are VERY unlikely to snap the chain in a TH425, been a long time since being inside one(I had 2 '67 Toros many moons ago) you may twist a shaft out of the sprockets(maybe) but you aren't gonna phase that chain assembly.
Will take far more than the stub shafts can, that I'm sure of.
Good luck with your project, should be one hell of a ride.

Kevin.





ps: If you are sure you want a "simple" transbrake, I might be able to help you out, provided someone has a spare VB lying around.

 

[SSedan64]

Guy on another forum built a 70 Chevelle w/350ci w/671-Blower & TH425. It spit the chain through the cover on the 1st WOT 1-2 shift. He did have some Pics in his Photo Album but, they're not there anymore.

 

[fog]

...ps: If you are sure you want a "simple" transbrake, I might be able to help you out, provided someone has a spare VB lying around.

It would be hard to find one in states, while I myself are 3000 miles away.
But what do you have in mind? mechanical lever to Lo type? Even that would do I think. Way better than nothing.

I need to find a 3.07 diff anyway for the project. So I should look for a complete trans that does not need to run.

Can you give me some more info on that. I would really appreciate it.

 

[SSedan64]

Th425

The 2.73 ratio was the most common.
A towing package equipped car had a lower 3.08 gear.
The FWD GMC motorhomes with the Olds 455 (or 403 after 1976) Olds and TH425 had many ratios.

Different Brand/Model motorhome gear ratio Info>> FRONT END
Final drive kits>> Jim Kanomata GMC Parts and Services
Links found here>> INDEX LIST

 

[fog]

Just trying out new signature while inpatiently waiting for KL Mallender to come online

 

[KL Mallender]

The 425 was used in Toronado's from 1966-1976, Eldorado's from 1967-1976, they were also used in GMC's 6 wheeler motorhome from 1973-1978, so finding a core shouldn't be real bad, but will need a little scrounging work, probably an "Old School" type junkyard.
The transbrake function would work in the same manner as the "Stage Right"
full automatic function in D or 2nd, but 1st would be locked.
To hear that someone spit a chain out of one on a 1-2 shift is suprising, but if the builder got carried away with his "Shift Kit" mods and eliminated it's accumulation, evidently it can be broken. I would hope you NOT make same mistake, and hopefully deal with modulator issues before other problems arise.


Kevin.

 

[SSedan64]

Not sure what Mods were done to Trans. He's worked for Hughes building Trans a long time tho' so probably didn't get carried away. He had a Pic of the Chain cover, Not pretty. I think the car was more for shows than anything.
I'll ask him what VB mods were done.
Todd

 

[fog]

If there is knowledge what to do and what to avoid in building, I'll be one big ear.

My kit that will arrive in a few days consisted:
rebuild kit, kolene steels, red eagle frictions, intermediate power pack(3->4), 34-sprag, BW band, 1" band, transgo shift kit.

But I dont have(and no-one here in Estonia) what TH400 modifications to do to that beast and what to avoid.

I'm right now looking for the spare VB on the Michigan area yards.

 

[SSedan64]

Th425

He replied with this about the TH425 VBody mods,

The usual TH400 type mods were performed...
1-2 check ball removed , direct clutch was dual fed.