I have heard how bullit proof the gear drive t'cases are for some time, I have a 21 spline case on the way for my 4.3 vortec/700r4 swap. I am running 33's and LSD diffs with stock burfs. I will never run larger than 33's
Do I really need to upgrade the 21 spline input to the t'case to a 23 spline?
Is it common for the 21 spline to fail?

 

23 spline input upgrade is only good if your tranny is 23 spline. From Marlin's web site:

"Accepts 5spd transmission outputs of R150F (3.0L V6) and R151F (2.4L Turbo I4)."

So to answer your question. I've never heard of or seen on the web (read about) anyone stripping their T-cases. Pirate would be your place to ask this question me-thinks, they've got a lot more guys throwing big motors infront of Toyota drivetrain than Yotatech does.

However, I gotta ask, why keep the Toyota drivetrain? I don't know squat about domestic tranny's/tcases but I'd imagine with that 4.3L , you might check a Toyota FJ40 forum ... dunno of any but im sure theres gotta be one out there! ... Since the 4.3 Vortec is a extremely popular swap in the 40's!

Cheers!

 

Marlin sells the gear you will need to upgrade your trans to the 23 spline, but its really not necessary.

With a 4.3 vortec, if you make enough power to snap a 21 spline, you are going to span that 23 spline just as easy IMHO.

 

People run a 21 spline input on the rear of a dual case setup all the time with no issues... hell A LOT of people run 21 spline outputs behind the dual cases.

You ain't making that much torque, EVER!

 

Thanks for the input guys, I did alot of research last night using the typical words " failure, 21 spline, trail damage...." and found many parts that failed but not one single t'case input shaft failure.
I have not ordered the adapter kit yet so I could have ordered it for a 23 spline and bought the stronger input shaft.
I think stronger is always better but based on this info and the fact that I run 33's with an auto tranny and LSD's I can think of better places to spend my $$ than upgrading to a 23 spline input shaft. the stock 21 spline should do the trick and with the extra cash I can buy some new shifter boots.

 

Note - the heat treated cromoly 23 spline OUTPUT might make sense with your setup if you're gonna run duals.

 

I'd worry about stock birfeilds long before the 21spline. They'll be your weakest link. Will also depend if you wheel by throttle. New a guy with a 350 driving his 21 spline with no t-case issues. All depends how you wheel.