[sherlock9c1]

I'm with you - Sprag is rolled over and is only gripping intermittently. The violence you felt in 4th was the sprag rolling over and temporarily binding. If you have a fully binding sprag, 4th gear will bind. Same thing as if the overrun clutches are on all the time like when the forward piston cracks and leaks pressure into the overrun piston.

FWIW, I've experienced broken sprags that would still grip enough to give 1st and 2nd gear in OD under light throttle but would slip with anything more than that. Putting it in D, 2 or 1 activates the overrun clutches which mask the broken sprag problem.

 

[sherlock9c1]

Replace the whole sprag assembly, first off. There was at least one GM sprag housing redesign according to the ATSG 4L60E update manual. Beyond that, it's beyond my experience. I would talk to a well-regarded builder about sprag failures, like Dana at ProBuilt or Shane Cobb; maybe PATC or Finish Line Transmissions and see what they say.

Or you could have just ended up with one of those statistical oddities where yours failed for no good reason.

 

[sherlock9c1]

Gregg, what's wrong with the stamped steel forward piston set that you made a billet forward piston?

 

[sherlock9c1]

Why would you put more overrun clutches in? I know they will wear out quickly if you try to downshift without giving throttle, but is there a time you need this?