Rear Diff Plug HELP

To All,
Welding square bar was quite easy and uneventful. The plug backed out with little force. The penetrating oil and heat combined with blasting air on the plug to "contract" it all contributed.

See Photos

The inspection hole appears non-tapered. The replacement plug (ordered from supplier) threads in but after about 3/16" to 1/4" I meet significant resistance.
When I compared it to the drain plug a noticed a stark difference.

The drain plug is tapered!


The old inspection plug does not appear tapered, but it is very short.

Please help?

Does anyone know if the housing is tapered for the drain or inspection plugs?
Could one be tapered and one be non-tapered?

I did not chase the threads. If they are tapered I can ruin the threads in the housing. None of the suppliers have tapered plugs. The are all non-tapered or cylindrical.

Should I just install teflon tape or orange Loctite and reinsert?

All the threads look good, both on the plugs and and housing.

A couple of posts said to wash the housing out with kerosene and refill. Is that necessary? The fluid was dirty but not congealed.

Please advise.

Diastole
 

Attachments

  • IMG_2879.jpg
    IMG_2879.jpg
    47.4 KB · Views: 64
  • IMG_2883.jpg
    IMG_2883.jpg
    45.7 KB · Views: 61
  • IMG_2884.jpg
    IMG_2884.jpg
    48.6 KB · Views: 64
  • IMG_2889.jpg
    IMG_2889.jpg
    39.8 KB · Views: 64
  • IMG_2895.jpg
    IMG_2895.jpg
    60.8 KB · Views: 68
  • IMG_2896.jpg
    IMG_2896.jpg
    69.1 KB · Views: 65
  • IMG_7897.jpg
    IMG_7897.jpg
    25.1 KB · Views: 63
  • IMG_7898.jpg
    IMG_7898.jpg
    36.9 KB · Views: 65
  • IMG_7899.jpg
    IMG_7899.jpg
    34.5 KB · Views: 61
Y-Blockhead,
Help me understand, pipe threads are tapered threads? Is that right?

I am new to all this.

The inspection plug I removed was non-tapered and ALL the suppliers sell only non-tapered plugs.

https://www.snydersantiqueauto.com/product.htm?pid=978061&cat=41823
https://www.brattons.com/DIFFERENTIAL-DRAIN-AND-FILLER-PLUG/productinfo/5920/

In this "new car to me" and my first Model A, the drain plug I removed is tapered. See last photos, the plug tapers in the calipers.

So, what do I do?

The only replacements are non-tapered. Do I reuse the old drain plug because it is tapered.
And, since the old inspection plug is now welded onto a piece of square bar, I have to replace it. All the replacements are cylindrical, non-tapered.
But, it looks like the plug I removed was non-tapered, but it is so short, I cannot be certain.

Diastole
 
The plug you link to from Snyder's states in the description "The original style. Can be used to fill and drain. This style of oil plug is 1/2" pipe thread."

Pipe thread is tapered thread.
 
yes, pipe thread is tapered. if you do run a tap in there, only put in a little ways. only enough to chase the threads. not cut them any deeper.
 
Wow, thanks for the help.

So, I can use the new plugs. Screw them in until snug. If they sit proud, with threads sticking out, above the level of the housing, that will be okay. Better than too deep

I appreciate yall
 
* pedant mode on *


sorry for being a bit anal and, hopefully not, confusing the original poster...


Not all pipe threads are tapered, that is plainly wrong. For instance most (1) female pipe threads are parallel, that is the case in NPT, NPS, BSP, G and R. But you also have parallel male threads, like G or NPS. It depends on where you want to seal the thread, within the thread itself or on, for instance, a shoulder with a sealing o-ring.


Sorry for the rant but this is my pet topic...



(1) i say most, because there are also tapered female threads, like Rc.


* pedant mode off *
 
Last edited:
The non tapered threads are used in Europe and the plugs have a flange and a gasket is used. All the pipe threads in the US are tapered. The threads can be cut too shallow or too deep, and that will determine how far in the plug will go in. The threads on the plugs can also vary, which will also determine how far in the plug will go in. You could try different plugs. If you have access to a pipe die you could recut the threads on the plug.

If you chase the treads in the housing, only do a little bit at a time and then test the plug. As I suggested earlier, use Teflon tape. You can use the white or yellow tape. The tape will act as a lubricant and a sealer. Make sure none of the tape extends to the end of the plug or beyond so that the tape does not end up in the differential.

If you chase the treads, make sure you get all the chips out of the housing. Use a solvent wash with the drain plug open. Inspect with a dental mirror or bore scope.
 
Last edited:
Careful you all ! Not all American pipe threads are tapered, some are parallel pipe threads, now you all have me wondering ? are the female holes in the differential straight or tapered pipe threads ? Just maybe that’s why hardware store pipe plugs don’t fit well in my differential and transmission ?
 
B H, If the threads in the differential housing were straight, then the plugs would have a flange and a gasket would be used. Ford used a normal tapered pipe plug. The straight threads could also have a different TPI, depending on the size.
 

Attachments

  • Screen Shot 2024-06-14 at 10.00.24 AM.jpg
    Screen Shot 2024-06-14 at 10.00.24 AM.jpg
    14 KB · Views: 24
Last edited:
You can get one like this at Home Depot. Easy to remove with the male end.
 

Attachments

  • pipe plug.jpg
    pipe plug.jpg
    55.7 KB · Views: 4
Y-Blockhead said:
The plug you link to from Snyder's states in the description "The original style. Can be used to fill and drain. This style of oil plug is 1/2" pipe thread."

Pipe thread is tapered thread.
Click to expand...

Not all pipe threads are tapered, NPT is tapered and NPS is straight. Typically, pipe threads are NPT (tapered), but one has to be careful.
 
Last edited:
There are straight pipe threads, think electrical rigid conduit.
NPSM – free-fitting mechanical joints Thread

NPSM is a lesser-known pipe thread standard series defined in ASTM B1.20. This thread series creates a non-pressure tight connection. It is mainly used in mechanical joints and special applications with no internal pressure and, consequently, no sealing requirements. It is possible to achieve a seal with the aid of an O-ring.
 
The taper for American National Pipe Thread is very slight and isn't measured in degrees, Its measured by length/diameter ratio. The two small sizes have 27 threads per inch so it doesn't match with normal standards for screw thread, The intermediate sizes are similar to screw threads at 18 and 14 TPI but they taper that little bit over the length/diameter ratio. At 1-inch they change to 11 1/2 TPI so they are odd.

There are so many different types of pipe that it was impossible to come up with a single thread type. I use a lot of AN type pipe thread for aircraft and most are straight but they use a 37 degree flair in the tube if using aluminum aircraft tube or aircraft hose fittings. There are AN fittings that are NPT on one end and AN on the other so that they can mate well with castings.

I've had to Heli-Coil buggered NPT threads in engine cylinder castings before and it requires a lot of care in drilling or reaming a bore for the tap to cut the Heli-Coil threads. There are tools to check for proper depth of cut and fit. A certain amount of clean up and dressing can be required to get a proper fit.
 
Last edited:
Diastole said:
I am new to Model As. I have a early '30 coupe.

I want to change the diff fluid. This car sat in a garage since 2015.
Owner recently passed, sadly. Rest his soul.

The fill-drain-plug has half of the lugs sheared off. I am looking for some help for a possible solution on how to remove it w/o damaging the housing.

See photos

I can try some vice-grips after heating the metal housing. Or, drill a hole, tap with left hand threads, and install a left hand thread bolt. Or weld a 3/8" square bar in what remains of the plug.

Being Model A's are new to me, and my interest to not reinvent the wheel, I am asking for any help or suggestions I can get.

I can do two of the three thoughts above, I do not have left hand tap and die set.

Thanks in advance

Diastole in the Smoky Mountains
Click to expand...
Any Results on removing Plug??
 
You must log in or register to reply here.
Back
Top