M21 Gearbox Oil change need advice

[bfhoon]

G'day

My m21 gearbox has been leaking forever I think it leaks around the front input shaft? Anyway its been a couple of years and I should top it up. I dont think I will bother draining it but i might. Do the filler plugs and drain plugs need replacing? I was hoping to just reuse them if they are brass or aluminium looks like the filler plug is just a thread do I need any kind of sealant on it? The filler plug also looks like a square bolt type do I use vice grips or something last thing in the world I want to do it strip the bastard assuming its gonna be friggen tight as I 'touched it for 7 years and before me who knows....

Also what the best oil to replace it with hoping to go something thicker maybe to stop the leak or am I dreaming?

Thanks

 

[krusing]

Hmm,

For what it is worth,
remove the Tail Shaft,
disconnect the reverse switch,
remove the shift,
remove the box, 4 x 9/16 bolts by memory,
"HANDLE THE FRONT OF THE BOX CAREFULLY, ESPECIALLY THE INPUT SHAFT EXTENSION COVER, AS THEY TEND TO BREAK especially on V8's, because they are longer and flex more."
If you break it, then you will be in the shite
drain it by removing the bottom bolt on the rear extension housing,
Remove the front bearing cover, 4 x 1/2 bold by memory, remove the seal and install a new one, place some gasket cement in the housing where the seal sits, and tap it in carefully,
While you have it out, maybe replace the rear seal, (easier to work on it with it out),
Refit the box,
and fill it with Castrol 85/90W (Oils aint Oils Sol)
Ahhh, the smell of Gear oil, brings back are lot of memories (My Torana Days)

The refill plug was a hex head plug, well it was on my M20,
but you say its a square head plug, open end spanner, not sure on the size,
Before you refit the re-fill plug, clean it up, especially the thread, and wrap some teflon tape on the thread,
don't use sealant, and you don't want to contaminate the oil,
And tension it to approx. 20nm.

PS: If your real keen, you could replace the o-ring seals on the shift shafts,
but that is a bit more tricky, as they have springs on the selector arms, (and can be like mouse traps)

 

[bfhoon]

Hmm,

For what it is worth,
remove the Tail Shaft,
disconnect the reverse switch,
remove the shift,
remove the box, 4 x 9/16 bolts by memory,
"HANDLE THE FRONT OF THE BOX CAREFULLY, ESPECIALLY THE INPUT SHAFT EXTENSION COVER, AS THEY TEND TO BREAK especially on V8's, because they are longer and flex more."
If you break it, then you will be in the shite
drain it by removing the bottom bolt on the rear extension housing,
Remove the front bearing cover, 4 x 1/2 bold by memory, remove the seal and install a new one, place some gasket cement in the housing where the seal sits, and tap it in carefully,
While you have it out, maybe replace the rear seal, (easier to work on it with it out),
Refit the box,
and fill it with Castrol 85/90W (Oils aint Oils Sol)
Ahhh, the smell of Gear oil, brings back are lot of memories (My Torana Days)

The refill plug was a hex head plug, well it was on my M20,
but you say its a square head plug, open end spanner, not sure on the size,
Before you refit the re-fill plug, clean it up, especially the thread, and wrap some teflon tape on the thread,
don't use sealant, and you don't want to contaminate the oil,
And tension it to approx. 20nm.

PS: If your real keen, you could replace the o-ring seals on the shift shafts,
but that is a bit more tricky, as they have springs on the selector arms, (and can be like mouse traps)

Thank for all the help buddy appreciate the reply I might pull the box out one day but I just don't have the time at the moment have done a few many many years ago under a jacked up car in the garage on my back swimming in concrete. I'm just to old for that **** these days. But yes once the tailshaft is dropped its relatively easy getting the box off the bellhousing.

 

[krusing]

have done a few many many years ago under a jacked up car in the garage on my back swimming in concrete. I'm just to old for that **** these days.

I know it all too well !!!

Things were so much easier to do yourself then, no electronics to worry about.
What we did back then in the mid 80's ! hahahahaha
(being I was in my mid 20's)
I know the Aussie 4 speeds very well, wasnt much to them,
as I did put a few reco kits through mine, and sycro rings a few times.

M20's behind a V8 didn't last too long if you drove it a bit hard.
The lay shaft lost it chrome surface, and the needle bearings got destroyed,
Thus then damaging the chrome in the cluster where the needle bearings are located.
I had an engineer friend that came up with an idea,
he machined up a bronze bush for the cluster, pressed one in at either end, and machined an internal oil groove for the lay shaft,
I never had a issue with it then, and sold the 4.2 manual LX with it in, and the buyer couldnt believe how quiet and smooth the 4 speed was.

I still have a spare bronze bushed M20 cluster that I had as a spare, may sell it one day.

 

[bfhoon]

I know it all too well !!!

Things were so much easier to do yourself then, no electronics to worry about.
What we did back then in the mid 80's ! hahahahaha
(being I was in my mid 20's)
I know the Aussie 4 speeds very well, wasnt much to them,
as I did put a few reco kits through mine, and sycro rings a few times.

M20's behind a V8 didn't last too long if you drove it a bit hard.
The lay shaft lost it chrome surface, and the needle bearings got destroyed,
Thus then damaging the chrome in the cluster where the needle bearings are located.
I had an engineer friend that came up with an idea,
he machined up a bronze bush for the cluster, pressed one in at either end, and machined an internal oil groove for the lay shaft,
I never had a issue with it then, and sold the 4.2 manual LX with it in, and the buyer couldnt believe how quiet and smooth the 4 speed was.

I still have a spare bronze bushed M20 cluster that I had as a spare, may sell it one day.

I'm suprised this one has lasted its behind a 308 that has an exedy brass button clutch so it grabs hard and has a 4.11 diff. Back in the day I just did the service exchange old box out new one in.

 

[Smitty]

I'm suprised this one has lasted its behind a 308 that has an exedy brass button clutch so it grabs hard and has a 4.11 diff. Back in the day I just did the service exchange old box out new one in.

if properly built (does anyone check the clearances these days?) and the nitride gear surface on the cluster etc checked for pitting
AND
the shifter properly adjusted, an M20 or M21 (same box ... different ratios) will last behind a 308, a hot 308 even (like a 380-390hp one)

I run one in the VK race car... in 20 years have not broken a gearbox. Diffs and engines .. yep, never a gearbox!

I spend time building/rebuilding mine .. quality bearings please (Jap or German, not chinese) and decent seals too (CBC do a good kit if thats what is needed)
The time is in getting the end float right ( I use HONDA valve shims of various thicknesses) and i have a metallurgical lab at work I can use to check out
the gear hardness/coatings.

 

[krusing]

Yes, the Cluster end float was VERY important, and the shims were a PITA tight fit to get in with the keyways.

 

[bfhoon]

Got under the car a few day ago I can only see on plug on the box about modway have I got the model where you have to drain it somehow by pumping it out?

 

[Smitty]

Got under the car a few day ago I can only see on plug on the box about modway have I got the model where you have to drain it somehow by pumping it out?

none no Aussie box needs pumping out
so how?
the drainage bolt doubles as a bolt that holds the extension housing on
undo the bottom/lowest bolt (note it has a thin copper gasket on it)

 

[bfhoon]

none no Aussie box needs pumping out
so how?
the drainage bolt doubles as a bolt that holds the extension housing on
undo the bottom/lowest bolt (note it has a thin copper gasket on it)View attachment 264519

Awesome you are an absolute life saver do you know if I can get a copper gasket to suit at supercheap or somewhere and what size etc? I'm assuming the fill plug is passenger side? Cheers for the info much appreciated