Widths of axles

Latest update: 2/4/12

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If you have read some of the other articles on this website you probably already know that I am not too happy with the rear axle gear ratio I presently have.

My stock 1983 RX-7 GSL rear axle is geared 3.933:1. The axle has LSD. There are very few other ratios available for the RX-7 "solid" rear axle (3.626, 4.10 and even lower ratios are all that are available).  Since I have a BEC (Bike Engined Car) none of them are suitable for what I want t do.

Most (all?)  motorcycle engine/transmissions have a reduction gear set between the engine and the input to the transmission. Modern motorcycle transmissions also have a very close ratio gear set in them and top gear is usually not geared 1:1 or an overdrive (.78 for instance) as are car transmissions.  

To give you an idea, here are the ratios of my GSXR-1000 transmission;
Gear        Pri Ratio    Gear Ratio  
   1 .......... 1.553 ...... 2.687
   2 ........................... 2.052
   3 ........................... 1.681
   4 ..........................  1.450
   5 ..........................  1.304
   6 .........................   1.208

As you can see 6th gear still has a reduction that is similar to 3rd or 4th of a 5 speed car transmission. My final ratio when the primary reduction and the lower geared 6th gear are combined with my 3.993 rear end is 7.38:1 .  Cruising at 8,000 RPM (72 MPH) in my car is not exactly fun after awhile. Not to mention what it does to the gas mileage. Although the acceleration is extremely "brisk" to say the least.

This final ratio is, to put it mildly, not too good. In affect my 6 gears are spread out between 1st and 2nd of a stock GSXR-1000 motorcycle! The original GSXR-1000 had a final sprocket ratio of 2.47:1 which gave it a  4.63 overall final ratio and a top speed around 180 MPH with the top speed governed by the ECU!

So I've been looking, measuring etc trying to come up with an axle that would be an easy replacement for the RX-7 axle I have now. My ideal axle would have ~3.2:1 to ~3.4:1 ratio, have LSD, have a ring gear no larger than 7-1/2"  and be slightly narrower than the RX-7 axle.

Most modern car rear ends are quite a bit wider than the RX-7 axle and commonly use larger (in diameter) ring gears than the 7" one in the RX-7 axle. The 7" diameter ring gear I have now can handle the power (165Hp @ 11,800 RPM) that the engine has.

OK, enough background, let's get on with it before the foreground goes underground!

What is presented here?
I have measured a few axles at my local junk yard to get an idea of what axle could be used to replace my present 1983 RX-7 GSL rear end. The RX-7 axle is 57.250" (+/- 1/8" or so) when measured from one WMS (Wheel Mounting Surface) to the other WMS.

Many sites and posters may give measurements between the brake mounting flange on the ends of the axles as the "width" etc of the axle. I feel that this method of measuring an axle is basically useless since what we are eventually concerned with is, will the tires rub on the chassis or wings (fenders). So if you see measurements that are outside the normal range on the low side, be aware that the poster might be giving the brake flange to flange dimension.

The axle shafts themselves on many Ford products have different lengths and many can be inserted into other housings to give different WMS measurements (with changes to the brake hardware in some cases). I'm sure other manufacturers do the same but I am more familiar with Ford products.

Some of the Mustang forums are a good place to get axle exchange information.

This link is a series of many posts that cover using the Ford 8.8" axles in Ford cars.  

There is a lot of good info on what fits what if you have the time to read it all.
This list at;  http://moose.ca/axle_widths  has many axles listed. I'd suggest you look there for an expanded list of what I have measured and posted here. There is a lot of other useful information on that site under "Vehicle Tech" that you might also want to read.

Axles I've measured:
This top view drawing shows where my measurements were taken from.
(Click on the drawing for a larger, clearer view.)

Axle dimension details

Axle measured WMS-
Ratio       LSD or open
 My '83 RX-7 GSL 7" 57.250 28.625 27.375 1.250 10 4 8  3.933     clutch plate LSD
 '86 Ranger 7.5" 56.500 30.500 26.000 1.875 10 4 14  4.10       open
 '93 Ranger 7.5" 58.250 29.500 28.750 0.750 ? 6 ?  3.45       open
 '86 Mustang 7.5" 59.500 30.500 29.000 1.500 ? ? ?  LocostUSA.com  post
 '92 Mustang 7.5" 59.125 30.500 29.125 1.375  9.75? 4 ?  3.73        open
 '80 Volvo 240 7.12" 54.00? 26? 28?   2? 10.5     3? 9  3.91        Torsen LSD
 '91 S-10 pick up 7.6"  54.500 27.562 26.687  0.875    12.0  Yoke    12  see Edit, 2/4/12 below

  1. Items with "?" were either not measured or the measurement was difficult to take and may not be correct.
  2. Axles will be added as I measure them.  I will be happy to add anyones similar measurements on other 7" or 7-1/2" rear axles to the list. 
  3. The list right now only has 7", 7.5" & 7.6" diameter ring gears. The 8" and up axles are way too big for my use.
  4. Most Ford axles I've measured have two shock mount brackets that are ~42" apart on the rear of the axle. This is the same measurements used in the "book" for the link brackets. It appears that one of the stock  brackets could be used as the pan hard mount.
  5. It looks like the '79-'92 Ranger axle would fit a book chassis with no width modifications except for the pinion offset.
  6. The '79-'93 Mustang axle is 3" wider than a the Ranger and should fit a + chassis as is.
  7. Ford standard gear ratios for the '79-'93 Mustang 7.5" axles are 3.08, 3.27, 3.45, 3.73, 4.10.
  8. The Ranger has the same ratios except that the 3.27:1 ratio is not used.
  9. I've read on the web that only the ring, pinion and spiders gears of the Mustang and Ranger axles are interchangeable. At the same time I've read that the LSD from a Mustang will go into a Ranger. So don't quote me either way, since I haven't tried to change them.
  10. Wikipedia says that the '86+ Ranger 4 wheel drive 7.5" diameter axle has LSD. The axle codes indicate that the following ratios were offered; 3.45, 3.73 and 4.10.
  11. The Volvo rear end is almost too narrow. It's the narrowest one I've found in a junk yard.
  12. If you find '79-'92 Rangers with the left side axles missing here's why. The left side '79-'92 Ranger axles are the same length as the left and right side '79-'93 Mustang 28 spline axles. People who are converting a Mustang rear axle from 4 bolt rims to 5 bolt rims will use two the left side Ranger axles (5 lug bolts) to replace their Mustang 4 bolt axles.
  13. PLEASE do your own research for facts on any of the axles listed here since a lot of us got burned on the mythical 7.5" Courier carrier that would fit into a 7" Miata (and by reference, RX-7) axle housing.  I'd like to see someone cram a 7.5" diameter carrier into the opening made for a 7" carrier.  It's hard enough to get the 7" carrier into the 7" housing.

[Edit, 8/20/11]   Update on internal parts interchange between Ranger and Mustang axles.
I called the Ford Racing website help number and was told that 1979-1993 Mustang 7.5" rear end gears and Trac-Loc will fit into a 1979-1992 Ranger 7.5" axle housing.  I caution you to call them yourselves or do some additional research to make sure.

[Edit, 2/4/12]   Additional information about the S-10 rear ends.
I've measured a neighbors 1991 S-10 two wheel drive p/u 7.6" axle and it is apparently 54.5" WMS-WMS with the brake drums mounted.  I can't guaranty my measurements since the axle was still in the truck and had the wheels mounted.  54.5" is what other people are quoting on the web.

This axle looks very promising since gears from the 1999-2002 F-body Firebird and Camaros are said to fit.  The normal S-10 ratios are 3.08, 3.23, and 3.42.  The 3.23 ratio is the normal ratio for a manual transmission.  The other two ratios are for the automatic.  Three different 7.6" (insert your favorite name here) LSD units are used on the F-body Firebird/Camaro axles which will fit into the housing.  From what I've found so far the S-10 axles of interest were made from 1985 to 1993 (see "Wikipedia S-10").

Start your search for S-10 info here.  I've found an article here that gives the pinion offset of the 7.6" S-10 axle as 0.875". The article is mainly concerned with installing a 7.5" S-10 axle into a Vega etc but it also has the dimensions of the 1988-1993 7.625" S-10 axles.  The article shows the WMS-WMS to be 54.25" without the drum brakes installed.

S-10 axle codes:

The axle code can be found on a tag on the drivers side door jamb.  There is supposed to be a metal tag on the axle housing itself but the S-10 I measured didn't have the metal tag. If the tags are missing you will have to jack up the rear end and count wheel/drive shaft revolutions.

The following are the axle codes for the three ratios that are normally offered for the S-10,  GU4 is 3.08, GU5 is 3.32 and GU6 is 3.42.  The 3.08 ratio also came as an option with the Eaton LSD (not the best for a Locost) as G80.  The Torsen LSD from a Firebird or Camaro is a better unit for our use.

There is a rather complete list of GMC axle codes here,  LocostUSA.com

Ford axle codes:

The axle code can be found at two places on Ford products. The easiest place to find the code is on a white tag on the drivers side door jamb. On the bottom of the tag is a series of codes that concern the vehicle you are looking at. The code for for the axle ratio is underneath the label, "AX:"

Mustang door jamb codes:
The following Mustang codes are for an axle without "Traction Lok" (Fords name for it's LSD equipped axles);
2.73 = 8, 3.08 = Y, 3.27 = 5,  3.45 = F,  3.73 = 6

The following Mustang codes are for an axle with "Traction Lok";
2.73 = M, 3.08 = Z, 3.27 = E, 3.45 = R, 3.73 = W

Ranger codes:
The following sites have Ford codes for several different vehicles including the Ranger;  

Ranger etc axle information   (This site has lots of info including how to decode the axle tag info.)

The more reliable place to look for the axle ratio is on a metal tag attached to the rear axle cover by one of the cover bolts. It's possible the rear end has been changed and hopefully the correct tag will be on the cover.

The ratio is listed on the bottom left of the tag right next to the bolt. Sometimes the bolt head covers up part of the ratio number. If the axle happens to be a 3.73:1 ratio the tag will have "3 73" on it for an open differential. If the axle has LSD it will have "3L73" stamped  on the tag.

As usual with cars/trucks, all of these statements have variations;  I have found '92 & '96 Mustang door frame AX: codes printed as "68". The "6" would indicate a 3.73 gear ratio. I don't know what the "8" means  since both cars had 7.5" diameter ring gear axles with "3 73" stamped on the axle tags.  The rear axles appeared to be the original factory installation.

So what was the result of all these measurements?

It would appear that the 1986+ 4WD Ranger LSD axle would fit into my BEC book chassis with no modifications other than welding the suspension link brackets to the axle, re-drilling the lug bolt pattern and the U-joint bolt holes. The drum brakes should be a reasonable match for the RX-7 front disk brakes.  The S-10 might fit also but the jury is still out on this one.  

If you swap out a drum rear brake axle for a disk brake axle you will have to use a brake master cyclinder designed for front disk and rear drums.  A mixed brake master cylinder applies the rear brakes slightly ahead of the front disk brakes because drum brakes have more brake shoe clearance than the front disk brake pucks.

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