What is this and why is my TH350 choosing to leak fluid therefrom?

So, fixed the selector shaft leak a while back.  Next up is this leak on the tail of the transmission.  What is this part called and what do I need to make it stop leaking?

I think that's the speedo drive. Probably an O ring leaking 

Peabody said:

I think that's the speedo drive. Probably an O ring leaking 

Exactly what I just learned on Youtube University.  It does bring up a useful question:  What am I using for a speedometer at this point in time?  Hmmm.

There is an oring there, and a seal in the driven gear.  If the sleeve is steel, it will be fine to replace the seals.  The originals were aluminum, and would wear out, get loose, and leak.

Any old timey trans shop will have a box full of them.

Speedometer bullet.

It has a seal on the inside of it and an o ring around the outside of it, Furthermore, the plastic gear that spins within it will get grooved from the seal over time. Replacement is cheap and easy. Shotgun it all, with a new bullet and gear both. Amazon can usually have you the stuff in hand within 24 hours. 

Count the teeth on the gear and replace with a like part.

So, my speedo bullet looks like this:

I'm guessing any lump of metal the same size with an o-ring will fill the hole?  The gear has 19 teeth.  I counted it three times, thinking it would be an even number.  LOL.  Dunno if that's relevant since I have no idea what this transmission came out of, or whether the PO's speedometer was anywhere near close.  I might buy that package that has a bunch of different gears, so I can pick the one that's closest.  I suppose vehicle speed will be dependent upon rear axle ratio and tire circumference.

I pulled the dash gauge cluster and found that my speedo is a mechanical Pro Comp Ultra Lite.  I put it in so many years ago that I've forgotten what is was and lost the instruction sheet.  Sigh.  No worries, though.  I just need to order a bullet and a speedo cable, and it will be on to the next problem.

Building a car is hard!  I don't know how some of you manage to crank 'em out like baking cookies.

Those are the ones that wear out and leak...

Streetwiseguy said:

Those are the ones that wear out and leak...

Please be more specific.  Do you mean the one pictured above?  Or the ones that ship with six different gears?

The aluminum sleeve itself wears where it mounts into the transmission.  The replacements are steel, and round all the way through the mounting area.  Like I said above, if there is an old timey trans shop anywhere near you, there is likely a dusty box full of the new ones there.

Speedometer cables are almost always 1000 revolutions per mile, or 1000rpm at 60mph.

So you find your driveshaft RPM at 60mph (tire rollout, gear ratio) and then you use the drive and driven gears to convert that to 1000rpm.

Okay, so rear tires are BF Goodrich Radial T/A size 225/60R15.  Circumference is like 80.5 inches.  Rear end ratio is 3.08.  What speedo gear do I need?

There are on-line calculators to help with this chore.

https://www.tciauto.com/speedometer-gear-calculator?srsltid=AfmBOoqSd951di5YGHoWkSWGg0F6AXc75VIiej9Bj5F1xTVqt11-Tgao

Don't even need one!

Per Tire Rack's website those tires run 813 revs/mile.  813 times 3.08 is 2504 driveshaft revs per mile.

To know the speedo gear you need, you also need to know the drive gear, which is a pain to figure out if you don't have the tailhousing off, but doable.  Fortunately we can guess since most cars with that trans had about that revs/mile so you should be in the ballpark and not have something weird.

18 divided by 2.504 (the drive ratio you need) is a little over 7.  17 divided by 2.504 is a little less. 18 is closer by a smidgen, but they don't make a 17.5 tooth gear   So, you probably could make do with either if you have a 7 tooth gear on the tailshaft.

If you have an 8 tooth gear, a 20 tooth speedo gear would be perfect.  If the trans came out of something with taller tires and/or 2.lol gears then you probably have an 8.

My suggestion, if you'd like to hear it, is run a cable with what you have now before you chase any parts.   If the odometer reads low relative to mile markers, like reads 18 miles over 20 miles driven, then you need to find a 20.

So, this car hasn't run in 15 years.  Every major system has been either replaced or rebuilt.  I've got a new bullet, gear, and cable coming.  If I need to buy one more speedometer gear to get it right, well that's okay.  For now, I'll be happy if it stops dripping fluid so I can finish up under the car and get this thing running.

Is the bullet gear dependent? I know they are for Mazdas.  I don't know for that trans.

I had to hunt down a GTUs gear holder for my FB after some rearend shenanigans.  And it always leaked into the drive cable because I forgot a seal,  so all of my right socks had permanent blue stains from Shockproof dripping out of the speedometer.

Pete. (l33t FS) said:

Is the bullet gear dependent? I know they are for Mazdas.  I don't know for that trans.

Nope.