Good info on weak points of the 4+3 OD unit.
Here are a few bad points of the OD unit:
underengineered carrier assembly bearing; poor lube to the bearing; and bearing pre load issues.
direct clutch thrust washer breakage issue will cause the overdrive piston to jam in the case and burn up the unit. the thrust washer endures about 1100-1200 lbs of thrust load and the clutch pack needs to spin on the washer at the same time.
I stopped tinkering with these units about 20 years ago and GM was still updating parts for them in the mid 90s. Twenty years ago the OD unit going rebuild rate was about $1200.00. I see that Gary mentioned Brian (S&K Speed) as a connection, I wonder if his shop still rebuilds them? Last time I talked to Larry, he didn't touch them any longer.
Hot rodding is about trying different things but, talk to some of the Corvette guys that have these units.
Thanks for the comprehensive reply on the achilles heels of the OD unit.
1. Re Carrier bearing: I did read that there was an updated bearing available and I think that I saw an oiling modification procedure but don't recall for sure with all of the stuff that I read last week. I don't know if preload can be corrected with assembly procedure or not, but, yes, I do remember reading that as well, now that you mention it.
2. Re direct thrust washer: I also read that there was a better hardened washer available for this thrust washer. You wonder why they did not put a torrington in this position? Do you know if the upgrade washer is a decent fix, or was it still a band aid?
I do hear all of the non-specific things you are saying though about the unit still having various problems that were never developed out of it. Sometimes a certain design is just beyond hope.
After seeing one lower quality exploded view of the OD "planet" I see that Nash used a two stage radial pinion arrangement, instead of a true planet. I don't like radial pinions as they don't balance the radial thrust loads out like a true planetary gear, and it is almost impossible to have all the pinions carry the load uniformly. Do you recall if the pinion/sun gear arrangement was fundamentally failure prone, or was that part itself pretty durable?
They guy that has this one for sale claims that the OD section was working perfectly, but the first gear in the T-10 lost it's synchronizer. When idling, the gear would just grind when he tried to shift it into first from neutral. When he started the car in gear, first gear held fine, and he could shift out of it, but not back in. That sounds like a totally smoked blocker ring, but I have never seen that before in a manual trans. Usually the slider and the slider teeth on the 1st gear get destroyed. I did rebuild (and explode) a couple of Muncies about 35 years ago, but never tore into a Super T. I have rebuilt 50+ various Hydramatics over the years, so tearing down the OD doesn't scare me at all. Would it be plausible for the ST-10 to just have a fried blocker ring, or would this likely be something more?
My plan would be for this to be behind a 300hp 327, gasser type build, street driving not planning to beat it like a dog. Yes, I like the idea of a 2.88 first gear, with a 3.42 rear gear (9.3 Olds) and a .67 Overdrive, but if it is just going to come unglued on me even with moderate driving, I am probably just beating my head on the wall considering this. Thanks for all the advice.
Driver.
