The absolutely cheapest and easiest way to make a long lasting repair on a stripped out oil pan

wr69 was kind enough to send me his stripped out oil pan, which I received.
The threads of the hole were stripped, as he posted in his photo. He did not send me the deformed drain plug

.I attempted to screw in a new drain plug with no success.

Method 1

I moved to an M12x1.25 retthreading kit from this kit off of eBay

.Lang Metric Rethreading Tap Set Thread Restore 6pc 6-12mm Made in USA | eBay

Lang Metric Rethreading Tap Set Thread Restore 6pc 6-12mm Made in USA

It worked like a charm and reformed the threads.

A new drain plug was tightened to 27 ft lbs with a new Toyota crush washer.

In that the hole may or may not be perfectly perpendicular to the pan's sealing surface, I recommend
either use a Viton or HNBR o ring (used by Ford and other manufacturers including Harley Davidson) and tighten it to no more than 10 ft. lbs.

Method 2

The old 1960's standard 1/2x20 fine threaded SAE drain plug used by both Ford and GM is widely available 1/2x20 fine threaded tap will allow repair of the stripped threads in the Prius Toyota M12x1.25 threads that have been stripped out.

The 20 threads per inch is very close to the metric 1.25 thread pitch. 25.4mm / 1.25 = 20.32 threads per inch, where 25.4mm = 1 inch, and the metric thread pitch of the original thread in the oil pan plug hole is 1.25 mm between the peak of each adjacent peak. The minor variation amounts to a variation of a negligible 0.016 mm for each thread. In effect, the 1/2x20 thread is a M12.7x1.25 equivalent.

The threads of the 1/2x20 tap only increases the diameter of the threads to 12.7mm which takes only an increase of no more that .35 or approximately 1/3 of a mm off what wr69 measured as 1.5 mm thick at the wall.

That is enough to make a permanent cheap and easy repair for $1.29, the price that I paid for a 1/2x20x3/4 inch lomg bolt at my local Ace hardware. The o-ring is one that I used from my auto air conditioning HNBR green o-ring kit that I already have.

 

As wr69 noted, the wall of the threaded adapter is too thin to be drilled and tapped to take a Helicoil.

 

Technical analysis of stripped oil pan and drain plug and how it can be restored.

Apparently, the photos posted be wr69, only a small portion of the threads were destroyed and stripped out. The rest were not cracked but apparently only stretched and deformed. The mild steel, like the pan is made is what metallurgist called "ductile." it stretches and deforms nut still does not crack and separate. That's why the pan can be stamped with dies and not crack. The same apparently happened to the threads of the drain plug. That's the reason why a new drain plug could not be threaded in the recess.

Basically, at least 90 % of the metal of the threads was still there. The retreading tap (not a thread cutting one) basically returned the threads to their original position. The pan's threads are comparable to that of a new pan. Almost no new material but a few splinters were removed.