I like to weigh in.I’m all about the gear vendors.That TH-400 and gear vendors is so much more durable than the 4L80.The only reason I haven’t went with one in my own pickup is the ground clearance.However,on a 2WD,I think it’s a non issue.
A 4L80 IS a old school 400. Everything from the forward clutch back is identical between a 400 and 4L80. There is a overdrive clutch pack in front of the forward clutch (very similar to a Ford 4R100 or GM 2004R trans). Other than that, most of the 400/4L80 parts are interchangeable.
Since GM stopped using the desirable sprag style direct drum in the 400 in 1971, the vast majority of 400 have much weaker roller clutch direct drum that can’t be upgraded. GM decided to bring back the tooling for the good sprag style drums for the 4L80, and many of the later model years came factory with a 34 element sprag. With that in mind, I’ll argue the 4L80 is more durable than the majority of 400 that people think are so indestructible. With some cheap hydraulic upgrades, a stock 4L80 will handle far more power than a late model 400 would.
As for junkyard transmissions, a little detective work and common sense goes a long way. Before doing anything, pull the pan. If the pan is full of trans and burnt fluid, look for another. Ideally, find one from a wrecked truck. If the trick was on the road when it was wrecked, chances are good it’s ok.
I recently bought a 4L80 out of the core pile from my local salvage yard for a 100 bucks. I needed a good case to fix a trans for a buddy that dropped his snd cracked the case. The core came out of a truck that caught on fire, there was so much melted plastic and undercoating on the case I couldn’t be sure exactly what year it was. When I took it apart it was absolutely flawless. Since the case connector was melted, I replaced the internal wiring harness, the front and rear seals, and my buddy used it instead of swapping cases.
Lastly, you could always pay a local trans shop a couple hundred bucks to open up a used trans for inspection.
