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M75 axle - change of gear ratio

Started by _peranaman_ , Feb 20 2012 11:31 PM

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12 replies to this topic

#1 _peranaman_

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Posted 20 February 2012 - 11:31 PM

Hi everyone - first large post here.

Just a little starter. I built the below car over 6 years and finished it in summer of 2010.

It's a millimetre perfect re-creation of the 1970 A2 Gunston Capri. Hope you like it?

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Anyhoo, I have been running the M75 axle with 2.92 CWP ration and LSD - and whilst it's OK - we all agree the axle ratio is too tall for optimum running. If I was getting 30mpg I'd keep it at 2.92, but as I'm not I may as well get some good performance!!

Here's my axle. It's an M75 with 2.92 crown wheel and pinion. (38 teeth and 13 teeth respectively).

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Now I'm told that if I want to change the ratio's in the M75 I can only get the following gears to fit the M75 -3.08 ,3.73 , 3.89 & 4.11 - (I've been informed that I could possibly get 3.23 gears for the M75 but they would new brand new and cost about 800 AusD).

But if I had the M78 axle I could have these gears: 3.23, 3.27, 3.45 & 3.89.

Now to the all importnat questions:- (bear in mind my axle has been custom narrowed and I have custom Moser shafts from USA, and I really don't want to change the rear axle)

My intention being to get somewhere around a 3.23 - 3.45 ratio in the M75 case.

1. Is the above info re. the ratios for the M75 accurate?

2. Is it possible to put the M78 gear set in the M75 case albeit with some machining? Not a problem for me.

3. Do the bearing sets ( Set 309 and set 67) work with the M75 gear and the M78 gears? (wondering what if any the main differences are between the M75 gear and the M78 gears?

I appreciate this is a heck of a thread for a first posting, but I've been stumped on this in the UK as no-one seems to have a clue about such axles.

In summary I want to change from 2.92 to 3.23 or 3.45 using my current axle.

Hoping someone can chuck some ideas my way.

thanks Chris

ps feel free to add any thoughts and suggestions to the ratios I have considered. My transmission is Tremec T5z with the following ratio's:

1st 2.950
2nd 1.940
3rd 1.340
4th 1.000
(5th) 0.730
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#2 _peranaman_

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Posted 21 February 2012 - 12:46 AM

Not sure if this helps but the markings on my Crown wheel are as follows:-

Factory stamped = ' 97 BW EB 2.92 1 D'

Etched by hand on opposite side ' 3 K 8771 '

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#3 Collo

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Posted 21 February 2012 - 01:15 AM

Nice Capri mate. I don't know how much help you'll get here with it, though someone may know. It's 2.15am here so you might have to wait a few hours.

Got any pics of your LX? Also, it's Holden, not Holdan

Cheers

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#4 TerrA LX

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Posted 21 February 2012 - 03:03 AM

Are you saying this diff is a borg warner 75 or a chev 7.5?

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#5 76lxhatch

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Posted 21 February 2012 - 03:55 AM

Its definitely a Borg Warner, 25 spline by the looks which is common in Fords and Valiants.

Everyone seems to call the Commodore diffs M78s, but they have 7.5 inch crown wheels (not 7.8 inch like the later Falcons for example). I know that the VK 1/2 to VS / VT1 Commodore and R31 Skyline gearsets are all interchangeable, and pretty sure that these also interchange with earlier Falcon (up to XF?) and Valiant. I would think that a 3.23:1 out of a Falcon or VK 1/2, or a 3.45:1 out of a VL (all of which are readily available for next to nothing) should fit.

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#6 TerrA LX

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Posted 21 February 2012 - 04:03 AM

There is a BW75 diff. Gemini and cortina I think ran them?

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#7 A9X

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Posted 21 February 2012 - 08:54 AM

Hello Chris.

Beautiful job mate, well done.

Just a quickie off topic quastion, your profile says you have an LX. Is it there in UK ? If so, what sort of response does it get.

Welcome aboard anyway.

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#8 _gearex_

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Posted 22 February 2012 - 07:51 AM

Looking @ the pics, its a early m75 diff. Ratios available are 2.92, 3.23 , 3.50, 3.70. They came out of the XT-XB fords etc & are righrt hand thread on the crown wheel bolts. All those ratios are getting hard to find now. However, if you change to the later model m75 ( VL Commodore, skyline etc) which has the left hand thread 5/8th bolts you can use 2.92. 3.08, 3.23, 3.45, 3.70. 3.89 & 4.11 ratios. All you will have to do is drill out the holes on your hemishere to suit the bigger bolts.

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#9 ls2lxhatch

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Posted 22 February 2012 - 11:00 AM

Gearex, is the KAAZ an option for the M75?

If it is then it would be worth considered replacing the hemisphere with a KAAZ as part of the ratio swap.

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#10 _gearex_

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Posted 22 February 2012 - 11:26 AM

I dont believe the are for the 25 spline axle only 28spline, but would'nt be too hard to make up 28 spline axles,

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#11 dattoman

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Posted 22 February 2012 - 08:45 PM

I saw a flier at work today for the KAAZ...I think it listed 25 and 28's for the 75 and 78 series diffs

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#12 _peranaman_

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Posted 13 April 2012 - 08:29 PM

UPDATE

I have a set of 3.45 and 3.23 gears from Oz now. All the initial dimensions seem to marry up with the 2.92 gears and bearings.

Will have to get the carrier machined to take the 7/16th bolts and then get the transmission shop to do the biz.

Will update on completion.

If anyone knows where I can get a spare M75 CW carrier that would be cool. I can get it machined before I strip the axle out.

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#13 _gearex_

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Posted 14 April 2012 - 11:41 AM

i have a few limited slip cw carriers to suit both the smaller right hand thread & larger left hand threaded bolts, your 3.45 & 3.23 gears are left hand thread on the crown wheel bolts, yeah?