39 transmission questions

LeroyM

LeroyM

Senior Member
Couple questions on a 39 transmission. How much gear oil does it hold? Mine has a leak at the drain plug, I took the plug out and wrapped it with pipe tape but it still leaks about a teaspoon or so every couple days. I always fill it thru the shifter tower because it's in a Model A coupe and it's hard to fill it from the side plug. When I drained it today I had about 3 pints in it, was that to low? Did all of those old transmissions leak or sweat at the drain plug?

Also what gear oil weight should I be running? Currently I'm using 75-90w.

Any help would be appreciated.

Thanks.
 
Leroy, those plugs are pipe threads and shouldn't need sealer of any kind. Not sure exactly how much goes in it, but use SAE 140 oil, not your 75-90 w. and you'll lose less oil. Also, fill only to the side plug, where it will run out if overfull. Much easier than through the shift tower anyway: Use a small pump bottle with a short length of clear tubing on it.

By the way, if the side plug is so hard to get at, what do you do about your U joint zerk? Out of sight-out of mind doesn't cut it.
 
The Sales Manual for 1940 shows 2.5-3 pints as the required lubricant capacity for the transmission. It sounds like you're in the ballpark at 3 pints. I add 3 squirts of Lucas HD Oil Stabilizer once a year to my 49 Merc O/D transmission to stop weeping thru the solenoid. I've also had good luck using NAPA "Never Seize" on threaded connections that are prone to leakage. How or why it works to stop leaks I have no explanation since that isn't what its designed to do.
 
If your drain plug is leaking either the drain plug is damaged or the casing is cracked. Closely examine the plug. Try swapping the drain plug and fill plugs. They are the same (I think).

Mart.
 
NPT threads do require sealant so they do not leak! Teflon tape is not a sealant, although it is often used that way. suggest you check at Home Depot they carry small containers of pipe sealant (paste) for gas. drain the transmission and clean the threads on the transmission and the plug first. Make sure there is not a crack in the transmission case.
 
Not to change the subject but is there an easy way to determine if I have a 39 rear end that is a 85 series or the 60 series. It is under my 32 and mates to a 33 trans. that bolts to a 48 merc Any wisdom before I start laying on that cement is appreciated.
 
My 38 three speed is said to hold 3pints. If filling from side it begins to run back out at that amount. I guess that is to protect us from ourselves.
 
Thanks for all the replies. I'm going to drain it this week and put NPT thread sealant on the plug and switch to 140w gear oil and see how that goes. Will check for a crack to but I'm almost 100% sure there is none since I went thru the transmission about 2 years ago.
 
Terry said:
NPT threads do require sealant so they do not leak! Teflon tape is not a sealant, although it is often used that way.
Click to expand...

NPT threads ARE designed to seal on their own. Trouble is that sometimes, significant deformation of the threads is required to reach that point and friction might prevent them from getting to that point. Teflon tape primarily acts as a lubricant to allow that to happen. There are some products such as Loctite PST that act as a lubricant and then, the nature of the product causes it to "set up" in the absence of air and fill the remaining voids or leak paths in the threads. It allows for fairly easy dis-assembly but should be cleaned off and replaced when removing and replacing a plug. By the way, whenever tightening NPT threads, particularly in iron or aluminum castings, be careful not to over tighten or you might break the casting!
 
Hey Bob: Get a new plug, standard 3/4 npt that's( National Pipe Thread ) find a shop that has a 3/4 inch pipe tap, and clean out the threads in the case, run it in by hand and then one half more turn, make some new threads, JUST a taste....With a new plug, tape / or sealant, if you must ?? do not over tight, snug is good, don't let your brother in law under there with a 12 inch pipe wrench, it should seal.... OLD....BILL
 
I can't envisage the exact setup, but if you are using a tap to refresh the threads, make sure it isn't trying to wind in against the cluster gear. It may be that the hole is clear of the gear, but without checking it cannot be assumed you can just wind the tap in without touching the gear.

Mart.
 
I wouldn't use a tap to clean up the threads. NPT is taper, and you could run the tap all the way through destroying the taper, then you'd never have a seal. Use a little pipe dope if you must, but that heavy oil won't leak as a water or gas line would anyway. Besides, if a Ford doesn't leak oil, it's empty.
 
Just thought I'd post an update. I drained all the old 90wt gear oil out and put in fresh 140w gear oil. I checked for cracks around the plug and didn't see any. I went and got a new roll of yellow teflon tape that's made for gas and oil and made two wraps around the plug. The 39 transmission holds exactly 3 pints and will run out of the side plug once you hit 3 pints. I used a long adjustable funnel this time to fill it.

I placed a pan under the drain plug and checked it a few days later and still had a tablespoon or so of fresh gear oil. After crawling under the car I found that my leak was not coming from the drain plug but was actually running from the back of the open drive tail shaft down to the drain plug and dripping off.

So my next project is to replace the tail shaft oil seal. It's always something. :(
 
expavr said:
The Sales Manual for 1940 shows 2.5-3 pints as the required lubricant capacity for the transmission. It sounds like you're in the ballpark at 3 pints. I add 3 squirts of Lucas HD Oil Stabilizer once a year to my 49 Merc O/D transmission to stop weeping thru the solenoid. I've also had good luck using NAPA "Never Seize" on threaded connections that are prone to leakage. How or why it works to stop leaks I have no explanation since that isn't what its designed to do.
Click to expand...

Just a thought regarding "Never Seize" on a tapered thread, the thread now has very little friction so it becomes a wedge because it is tapered, so be careful with Never Seize on tapered threads into cast/forged trans cases. I use pipe sealant on tapered threads and Never Seize on straight threads. Just my thoughts. Paul J.
 
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