[sapsz28]

I have a m-21 in my Z28 there is no access to the drain hole,there is a plug where the regular drain pug location is,but it is a flush plug .I dont mind tapping it to remove the plug as long as I can replace it with a functional drain plug has anyone encountered this dilema and what fluid should I go with,GM Syncromesh or royal Purple 75w90 or 75w140 I dont mind spending a few bucks as long as its put to good use. any help would be thankfully received and faithfully applied.

[cobalt327]

Some history on the drain plug, courtesy of Joe Lucia of the corvetteforum.com:

As far as drain plugs go, during the '65-'69 period, only M-22 model Muncie 4-speeds were originally equipped with a drain plug. Beginning in 1970 with the introduction of the "661" main case, all Muncies got the drain plug. It is very possible that very late '69's also received the "661" main cases and, consequently, the drain plug. For most Chevrolet car lines (except Corvette and Camaro), the 1970 model year began in the fall of 1969. So, the "661" main case may have been the only one in production by the fall of 1969. My original owner, mid-September 1969 built Corvette has the "660" main case and does not have the drain plug. However, later build '69's (which were built through December) may use the "661" case.

There is a boss below and forward of the fill plug where the drain plug goes. Are you saying your drain boss already has a plug w/o a square or hex to remove it, or do you have just the boss and no plug of any kind?

If you have no plug at all, the boss can be drilled and tapped for a pipe plug. Start w/a small pilot hole and work up to the final tap size drill bit.

Below shows a case w/both drain and fill bosses drilled and tapped, the other case has just the fill boss tapped.

BTW, some fill holes had a brass plug w/o any way of removing it- it was basically flush w/the boss in the case. Don't mistake this for a drain. You could try removing it by slotting the plug for an impact driver, use care so as to not damage the case. Carefully applying heat may help.

Any work like this- especially drilling and tapping the case- should be done w/the trans removed if at all possible, so the chips can be cleaned out and just to make things easier in general. Otherwise, use a suction pump to pull the fluid out of the fill hole and refill. You could do this several times to flush almost all of the old fluid out.

Hopefully AutoGear will chime in on this to keep me honest.

[sapsz28]

after looking at the pictue of the 2 different cases mine resembles the forward of the 2 with only a fill hole,the drain boss is flush ,however there is a stamped circle on that flush drain cover I would rather tap it for a drain plug.
or could I weld a square nut on that drain boss and then nice and easy try to see if I could gently unscrew it.I tried the suction meathod but to no avail.If the bosses are in the main spot why would muncie not install drain and fill plugs on all m-series of transmissionsanyone ever try any of those 2 missions.

[cobalt327]

I'm having a hard time picturing exactly what you're describing.

Your case looks like the one on the right?

Or are you saying the drain boss has a plug, but w/no way to remove it? If so, check to be sure it's steel before you attempt to weld anything to it.

In late '69, ALL Muncie 4-speeds got both a fill and drain plug. This means that the dual drain plug cases were used in '70-up production although there might have been a few that made it into super-late '69 production, who knows. Before then only the M-22 "Rock Crusher" had them both.

For using a suction pump to remove the old fluid- you can use copper tubing bent carefully to clear everything, then use a length of rubber hose connected to the pump to connect to the copper tube.

It's some work, but you could also drain it by removing the side cover to get a suction tube in there better than through the fill plug.

[AutoGear]

As far as we can tell; the 660/661 cases all came with the boss for the drainplug. What do you mean by 'flush plug'? Like a freezeplug or like a socket headed plug someone cranked in there long ago?

Any competent machinist can open the boss to put the proper plug in there. I can only recommend removing the unit from the car to do this, however if you can't, make sure to use plenty of grease to catch the chips and flush repeatedly with a suction gun. The proper plug is a 1/2" pipe tap; with a square head and is magnetic. Don't go through a restoration house, you should be able to get a replacement for a couple bucks.

Thanks to Cobalt for the picture of our case, we're really proud of it.

RE: fluid

You should really have a GL4 non synthetic. Even non synthetic blends are problematic.

The problem is the 'threads' on the inside of the bronze synchro ring are used to cut through the oil film, and the trans uses the lube as a bit of a friction brake. Really slippery lubricants mean the ring will skate across the film and the gearbox won't 'slow down' quickly enough.

If you must use a synthetic, the only one I can recommend is Royal Purple MAX GEAR ( -NOT- Synchromesh) in the 75w90 range ( 140wt is not necessary). 1 QT is all you need. (there was a lot of casting core problems and the fill and drainplugs migrated, which makes the 'fill till it pees out' problematic.

If you want to go non synthetic; Brad Penn Racing's 90wt GL4 or Sta-Lube GL4 75w90 sold through Napa. Again 1 QT.

[cobalt327]

AG, would you mind me using the info above on manual trans lube (w/credit) in the Category:Transmission section of the Wiki Tech Articles, aka Crankshaft Coalition? I believe it would be a useful addition and has been on my "Wiki to-do list" for a while now.

Thanks in advance, and check your PM, pls.

[AutoGear]

Not at all. Feel free to clean up the explanation, Im not a very good technical writer.

To the OP:

Are you sure its a Muncie M21? The 'M-series' codes (M20, M21 and M22) are GM 'RPO' codes, like Z28 and WS6...right down to the seatbelt retainers.

RPO M20 is wide-ratio 4spd; M21 is close-ratio 4spd; M22 was a specialty Heavy Duty close-ratio 4spd (I personally do not know if RPO M22 was used to designate anything other than the Muncie 2.199 1st gear heavy duty box)
an '80 camaro would have had a T10 or saginaw. NastyZ28.com has a great forum on decoding RPO codes.

Nate

[sapsz28]

yes its a M21 no its not original by flush plug ,I mean where the drain hole should be with a square nut the supposed location of the hole is flush with the case as in cobalts picture of the 2 trannys ,mine resembles the one on the right,the one that has no visible bolt to drain the fluid . FLUSH meaning a 180 degree plain ,flat ,that is the best that i can describe it.
would GM Syncromesh be better to use right know I'm running royal purple 75W90,Thanks guys

[cobalt327]

Originally Posted by sapsz28

yes its a M21 no its not original by flush plug ,I mean where the drain hole should be with a square nut the supposed location of the hole is flush with the case as in cobalts picture of the 2 trannys ,mine resembles the one on the right,the one that has no visible bolt to drain the fluid . FLUSH meaning a 180 degree plain ,flat ,that is the best that i can describe it.
would GM Syncromesh be better to use right know I'm running royal purple 75W90,Thanks guys

OK, I believe I'm finally getting the picture here, I get confused sometimes so thanks for bearing w/me.

after looking at the pictue of the 2 different cases mine resembles the forward of the 2 with only a fill hole,the drain boss is flush ,however there is a stamped circle on that flush drain cover I would rather tap it for a drain plug.
or could I weld a square nut on that drain boss and then nice and easy try to see if I could gently unscrew it.

I'm going to say there's no actual plug at the location where the drain plug should be, the "stamped circle" is very likely just cast into the case as a designation of where the plug would be machined on a case that went through that process during manufacturing, so there's no plug there that can be removed, period.

So you're left w/using the side cover or a piece of copper tubing snaked down the fill hole to drain it, at least until you install a plug.

I'll leave the lube question and recommendation to AutoGear but I use Amsoil 75w-90 GL4 in my Muncie M-20.

[AutoGear]

I gave up on the snakey tobey thing a long time ago. I bought a 'suction gun' and never looked back; although Id kill for a Made in USA 'Supercase'

Cobalt is correct, thats a casting pad or boss for locating the hole for the plug.

I believe only the 660 and 661 casting cases were factory drilled for the drainplug. Incidentally; if you follow common practice and fill the case till it drips out the fill hole; you may be over-filling it. The plugs sort of migrated as time went on with these castings (core shift) so its all over the place. We recommend 1 quart of GL4 or GL5 MT1. Don't use 'Synchromesh' from Royal Purple...use their Max Gear. Other people have had good luck with Sta-Lube GL4 from Napa. I don't know enough about AMSoil to say either way.