Discussion of built driveline

[carnuck]

I just ordered a fuel tank for my son's Comanche in Canada from carparts.com

[The Dark Side of Will]

My dad and uncle snagged a '70's GMC Motorhome (aka "Trans-Mode") for *FREE* a while back--it was sitting at my uncle's brother-in-law's house.

It uses the longitudinal FWD TH425 tranasaxle with an Olds 455 engine.

My dad wants to put a Duramax in it.
To that end, he bought a '66(?) TH425 with the "switch pitch" converter, expecting that he'd be able to use the hydraulic control for the switch pitch to operate a lockup converter. I looked that unit over today... It looks VERY similar in concept to the Eagle Dana 30.

The diff for this unit is
A) HUGE - designed for big block torque behind Cadillac 500's and Oldsmobile 455's.
B) Set up for independent suspension almost identically to the Dana 30 at the front of an Eagle.
C) Very narrow across the output flanges, with the pumpkin itself being very narrow left-right, in order to fit between the frame rails of the Olds and Caddy bodies of the day.

IOW, it's the ideal diff architecture to swap into the front end of an Eagle.
Stock ratios included 2.73, 3.07 and 3.21, but there are aftermarket 3.54's available...  :-D

I'm going to finish up the D30 and get the car on the road, but this option makes WAY more sense than trying to adapt the Jaguar Dana 44 to the front of an Eagle.

[rollguy]

My dad and uncle snagged a '70's GMC Motorhome (aka "Trans-Mode") for *FREE* a while back--it was sitting at my uncle's brother-in-law's house.

It uses the longitudinal FWD TH425 tranasaxle with an Olds 455 engine.

My dad wants to put a Duramax in it.
To that end, he bought a '66(?) TH425 with the "switch pitch" converter, expecting that he'd be able to use the hydraulic control for the switch pitch to operate a lockup converter. I looked that unit over today... It looks VERY similar in concept to the Eagle Dana 30.

The diff for this unit is
A) HUGE - designed for big block torque behind Cadillac 500's and Oldsmobile 455's.
B) Set up for independent suspension almost identically to the Dana 30 at the front of an Eagle.
C) Very narrow across the output flanges, with the pumpkin itself being very narrow left-right, in order to fit between the frame rails of the Olds and Caddy bodies of the day.

IOW, it's the ideal diff architecture to swap into the front end of an Eagle.
Stock ratios included 2.73, 3.07 and 3.21, but there are aftermarket 3.54's available...  :-D

I'm going to finish up the D30 and get the car on the road, but this option makes WAY more sense than trying to adapt the Jaguar Dana 44 to the front of an Eagle.

If I understand you correctly, you will be using the GM "third member" in your Eagle?  Are you planning to use the transmission as well?  I have had many a Cadillac ElDorado, and know that the third member is bolted to the trans, and not separate.  I don't believe the third member can be divorced from the trans without a lot of fabricating.....Rich

[The Dark Side of Will]

"Will be using" might be a strong term... More like "That looks like the most realistic of a set of far-fetched options".

I've seen photos of the transmission in the Fiero community before. Do you have a photo of one separated?

As long as the pinion bearings are in the pumpkin and not the transmission, then it *may* just be a matter of machining a block off plate to bolt to the pumpkin in place of the trans and hold an input shaft seal. I have no idea if there's a yoke on the market that would work or if I'd have to get Mark Williams or another outfit to make one custom for me.

Buuut... the intermediate axle, the brackets, the CV joints, the overall size and profile of the diff... all of that looks at first glance to be very compatible with the Eagle.

EDIT: This style of transmission uses a male spline on the trans and a female spline in the pinion. According to this: http://www.fiero.nl/forum/Archives/Archive-000001/HTML/20130314-2-104990-3.html The TH325 uses the same 27 spline interface as the yoke on a TH350 or Th700. The 425 may use the 32 spline interface... but being somewhat standardized makes it easier.

[rollguy]

"Will be using" might be a strong term... More like "That looks like the most realistic of a set of far-fetched options".

I've seen photos of the transmission in the Fiero community before. Do you have a photo of one separated?

As long as the pinion bearings are in the pumpkin and not the transmission, then it *may* just be a matter of machining a block off plate to bolt to the pumpkin in place of the trans and hold an input shaft seal. I have no idea if there's a yoke on the market that would work or if I'd have to get Mark Williams or another outfit to make one custom for me.

Buuut... the intermediate axle, the brackets, the CV joints, the overall size and profile of the diff... all of that looks at first glance to be very compatible with the Eagle.

EDIT: This style of transmission uses a male spline on the trans and a female spline in the pinion. According to this: http://www.fiero.nl/forum/Archives/Archive-000001/HTML/20130314-2-104990-3.html The TH325 uses the same 27 spline interface as the yoke on a TH350 or Th700. The 425 may use the 32 spline interface... but being somewhat standardized makes it easier.

Also not that it is a big deal, but the trans dipstick goes through the diff as well.

[eaglefreek]

I have had many a Cadillac ElDorado,

What didn't you own many of? 

[carnuck]

I had a FWD '85 Buick Riviera with a supercharged Buick 307 V8 (Canadian model before emission testing on the West Coast) and it had a TH325-4L O/D automatic that had issues (cost me $1500 to fix in parts alone when I worked in a trans shop) and it had a separate 10 bolt front diff with a CV coupler to the trans. We ended up modding the car to take a 455 I had kicking around and put the turbo motor in an '81 Pontiac Firebird (small body) till it blew, then swapped a 455 Pontiac SJ motor from a '74 Gran Prix in the Firebird.

[The Dark Side of Will]

Also not that it is a big deal, but the trans dipstick goes through the diff as well.

I guess I didn't really say it, but I would be using the GM diff by itself... block off plate, input shaft seal, yoke and a driveshaft to my NP229.
Yes, custom brackets would be required, but I have the capabilities to make those without much trouble.

Does it share an oil supply with the transmission? IE, the dipstick in the diff checks the fluid level in both units? I see what looks like a dipstick tube on the photo you posted.

[The Dark Side of Will]

I had a FWD '85 Buick Riviera with a supercharged Buick 307 V8 (Canadian model before emission testing on the West Coast) and it had a TH325-4L O/D automatic that had issues (cost me $1500 to fix in parts alone when I worked in a trans shop) and it had a separate 10 bolt front diff with a CV coupler to the trans. We ended up modding the car to take a 455 I had kicking around and put the turbo motor in an '81 Pontiac Firebird (small body) till it blew, then swapped a 455 Pontiac SJ motor from a '74 Gran Prix in the Firebird.

I've never heard anything good about the 325; it's a light/medium duty 4 speed, while the 425 is a heavy duty 3 speed... a TH400 folded in half.

[rollguy]

Also not that it is a big deal, but the trans dipstick goes through the diff as well.

I guess I didn't really say it, but I would be using the GM diff by itself... block off plate, input shaft seal, yoke and a driveshaft to my NP229.
Yes, custom brackets would be required, but I have the capabilities to make those without much trouble.

Does it share an oil supply with the transmission? IE, the dipstick in the diff checks the fluid level in both units? I see what looks like a dipstick tube on the photo you posted.

I believe the diff has a separate oil sump, and does not co-mingle it's lubricant with the trans.  The dipstick is for the trans, but it goes through the diff into the trans sump.  It probably won't be an issue for you because of the adapter plate and all that you are making.....Rich

[The Dark Side of Will]

So I took the opportunity to play...

This is the diff from my dad's TH425 put up where it would go on my 4.0 (along with a piece of aluminum bar for some reference points). The upper left bolt on the TH425 diff should be 6" back and 4" down from the forward bolt attaching the aluminum to the block... at least as a starting point.



Here's the side of the diff at its point of closest approach to the oil pan:



Some extra views:



The axle relative to the oil pan:



The right side axle bracket:











Here's the TH425 unit next to the Dana 30. The D30 is a 7.375 or so ring gear. I haven't seen a number of the TH425 unit, but given that it's used with Caddy 472's and 500's in 4000+# luxobarges and Olds 455's in 11,000# motor homes, it is *AT LEAST* a 10 inch ring gear. This makes it bigger than the 8.5" Dana 44 I have for the rear and definitely in the Dana 60/Ford Sterling/GM 14 bolt class of axles.



The TH425 diff is actually *NARROWER* across the output flanges than the stock Dana 30:



Output flange size difference:





The input to the TH425 diff is 1.415" or so and 33 splines... *NOT* 32 as I conjectured.

*Image links may be broken until Google Photos pulls its collective head out of its@$$.

[The Dark Side of Will]

I believe the diff has a separate oil sump, and does not co-mingle it's lubricant with the trans.  The dipstick is for the trans, but it goes through the diff into the trans sump.  It probably won't be an issue for you because of the adapter plate and all that you are making.....Rich

Of course I completely forgot to take a photo of the input side of the unit, but it's sealed. It has its own input seal and everything. I don't even have to make a seal carrier or block off plate. I just have to cover up the pinion bearing adjuster.

[eaglefreek]

Is it worth the trouble? I've never heard of anyone breaking the front Dana 30.

[amcfool1]

x2 on that. also whole crossmember will have to be modified, it also looks very heavy, not to mention kinda ugly:} good luck regardless! gz

[The Dark Side of Will]

Is it worth the trouble? I've never heard of anyone breaking the front Dana 30.

Ever heard of anyone running an Atlas 4 speed and 42's with an Eagle D30? ;-)

Considering that a 3.54 for the TH425 diff is going to run me $1500, this is not a path to pursue lightly, so I'll probably have to break the D30 first.

x2 on that. also whole crossmember will have to be modified, it also looks very heavy, not to mention kinda ugly:} good luck regardless! gz

Of course it's heavy, there's a lot of steel in it. That's the whole idea!
The whole crossmember wouldn't need to be modified,  just the part that interferes. ;-)

Big drawbacks: no locker available; geometry of the cover means that if I slide it over a rock I'm likely to get a fluid leak.