'51 O/D question

[Shoebox]

So, as my '51 project continues, I'm filling the O/D trans with fluid and I want to do the right thing. The shop manual says (see pic) well there is two drain points and two fill points, I'm only seeing ONE drain point, so what does a guy do?....Thanks!

 

[petehoovie]

So, as my '51 project continues, I'm filling the O/D trans with fluid and I want to do the right thing. The shop manual says (see pic) well there is two drain points and two fill points, I'm only seeing ONE drain point, so what does a guy do?....Thanks!

 

[38 coupe]

In all the overdrive transmissions I have seen from the era there is a drain plug on the bottom side of the transmission case, and a drain plug in the adapter casting between the overdrive and the transmission.

 

[Mac VP]

I’d try to take some pictures for you but I’m not at the shop today. On the passenger side of the tail housing you can easily see the filler plug (with the square head for a removal by a wrench). To the right of that, and down low near the casting for the motor mount, is the drain plug. It’s smaller than the filler plug and has a female hex opening for an allen wrench. Often its socket is full of old grease and road dirt, so you’ll have to use a pick to clean that stuff out. I forget the hex size….maybe 1/4”. Don’t try to get a 90 degree allen wrench in there. We use a short allen that is made with an adapter for a 3/8” socket drive. You have to attack it straight on. This is normally adequate to loosen and remove the plug. Once in a while we’ll find that the plug is rusted in place and the allen wrench will strip out the hex socket of the plug.

It’s a pain in the rear end to get it out now so you can choose to leave it in place or end up pulling the extension housing off the transmission and using heat or drilling it out etc. The oil sump in the overdrive section is common with the front section and will drain out through there as well…..just not as completely or quickly as it would through its own plug.

If you are successful in removing the plug, it should allow the gear oil to flow. If nothing comes out, try poking a pick tool in the opening in case some internal debris is blocking the drain. If nothing comes out, I’d be very concerned that the overdrive gearing has been running dry and may be damaged.

 

[wga]

There is a plug that takes a Allen wrench to remove. On the passenger side as you look
to the left of the larger fill plug, it is near the area where the cases join.
It is a mess to refill. Obviously when the transmission is out, it is easy to see. The plug is slightly recessed into the case. I used a long end of the allen wrench to remove.

 

[Shoebox]

I’d try to take some pictures for you but I’m not at the shop today. On the passenger side of the tail housing you can easily see the filler plug (with the square head for a removal by a wrench). To the right of that, and down low near the casting for the motor mount, is the drain plug. It’s smaller than the filler plug and has a female hex opening for an allen wrench. Often its socket is full of old grease and road dirt, so you’ll have to use a pick to clean that stuff out. I forget the hex size….maybe 1/4”. Don’t try to get a 90 degree allen wrench in there. We use a short allen that is made with an adapter for a 3/8” socket drive. You have to attack it straight on. This is normally adequate to loosen and remove the plug. Once in a while we’ll find that the plug is rusted in place and the allen wrench will strip out the hex socket of the plug.

It’s a pain in the rear end to get it out now so you can choose to leave it in place or end up pulling the extension housing off the transmission and using heat or drilling it out etc. The oil sump in the overdrive section is common with the front section and will drain out through there as well…..just not as completely or quickly as it would through its own plug.

If you are successful in removing the plug, it should allow the gear oil to flow. If nothing comes out, try poking a pick tool in the opening in case some internal debris is blocking the drain. If nothing comes out, I’d be very concerned that the overdrive gearing has been running dry and may be damaged.

Thank you for the response Mac. I could have asked this question directly to you as this trans is one you built for me. So, with no need to drain anything, how much of the 4 1/2 pints should I put in the main fill and how much in the OD fill hole?

 

[Mac VP]

I’m not sure how much gear oil lives in the overdrive section when properly filled. I suggest pouring about a half quart into the OD section as a prefill measure. The rest of the filling process (and any further topping off) should be done through the side filler plug in the front case. Leave the transmission level for at least a half day in a warm environment to allow the gear oil to settle in and level out. Check and top off as needed through the main filler plug in the front case.

 

[Shoebox]

I’m not sure how much gear oil lives in the overdrive section when properly filled. I suggest pouring about a half quart into the OD section as a prefill measure. The rest of the filling process (and any further topping off) should be done through the side filler plug in the front case. Leave the transmission level for at least a half day in a warm environment to allow the gear oil to settle in and level out. Check and top off as needed through the main filler plug in the front case.

Makes sense, Thanks Mac.