what's the weakest link in a th400?
 

I know TH400's are beefy trannies, but everything has it's limits. When a 400 decides to let go due to too much power, what's the most common thing that breaks or burns up? I'm thinking of rebuilding my trans with TCI's rebuild kit but what else should I upgrade while It's apart? I don't really have a hp goal, but whatever it takes to get my truck in the low 9's.

Btw, I was told it was a hd th400 out of a huge truck, if it makes a difference.

I would say the intermediate sprag and then Intermediate shaft .

Intermediate sprag is a known weak point and is the weakest hard part.
Next would be the forward hub and/or input shaft to drum.

The intermediate shaft is not a weak point IMO. I have TH400s that have ran 7 second 1/4 mile times with a stock intermediate.

The sprag is the biggest failure I have personally seen? Jake was is the pros and cons of goig sprag less?

Spragless is a good solution for a combo that's north of 1500 HP or some offroad applications.

I use a spragless VB for some of the offroad stuff where they are going to be landing on the throttle in 2nd gear from a jump.

The 34 element sprag that is standard in the industry works fine, even at power levels above 1000 HP as long as the operator is knowledgeable and understands how to do their burnouts and some of the things they cannot do while driving.

The larger 36 element sprag that we call a Super Drum. We make our own cast iron version of this setup. It's good to the 1500-1800 range or more. Above that we recommend spragless or a 2 speed TH400 setup.

Thanks jake. Heard of ppl flipping the sprag after the burnouts and such, but I still row my own gears :)

So can the input shaft/hub and sprag be added to the trans without machining work to the case? Can they just be swapped out?

Yes,
The input shaft is typically an assy with the forward drum. The ones we have made have a rebroached drum, and the input shaft spline to drum interface has much more area to prevent failure.

The hub doesn't require any machine work unless you want to rollerize it and even then the machining can be done elsewhere.

The 34 element spray can be installed on an early 400 drum or 4L80E drum. The Super Drum doesn't require any case machining, just special clutches and close check of some clearances.