4 speed Aussie M20 rebuild

[craigvk]

Yo,

Just wondering the steps to rebuild or put a kit through the M20/21 4 speed? I want to get it back to excellent condition again. Not sure of the condition of it but I am sure it's a good runner but I would like to know what I need to rebuild it. I don't want it in my car unless I know it's been serviced and in top condition.

Thanks

[craigvk]

Also as I have seen, the M21 was in a lot of Brock Commos 304's with the only difference to mine being the gear ratio differences between the M20 & M21. Would I be right that there shouldn't be an issue with it bolted to a 308?

I was going to get a heavy duty performance clutch kit before I put it in my car. I have seen them for about $200 compared to about $100 for a standard kit.

[SL/ENUT]

Usually the input shaft in a M20 is longer than an input shaft for a M21. The M20 gearbox is good for a 6 cylinder/253 but after that, can struggle. The M21 does have a little bit stronger gears for the 5 litre. Also you are right with the gear ratios, the M20 is classed as a wide raito, M21 a close ratio gearbox.

The heavy duty clutch is a good idea, got one in my gold sle, best thing i put in it i reckon.

Now for the gearbox service, don't get parts made in China/Taiwan, these parts are very weak, specially the gear cogs!. If you read your car service manual, it should have a section on M20/M21 4 speed gearboxes. If it does read step by step on how to dismantle it and rebuild it. I have heard that these gearboxes are a prick to rebuild.

[les82]

these gearboxes are ****e. for the money u will spend on a rebuild spend a few extra bucks and pick up a t5 of supra box. ive killed 2 of them so far and i think the third one is on it's way out

[SL/ENUT]

Well there is reason why your killing them, the cheap parts or how you drive. The M20/M21 don't like flat shifting and rough treatment. I've had my M21 in my commodore since it was built, it has never ever had a rebuild and still tickling along fine. But having said that, my M21 is due for a rebuild, same as the donk too.

[craigvk]

Thanks SL/E you built my confidence back

I don't see why the Aussie boxes wouldn't be fine. Overall, I never thrash my car. My car is simply a daily driver that I treat with respect and I simply like the power of it. Boy I'm going to be in the crap when I start pulling it apart. I was told by the previous owner it was a good runner. Who knows. Maybe if I stick a H/duty clutch kit in, it would go like a dream But overall, I don't really want to go to the effort and need to pull it out 6 months later. If the cogs are ok then maybe I could just do some minor crap to get it nice again.

[vkberlina]

The M2x boxes were good boxes and are reasonably strong as long as you dont start abusing them then they generally wont last.
The M20 was used on 202 & V8s and the M21 was only used on V8s. The V8 models of M20 & M21 have a longer input shaft compared to the 6cyl version.

Ratios for the boxes as follows


M20
1st - 3.05
2nd - 2.19
3rd - 1.51
4th - 1.00
Rev - 3.05

M21
1st - 2.54
2nd - 1.83
3rd - 1.38
4th - 1.00
Rev - 2.54

If your going to rebuild your box yourself, it isnt hard get yourself a copy of the Holden HQ Vol 3 service manual (M37017) which will outline full rebuild instructions, or the next best is a HQ-HX manual from gregories or Max Ellory

You should check the following when rebuilding the box.
Check for pitting in the gear tooth surfaces & bearing contact surfaces. If the case hardening begins to pit it will get progressivly worse with use and cause premeture gear or bearing failure.

Check the internal diamater of the cluster with a micrometer and make sure it is not out of round. if it is you will need to have it machined and a insert made to fit & pressed in. Make sure the cluster is heated and the insert cooled and is pressed in with a couple of thou of interference.
check the surfaces of the syncro rings for wear.

Give all the gearbox parts a really good clean including your work area prior to reassembly, you can paint your gearbox housing at this stage as well.

When assebling your bearbox check endfloats of gears to factory spec and shim and required.

When building your box I use Mobil Waxrex (basically industrial vacaline) to help hold the needle rollers inplace in the cluster. Cut down your old lay shaft out of the cluster to be the same length as your cluster and round the edges. Use the shaft as a dummy assembly shaft so the needle rollers dont fall out of place whilst you assemble the box till you insert the new lay shaft.
Spray a degreaser into the cluster to try to wash out as much waxrex as you can. It will melt with heat (approx 30deg+) but may affect cold gear changes.

I recommend you use Castrol VMX 80 in the gearbox, when you first drive the car with the box replaced, take it easy take it for a bit of a drive to heat up the oil put about 50km on it then whilst hot drain the oil and refill it. This should remove the last of the assembly waxrex. The gearbox should take 2L of oil.

cheers
Scott

[jasman]

To answer your question: first drain the oil. Grab your self a scone tray for bolt identification. Undo all the bolts holding the selector cover, gently give a slight tap to the cover it helps to free the cover. you may need a small srewdriver to help being careful not to damage surfaces.Take off the cover you should notice two levers connected to the cover. Note the one to the front of the gearbox can go eitherway around however the back one has a section removed and must go towards the center. Undo the front bearing retainer 4 bolts and gently remove and put aside. Next there is a lever and reverse tension bolt ball and spring on your extension housing. undo the 5/8 bolt and take out spring and ball then loosen off the nut but not all the way and gently (soft hammer) tap the top this allows for the extension housing to come away with out getting caught up in the reverse gear. next remove the extension housing bolts and put aside, one of those bolts has a coper washer on it goes to the bottom oil drain. gently remove the extension housing, you will notice that there is a bronze washer and a small metal object like a T that fits inside the reverse lever, take out and put aside in tray. You should now have all the covers off and be looking at your gear box. Next take off your speedo and clip and put aside, then slide you reverse gear off. next on the bearing retainer end (4 bolt end) you will notice a metal looking shaft about 50ml from your input shaft, with care us a suitible drift or punch tap out from the front (input end) to the back (this is important the shaft is the same size either end yet the housing is slighly smaller at the front) drive out shaft and put aside dont worry about rollers more info to come. the push down on the cluster gear it has 4 hears on it to the bottom of the case. when this is pushed down you can now remove the mainshaft as a whole and put aside then the input shaft followed by the cluster gear. iside the case wil be two bronze washers take out and put aside, you should have a bear case. If you have a digital camera it will help you to take some pics. The cluster gear has a lot of washers, a spacer and needle rollers. (dont be concerned its quite easy if you follow my instructions) remove and put in tray. Next remove all the large needle rollers from inside the input shaft and put in tray. Ok you should have done your cluster and input shaft. Next lay down the mainshaft with the small end pointing towards you with a pair of circlip plyers remove the circlip and put in tray, then you will notice a hub with a slot a brass ring and a gear, on the back side of the gear gently with a scewdriver, lever up the gear. lay out in order. then go the the other end and remove also. the main shaft disasembles in two halfs. you will need to remove the large bearing retainer with a soft hammer, take off the next gear (first gear) and ring a put aside then you will notce another circlip that needs to be removed, take off and put aside. you should now have a mainshaft and one gear, brass ring and a hub, at the gear end with a foft hammer hit on the gear till the gear and hub is off the mainshaft. you should now have the mainshaft free of all gears. Mmmm what to do now. You must check all internal bearing surfacse where bearings run, with you brass rings, they must be checked on the gears they came off. to do this when clean you with your hand push down on the ring and try to turn it it should bit and stop. make sure the gap between the ring and gear is not touching each other. in genral if you are not replacing the rings do this, Grab you inputshaft ring and swap it with your secong gear ring, this will help 1-2 shift.
You will need to remove all large bearings and replace them being careful to notice which what they go. with your cluster gear check for surface wear on gears and on internal bearing areas. To assemble bearings in cluster in the cluster gear is one washer a set of needles another washer another needles another washer then a spacer then a washer a set of needles the washer then needles the washer. in total you should have 6 washers 1 spacer and 4 sets of needles. use a vasoling to hold together. avoind donig when its hot weather. after checking the box for wear repeat assemble. as per removal.I was working as an apprentice some 25 years ago as a transmission spelist in man the automatics. I built the brock gearbox x2 that went in the repco relibility challange 1979. he took two new and the original i buily lasted the whole journey. i was pretty excited i was just seventeen when i built it. hope this ramble helps.