253 motor M21 gearbox

[HX76King]

Just checking to see if I understand this completely.

I already have the m21 and a 253 that I am planing to warm up so the best diff ratio for this combo is a 3.55 from what I can gather from what you have said.

Also why is this, does the 253 not have enough torque/power?:knockout:

[wbute]

Its because the M21 has such a high first gear ratio and the 253 will really struggle with it if you have a tall diff ratio as well. Yes because they are not as torquey as a 308. M21 was the sporty option with closer spaced ratios.
So you have to go with a lower ratio dif and then you loose out on highway economy as all the gearboxes have 1:1 4th gear.

[HX76King]

So probably best to swap for an m20, as I plan to take it on road trips, ay?

So anyone after a V8 M21? ha ha

[adam perth]

the V8 M20 is harder to find than the V8 M21.

may be easier for you to get a good 6 cyl M20 and upgrade it to V8 with V8 imput shaft ($185)

[HK1837]

Or just rebuild the M21 as an M20. Input shaft, cluster and 3rd gear. They normally need a kit in them before they are used anyway.

[GNW1974]

3.08 was always the standard diff with 253 and M20 in almost all HQ and HJ, there is no economy axles listed (except where 3.55 is standard eg 253 M20 HJ LS). 3.55 was standard in HX and HZ for the same engine/box. HQ had 3.36 as standard if 253 and M21 was ordered, but GMH obviously realised this as a mistake as HJ-HZ got 3.55 with 253 and M21. All of this is reflected in the vehicle ID I have recorded. Rarely do vehicles have diff ratios other than what is standard. I don't think you could get 253 and M21 on commercials though, only on passenger vehicles.
3.36 was always standard with 308 and M21 in HQ-HZ.

There was also the M22 which was the standard box on HQ 6cyl commercials and optional on all other HQ 6cyl. Also fitted to many HJ-WB tonners. Ratios are:
3.74:1
2.68:1
1.68:1
1:1
3.74:1

I have just re read the information I have about standard diff options on HQ 253 cars and with the M20/3.08 option you are totally correct, but it also lists the high performance transmission option, which I assume means a M21 box. This was standard with a 3.36 diff. The HQ ute I have bought is an M20 /3.36 combination so I am assuming this diff would have been an upgrade when bought. The 3.55 diff on this combination was then specified as the performance option.

I was looking a a reproduction of a Holden brochure and it lists the 308 and 350 with a RAC engine rating of 51.2 HP so we can't believe everything we read.

[HX76King]

Or just rebuild the M21 as an M20. Input shaft, cluster and 3rd gear. They normally need a kit in them before they are used anyway.

I was planing on getting it rebuilt anyway so that could be a great option. Has anyone dealt with, aussie 4 speed specialists, "http://www.freewebs.com/aussie4speeds/".

I might drop him an Email and see what he says. You guys might be able to keep him honest :shifty: ha ha.

[HK1837]

They are easy to do but tedious.

[adam perth]

offload you M21 for $500 and get a 6 cyl m20 for about $150 + imput shaft $185 and take to a gearbox specialist. You should come out about $100 down.....

[HK1837]

I disagree Adam. Add $250 for the M20 cluster and another $125 each for 1st and 2nd. Then $100 for the cover over the input shaft. Often the main shaft will be stuffed too. If the M21 is a known good working box it'll be cheaper to replace the input shaft, cluster and 3rd gear. Rebuild cost will be identical otherwise. That is assuming the M21 is any good, it could be as stuffed as a 2nd hand 6cyl M20!
I won't pay more than $50 for any aussie 4spd secondhand as half the time all I get out of them is the same stuff: case, reverse, 3rd gear and extension housing. Nearlly always the cluster, input shaft, 1st gear are stuffed and then 60% of the time 2nd gear and the main shaft are stuffed.