anyone ever heard of this happening?

[TICK]

The 8.5" ring gear rear axle should never have been put in a full-size truck. It was designed in the late-60's and used in the early '70s in COMPACT and INTERMEDIATE cars. Full-size cars didn't get the 8.5 until engine power was dramatically reduced, and the full-size cars had been downsized into what had been "intermediate" size and weight.

The 8.5" axle assembly did get an upgrade in '88--'89, the axle shafts were made larger, which required different spider gears. Wasn't enough.

In addition, the 8.5" axle as used on GMT-400 pickups also got crappy leading/trailing shoe rear brakes of a mere 10" (254 mm?) diameter. The best thing you can do for your truck is to find a K2500 with 6-lug axles and the same gear ratio as what you have now, and swap the 9.5 axle in place of the crappy 8.5. The 9.5 axle is MUCH stronger, and has real (11.x, Duo-Servo) rear brakes as a wonderful bonus.

You'll need new U-bolts and a conversion U-joint on the rear of the driveshaft. The driveshaft will poke into the transmission about 3/4 of an inch more.

I did this years ago. Great upgrade!

i used the yoke from the 10 bolt, so no ujoint, or drive shaft issues.

 

[GoToGuy]

Yeah like my 95 K2500 xcab, sb 9.5 14 bolt, 6 lug, factory rallies in garage. Owned since new. If it has 8 lug wheels it's a 10.5 , 14 bolt, AAM

 

[TICK]

i should add, that i took out the offending tranny, gave it back. and built the snot outta my origanal tranny. i put every upgrade, used the red alto clutches, ext.ext. the important thing is while i still made a firm shift, the accumalaters are still in line to obsorb the shock. it works great. i feel like i now have a bullet proof tranny, and rear end.

 

[Erik the Awful]

Clean, crisp shifts with no overlap is good for a streeter. Leave the crash/bang shifts kits for the race cars.

Years ago I had the TH400 from my Cadillac rebuilt and converted to a short tail. I told the builder I wanted it to get the shifts done quickly, but I wanted it Cadillac smooth. He delivered, and that is my best description for how a street transmission should be built.

 

[stutaeng]

i should add, that i took out the offending tranny, gave it back. and built the snot outta my origanal tranny. i put every upgrade, used the red alto clutches, ext.ext. the important thing is while i still made a firm shift, the accumalaters are still in line to obsorb the shock. it works great. i feel like i now have a bullet proof tranny, and rear end.

Good to hear that. So you were thinking that the transmission shifting too hard caused the rear end to crack?

The tail housing geartrain endplay of these transmissions is around 0.015" or so. I can't imagine having this movement would damage the rear end, since there's a slip yoke that allows the driveshaft to "stretch." Was the yoke maybe binding? Was it perhaps a coinsidence or I'm missing something?

I'd think maybe a drum, shaft, sprag or bearing inside the transmission is what would fail if shifting is too harsh.


In any case, glad the problem was resolved.

 

[someotherguy]

Yeah like my 95 K2500 xcab, sb 9.5 14 bolt, 6 lug, factory rallies in garage. Owned since new. If it has 8 lug wheels it's a 10.5 , 14 bolt, AAM

Not true. Plenty of 8 lug 2500 GMT400's are 9.5 SF's.

Richard

 

[stutaeng]

Years ago I had the TH400 from my Cadillac rebuilt and converted to a short tail. I told the builder I wanted it to get the shifts done quickly, but I wanted it Cadillac smooth. He delivered, and that is my best description for how a street transmission should be built.

Well, you are only comparing possibly one of the best transmissions in modern times when it comes to those characteristics, LOL.

We had a C20 Custom (with a Buick 402?) my oldest brother drove with all our belongings when my family moved from CA to TX back in the early 90s. My Dad bought it from him, but eventually traded it for some drywall work on our house. I always remembered how smooth that truck shifted....fast forward to up until 4 years ago, the only thing our family had between my Dad, brothers and I in trucks were 1/2 tons with the 700R4 and 4L60e, and although reliable (for the most part), I don't think any one of them compared to the TH400 in that 'ol Chevy 3/4 ton the way it shifted.

That's why the dragster guys love that tranmission: easy to work on, can take a TON of abuse and the shifting can be made really aggressive with less risk of blowing it up. A 1,000 HP build only requires some internal parts swap. I've watched a video on a guy that rebuilds them and he mentioned they haven't found the upper limit on how much power it can handle, LOL. Obviously, with spendy aftermarket parts...

 

[Schurkey]

So you were thinking that the transmission shifting too hard caused the rear end to crack?

The tail housing geartrain endplay of these transmissions is around 0.015" or so. I can't imagine having this movement would damage the rear end, since there's a slip yoke that allows the driveshaft to "stretch." Was the yoke maybe binding? Was it perhaps a coinsidence or I'm missing something?

I'd think maybe a drum, shaft, sprag or bearing inside the transmission is what would fail if shifting is too harsh.


In any case, glad the problem was resolved.

The issue is not the length of the driveshaft, and the end-play of the transmission getting "too long".

The issue is the shock-loading at gear change: fairly high engine torque, slam-shift so that rotational inertia of the trans guts adds to the torque, plus the converter is probably forced into torque multiplication--all happening in a split-second. The shock of that instantaneous torque increase cracks the rear axle guts. Guys love that the shifts make the tires spin--sticky or heavily-loaded tires that don't spin, don't reduce the torque going through the axle assembly.

Yes, all the rotating parts are at risk from the shock load--trans, driveshaft, axle. Not so much the engine, as there's no geared or hydraulic torque multiplication there. Turbocharged engines can be at risk, though--the turbo gets spooled-up, then engine RPM drops so there's a pressure surge at the shift point. The pressure surge massively increases cylinder pressure--and therefore load on the pistons, rods, and crank.

 

[GoToGuy]

Some years back a gentleman in a parking lot couldn't understand how I have 2500 emblems on my truck yet have six lug rally wheels? Swap the doors? Fake emblems? No its' factory, just not as high gross weight rating. Oh well. As for Tubocharging a pressure regulating valve assembly would ensure an optimal effeciancy, and overpressure relief for pressure wave. Another design calls for a flat rated power increase utilizing smaller high effeciancy turbo, some small diesel engine designs use a version of this concept.

 

[smdk2500]

Not true. Plenty of 8 lug 2500 GMT400's are 9.5 SF's.

Richard

Yup have one sitting in my driveway.