Technical TH350 leak..drive gear

I've got a TH350 behind a 383 stroker in my 64 f100. It was leaking like a sieve from the speedo cable bullet that the cable plugs into. The bullet itself is fine, it was the drive gear that had a groove near the end of it allowing fluid to pour out. Very easy replacement but my question is this....

What causes the drive gear to get the groove? Do they just wear out over time or could there be an issue where I'm replacing drive gears constantly?

Thanks

 

never seen one grooved , but anything can happen. put in a new driven gear , outer O-ring and the inner seal and you should be good

 

They get grooved, yes, it's pretty common. It has to do with being 40 years old, I think. Oh...and the rubber seal gets hard over time, which doesn't help.

You should not need to replace it again, unless you plan to put another 100k miles on it over the next 15 years.

 

Thanks squirrel. I was hoping it was just a case of being old and worn. I went ahead and bought the bullet and the gear to cover all bases. Parts should be in today!

 

Just for reference in case anyone was confused by my question. The one on the right has a groove worn very close to the tip. The blue one on the left is brand new.

 

I have seen TH400 gears worn like that, but it's not very common on the 350 gears...only a few of mine are worn, and not that much. Thanks for the pic

 

While you are at it, I suggest you replace the gasket for the shifter shaft. I found that there were 2 installed (one on top of the other)in my 350 as there is room for 2 make sure you install it without nicking the rubber seal when you install the new one

 

FYI some TH 350's had a "bullet" speedometer driven gear fitting made from cast aluminum. Those could and would wear the shank bore into an oval shape, once that happens the fitting is junk. If you come across one of those replace it with a steel replacement, the GM part number is 345215. Also would recommend replacing the gear at the same time as the shank of the gear may have scoring on it.

If you have one of the larger ones similar to the type used on TH 400's, disregard this part number. Those TH 350's were Buick Motor Division produced transmissions, otherwise same as the other 350's.