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The "drain" plug is halfway up the passenger side of the transmission body. You need to suction out the old fluid and re-fill at the same port. Not ideal but that's the way it is.
And, welcome to the wonderful world of C2 Corvettes!! Pictures are, kinda, mandatory.
Syphon lube from fill port using suction gun or straight syphon.
For C2 cars only the M22 special high performance box received both fill and drain plugs. Many here drill and tap their Muncie for a drain plug when they can.
Thx all. There is definitely no plug on the underside. Fill plug is 1/2 way up case on passenger side. Tried draining with a hand pump but don’t think I got to the bottom of the pan. Hard to tell.
I assume that the L72 could only have a heavy duty fourspeed like the M22.
If so it's a square bolt head on the side of the box. Fill is high drain is low.
You know what happens when you "assume"... For the 1966 Corvette equipped with the L72 427ci/425hp engine, the available transmissions were: the M20 wide-ratio 4-speed manual (was the standard transmission), the M21 close-ratio 4-speed manual, and the M22 heavy-duty close-ratio 4-speed manual (both available as optional equipment). The Powerglide automatic was not available with the L72.
Wide M20 and Close M21 were both optional transmissions with the same price. Take your pick but typically tired to recommended rear axle ratios. 4 speed was a mandatory option with some engines. M22s sold in 1966 were so rare as to not even be relevant,
Some sorta good info above, some not quite correct.
As far as a 4sp tranny in 66, ONLY, ONLY, ONLY the HD M22 got a drain plug.
The 1in cluster shaft was introduced in late 65-------------FOR THE 66 MODELS! ALLLLLLLLLLLLLLLLLLLLLLLLL Muncie cases, beginning with the first one in 63, have a boss for a drain plug. But ONLY 66-69 M22s got a drain plug,
Then, in 70, ALLLLLLLLLLLLLLLLLLLLL Muncie cases got a drain plug.
A drain plug CAN BE ADDED with the transmission in the car. I have only done 2-3 in the car------------BUT I cannot count how many I have added in the fixture in the past 50+yrs.
Here is how I do it.
This is the case before drilling a hole in the lower boss.
First, I drill a pilot hole.
Then I drill a big hole with an 11/16 drill bit.
The hole is threaded with a 1/2in pipe thread tap.
I ONLY use a magnetic plug in the drain hole.
Prior to 1970, Muncies had a magnetic donut glued to the bottom of the case. It is good at removing metal particles from the oil, but there is no simple way of cleaning it off. Thus, when I add a drain plug, I always remove the magnetic donut.and toss it in the top of the tool box
And that's how I add a drain plug to 63-69 Muncie cases.
easy way to fill the transmission. put a tube in the fill hole, run it up the firewall (like an automatic filler tube would be). fill from up top. much easier than trying to fill from under the car.
Good advice makes sense. My issue is removing it since I have no drain plug. Not sure if I’m hitting bottom with extraction tube. Trouble fishing it there.
I don't know what I was thinking years ago when I put my 69 Camaro together. Rebuilt the trans and put the car together. Guess I forgot to add the drain plug! Anyway, I drilled the case in the car and added the plug. Can be a little messy but It worked. Don't forget to stick a magnet in the case to get any stray metal.
