I found a used M46 transmission and was told it came from a '86 240.
The case is cast iron opposed to the cast aluminum one it is replacing.

A PO stripped the drain plug thread. I think I can chase out the case with a tap but want to verify the thread size.
It looks like 18mm x 1.5 pitch. Does that sound right? I don't want to just run the tap through and then find out it was the wrong size.

It is not the same size as the aluminum case drain plug...I tried already.

Thanks
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'60 544, '68 220S, '70 145S, '72 144E, '86 745T

I can tell you that the plugs in the '84 M46 I put in my '80 242 are 1/2" NPT with a square head. So were the plugs in the M45 which was original to the car. Others here have described iron box plugs with hex heads but no one has convinced me that Volvo used another type of thread before the metric (?) plugs found in the aluminum gearboxes. I'm under the impression that all the cast iron gearboxes had pipe thread. But I can't say that this is absolutely the case, no pun intended. I would like to know, one way or the other.
It sounds like you know your threads. Maybe if you remove the fill plug you can get a better idea. It should be the same. Or is that what your observation is based on?
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'80 DL 2 door, '89 DL Wagon

I am familiar with the 1/2" NPT square head plugs. They were used in the M4, M40 and M41 also. I have plenty of plugs from the older gearboxes and they definately are a different size than the cast iron M46 that I have.

The box that I have doesn't look like a pipe thread...no taper. I also have the hex head plug (maybe I should call it a short bolt). Sure looks like factory equipment to me. The fill and magnetic drain plug (bolt) appear to be the same (with a 22mm or 7/8" head - they are almost the identical) excepting the magnet.

I thought it was strange that the aluminum case and cast iron case used different plugs. Everything else on the case appears to be the same. Thinking about it though, I guess it is logical that the aluminum case would use a coarser thread.

You are correct about my observation and comparason of both plugs. I am just trying to double check my measurements before I re-tap the case. I'm crossing my fingers anyway regarding leaks on this box. Re-tapping the hole will create the potential for more leaks as it is. I'll probably end up with a few turns of teflon tape on the plug when I reinstall it.

Thanks for the help.
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'60 544, '68 220S, '70 145S, '72 144E, '86 745T