Stripped transmission drain plug - Ford 4100

[Xfaxman]

Tap drill for a 3/4-14 is a 29/32" drill. Depending on how well you get a 1/2" drill centered in the plug, I would then drill it to 7/8" and pick out the remainder of the plug, clean threads, and install new plug. If you are off center, you can still stay undersize so you don't damage the threads, then tap out the remainder using a punch.

If your plug is 3/4=6/8 how can you you stay undersized at 1/8 over the plug size? I believe there is some math error going on!

Pipe size is the inside diameter of the pipe. It is a 3/4 pipe plug. Here is the chart that ruffdog linked above:

And fraction to decimal:

 

[CoyPatton]

Pipe size is the inside diameter of the pipe. It is a 3/4 pipe plug. Here is the chart that ruffdog linked above:

View attachment 671800

And fraction to decimal:

View attachment 671801

You folks are putting a whole lot of trust in centering a drill bit. Yes the inside of the pipe is 3/4” and then the wall thickness of the pipe of which if truly centered you have taken up 1/16” of that wall thickness. But if you are 1/8” off center (or more) just how much will you be away from those threads? Besides the goal of the OP is not to install a plug but to get the old one out and save the threads in the pan!
Personal experience, threads drill much easier than solid metal and a variation on perfectly centered and perfectly straight working under a tractor indicates some bad advice!
Try a left handed frill bit and it might grab a back out the plug. Better is to drill it in progressively larger sizes unsung a taperer extractor and avoiding threads altogether.
So many things to try before suggesting to drill and tap and damaging the existing threads when it is totally unnecessary.

 

[fried1765]

You folks are putting a whole lot of trust in centering a drill bit. Yes the inside of the pipe is 3/4” and then the wall thickness of the pipe of which if truly centered you have taken up 1/16” of that wall thickness. But if you are 1/8” off center (or more) just how much will you be away from those threads? Besides the goal of the OP is not to install a plug but to get the old one out and save the threads in the pan!
Personal experience, threads drill much easier than solid metal and a variation on perfectly centered and perfectly straight working under a tractor indicates some bad advice!
Try a left handed frill bit and it might grab a back out the plug. Better is to drill it in progressively larger sizes unsung a taperer extractor and avoiding threads altogether.
So many things to try before suggesting to drill and tap and damaging the existing threads when it is totally unnecessary.

I might first choose to use a left hand drill bit, on a job like this.

Largest size I have is 1/2", but if that didn't work you could then use a large (1/2") extractor.

 

[FarmaBro]

If it's stuck enough that a 1/2" square drive chewed up the hole, I don't expect any type of extractor device to work on this.

I suggest you drill a hole in the plug just to drain the fluid. Next heat with a torch until red then spray it with the hose. Thermally shocking could break the thread loose. Finally, weld a steel bar to the plug and turn it with a pipe wrench. Try to find a good balance between a fat bar and large weld area.

In the interest of safety, maybe you don't want to heat it so hot. Vaporized transmission fluid could explode. Maybe someone else can chime in on that possibility.

One safe way to do this would be to remove the pan and take it to the bench.

 

[CoyPatton]

I might first choose to use a left hand drill bit, on a job like this.

Largest size I have is 1/2", but if that didn't work you could then use a large (1/2") extractor.

Yes! I would do that long before drill out at a size to retap threads. Your chances of being exactly centered on drilling would be slim to non-existent in my opinion under a machine with less than machinist tools!
If the extractor does not work, try a chisel and hammer driving the plug to unscrew. The plug is destroyed so beating on it with a chisel is a non issue. The hammering effect may well break the corrosion. With care this cam be done with just a small clearance to the threads. Once moving, other tools can be used to remove the plug and not damage threads.

 

[Jstpssng]

If it's stuck enough that a 1/2" square drive chewed up the hole, I don't expect any type of extractor device to work on this.

I suggest you drill a hole in the plug just to drain the fluid. Next heat with a torch until red then spray it with the hose. Thermally shocking could break the thread loose. Finally, weld a steel bar to the plug and turn it with a pipe wrench. Try to find a good balance between a fat bar and large weld area.

In the interest of safety, maybe you don't want to heat it so hot. Vaporized transmission fluid could explode. Maybe someone else can chime in on that possibility.

One safe way to do this would be to remove the pan and take it to the bench.

Yours is the best suggestion yet, although I would be worried about warping or otherwise compromising the pan before it got as hot as you suggest.

 

[FarmaBro]

Yours is the best suggestion yet, although I would be worried about warping or otherwise compromising the pan before it got as hot as you suggest.

If it is as some suspect, that a tapered plug was forced into a cylindrical hole, the threads are already ruined. Our friend will likely be buying a helicoil kit regardless.

You are right that he should be wary of damaging the casting by heating. Beware Bob.

 

[boatmoter]

you can chance it by welding a nut or a bolt on to the stripped plug,let it cool then wrench it out

 

[radios1]

is the pan easily removable?. you could drill a hole in the plug to drain the fluid, remove the pan and bring it to a machine shop to remove the plug.. I'd use anti-seize compound on the threads of the new plug, and not make it too tight!..

 

[Other Bob]

Sorry I've been AWOL, been working a lot of hours and busy on other projects. I did find a picture of the plug from before I started trying to get it out with the extractors, it's round now. . I think I've given up trying to get it out for now. I had the thought of just getting an extractor and get out as much old fluid and water as I can before a heavy freeze sets in. I've never used an extractor before, seemed like it would be cheating or at least not sufficient. I think if I run it a bit and let the fluid and water mix and warm up I'll get most of it out, at least enough it won't crack anything this winter.

About a day after I got the idea this video poped up on my youtube home page : This Sucky Tool Has Saved Me 100s of Hours!!! - YouTube

It looks like it will work pretty slick, I ordered one the other day and it should get here next week. I'll update with the results.