Do you mean you stripped the 3/8" internal drain bolt housing? Are you sure that you were attempting to loosen it, and not tighten it? Depending on how bad it is stripped, you may be able to rent an electric impact wrench with 3/8" tip, and get it loose with the car jacked up enough and some finesse. We have Sunbelt Tool Rental in my area, and even if they only have a 1/2" drive tool, you could buy a 1/2-to-3/8 adapter. Haven't seen your 'damage', but that's what I would try, and surely what a shop would try if you took it to 'em. I would seriously suggest soaking it several times through the day prior to the next removal attempt with PB Blaster or equivalent. As mentioned before, also make sure that you've cleaned out the inside of the drain bolt to remove any grease and gunk that would keep your removal tool from seating all the way and not slipping as much. *So after soaking with PB Blaster or equivalent, prior to final attempt at loosening, maybe spray well with some brake cleaner or equivalent. Just a thought. And definitely 'tap with a hammer' to seat it firmly as advised before.

 

If a 3/8" drive will no longer work, you are down to the visegrip and hammer method. Don't know if a visegrip will fit over the bolt or not.

 

Vise-Grip method sounds worthwhile. I would definitely soak bolt threads multiple times prior to breaking it free. You won't have any leverage, so you'll need all the help you can get when using the hammer.....a real hammer, not a rubber mallet.

 

Originally I had lightly tapped the 3/8” extension into the transmission drain plug. I thought it was properly seated. In fact it had only gone in a little and while trying to loosen the plug for a couple of minutes it slipped out enough and the force I was applying rounded out the outside face of the drain plug.

I took a small rubber mallet and tapped the extension in until it seated properly. Great I thought, but after nearly wearing my self out trying to remove the plug I gave up. I already sprayed it with WD40 and PB Baster over days. I have a large 3/8 ratchet, bigger the standard ratchet but is not provided enough leverage or that plug is really super tight. I’ll be picking up a breaker bar.

A silly question, but this transmission drain plug does loosen to the left or is it different on Hondas? Damn that thing is on tight.

 

It's a normal right-hand thread just like any normal hardware-store bolt.

It's really tight, but when it lets go it becomes loose very very suddenly. Don't knock the car down on top of your arm.

 

I don't think WD40 or PB blaster will get past that crush washer. I would clean that stuff off, with some brake cleaner, so it doesn't make the breaker bar drive slippery.

 

I've had mixed results with some of the "freeze off" type stuff. Not sure if anyone else had used this type of stuff before and what their results where. Just thought I'd suggest that as another method to help get that out.