1960 f100 4x4. I have a small leak coming from the rear differential seal. This being said I would like to be able to check the level of the fluid in the rear diff. There is no fill plug but there seems to be a flat spot on the third member that was almost meant to have a fill plug. Has anyone else ran into this problem? Is drilling a fill plug a difficult task?

 

Are you sure there isn't one? I think mine doesn't have a head on it, if I remember correctly, you stick a 3/8" ratchet (or extension) into it and turn it out. If that's what you have, it might be flush with the case and it might be caked in grease so it might be hidden. If so, you can always replace it with plug that has a head on it for easier removal.

 

I think some had the plug on the housing, some had it on the third member, so if changing the housing or third member can leave you without a check or fill plug.

 

How do you fill it without a plug? Pop out an axle and dump oil down the tube? Just curious.

 

Thats exactly what I had to do many years ago when I swapped third members in a 57 Ford passenger car. The later model third member didn't have a fill plug and car housing didn't have one either

 

That's what I've had to do a couple times now. A fill plug would be very helpful. I did swap the housing because it had snapped and if I remember correctly it did not have a fill plug either. The third member for the 59 I swapped with has a fill plug drilled in the very spot I was talking about. I don't know if this was factory or not. I actually called a machine shop how much it would cost to have them drill one and they estimated without seeing the project around 120$

 

I don't know why it would cost $120 just to drill and tap a hole.
I think you could add a plug to wherever you wanted it. If it were me, I'd probably do it in the back of the housing, just so I didn't mess up the third member...if I screw up a hole in the housing I can always weld it closed, but a new third member might get pricey. The metal on the back of the housing is probably too thin on its own to just drill and tap. I don't know that for sure, though. If it were mine, I'd probably buy a weldable filler plug bung (summit or speedway?) then drill the hole where it won't be in the way of any gears and weld the filler plug bung over that hole. If I were to go that route, I'd probably pull the third member first so I could set down some towels and really get inside and get any metal shavings from drilling out of there.
Then you'd need to add up the cost of the tap and the drill bit and your time and compare that to the $120, or find another price. I would make sure it was a tap specifically for pipe thread, so it should be sure not to leak all over...but then again, it's the fill plug so it won't leak once it's below that point anyway. Is the other third member in working condition? If so then you could swap that in, if not, maybe price a rebuild with gears you want and then you'll have exactly what you want.

 

I could probably do that. The other third member has different gears so I can't use that third member. I could have someone switch all the gears out but that would probably be even more expensive yet. I checked the oil yesterday and it looked more then full so I may just wait on the fill plug for awhile.