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waterbucket
Philip Waterman
England
(70 posts)

Registered:
07/30/2011 01:08PM

Main British Car:
1978 MGB GT

GM 10 bolt rear axle S10
Posted by: waterbucket
Date: May 05, 2016 01:03PM

I have a question regarding these axles, many of you recommend these axles because of low weight, standard LSD on a lot and choice of ratios. In the UK we do not see these that often but on checking I find that they used the 8.5 inch axle on the S10 from 1995 and the only ones I find in the breakers yards are late 1990's, my question is this is this a much heavier axle than the 7.5" and if so does anyone know how much heavier.
regards Philip


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5761 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: GM 10 bolt rear axle S10
Posted by: BlownMGB-V8
Date: May 05, 2016 01:54PM

Philip, you should base your selection on engine horsepower, and personally I would be very wary of going to a smaller axle than the OEM MGB axle. I know, a lot of guys have done it and that concerns me. I worry that we have a bunch of vehicles out there that might experience axle failure after a few years. The long term durability is what bothers me.

So the question must be asked,"Is the Salisbury axle overkill?" If it is, then the 7.5" axles are probably justified. The Salisbury was after all, a truck axle. It's well known weakness are the spider gears and the axle shafts, but is the R&P set overly large?

I would say that for HP levels below 200 the answer could be "yes". And for those applications a well made 7.5" axle may be adequate. Over 300 HP I would say that the MGB R&P is undersized. I have personally worn out a set, which were initially in good nick with the proper contact pattern and backlash. Behind my engine those gears lasted MAYBE 20,000 miles at most. Now granted my power level was perhaps a good bit over 300, it's hard to say what you really get when you push 16 lbs of boost through a stock 215 Olds. Certainly less than 400 I would think. So that is a fair representation of a practical upper limit.

If you think of a graph with horsepower on one axis and miles on the other, it's easy to see that any axle is going to give you a curved line. At one end the HP is low and the mileage is near infinite, at the other HP is high and mileage is very short, terminating in breakage. That curve will be similar for all axle sizes by simply scaling up or down the horsepower numbers. You want to be somewhere on that line that gives you an acceptable mileage expectation at the horsepower you plan to have, without excessive size and weight. This is basically how automakers choose axles, plus factoring in additional percentages to reduce warranty and accident claims. This "safety factor" can easily at least double or triple the miles driven figure, which is why high mileage junkyard axles generally have quite usable R&P sets. It is cheap for an automaker to oversize the axle a bit compared to dealing with recalls. Plus, they have to design for abuse. They must assume that the buyer is going to hammer the axle mercilessly, and some do. The foregoing is obvious from what we see in terms of axle failures. The mileage of this curve rises rather sharply in the region most commonly used, which is both why small increases provide great durability improvements, and why it is cheap insurance to go a bit larger.

So in practical terms, axle selection is based on size. The metallurgy hasn't changed in many decades, the alloys used a century ago are still being used, at least to some extent, and the last half-century has shown only very minor incremental improvement. So a new axle, or a European or American or Oriental axle, are all very much the same. Size is the determining factor. So using the smaller axle is exactly the same as moving towards the shorter end of the mileage curve. You are trading your safety factor for less weight. How you drive will determine if that was a wise decision or not, but in terms of R&P life, it is full throttle acceleration in any and all gears, not hard starts, that primarily determines gear life. In other words, the volume of horsepower transmitted through the gears determines heat generated and acceleration of wear.

From what we've seen with the MGB, 300hp is a reasonable practical limit for the stock size R&P, which is about 7-7/8 or 8" in diameter, but a good bit of that safety factor is gone at that level so you may have to do repairs. (Ford 8" axles were very rarely used behind V8 engines for example) 8-1/2" is a good axle for up to 500 hp. Above that you may want to consider something larger.

Jim


waterbucket
Philip Waterman
England
(70 posts)

Registered:
07/30/2011 01:08PM

Main British Car:
1978 MGB GT

Re: GM 10 bolt rear axle S10
Posted by: waterbucket
Date: May 05, 2016 04:10PM

Jim
I will only have 250bhp so any of them should suffice and I totally take your point regarding longevity, but as I hope to lose nearly 100lbs from the front end I do not want to add more to the back if I can avoid it. As you may recall a few weeks ago I posted about Jaguar diffs, The Jaguar IRS has great appeal for ride as well as handling but it is heavy, I can also easily obtain a Salisbury 4HA axle which will fit straight in (I think) but is very heavy but comes with a good ratio. If the GM 8.5 is lighter and comes with a LSD and the right ratio it would be well worth considering as an alternative to the Jaguar IRS.
best regards
Philip


MGBV8
Carl Floyd
Kingsport, TN
(3639 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
79 MGB, Buick 215

authors avatar
Re: GM 10 bolt rear axle S10
Posted by: MGBV8
Date: May 06, 2016 11:14AM

Philip,

Is that 250hp at the flywheel or the rear wheels? You mentioned the weight of the rearend a couple of times. I would like to find lighter than the stock Salisbury. Seems the best route there is the 8 inch Ford with Wilwood calipers & the now discontinued aluminum center case.

The 8.5 inch rear is going to be right at 200lbs before narrowing. The 7.5 will be virtually the same as the MGB Salisbury. I think the 7.5 will hold up fine. I know of one behind a 300hp Buick 300 in a B. I would go with the later 7.625 with the 28 spline axles (vs 7.5 w/26 spline). Both are still called 7.5. I think the 7.625 started arounf mid '89 in the S10, earlier in the Camaro.

The Ford Ranger 7.5 is an other option. It's likely strong enough, as well, & possibly a bit lighter than the S10 or MGB.


mgb260
Jim Nichols
Sequim,WA
(1962 posts)

Registered:
02/29/2008 08:29PM

Main British Car:
1973 MGB roadster 260 Ford V8

Re: GM 10 bolt rear axle S10
Posted by: mgb260
Date: May 07, 2016 06:39AM

For under 350 HP I like the Ford Ranger and S10 7.625 for availability. Here is a picture of Chris Gill's with Tru-Trac and Mustang disc brakes.
Chris's 7.5.jpg



Edited 1 time(s). Last edit at 05/07/2016 06:41AM by mgb260.


MGBV8
Carl Floyd
Kingsport, TN
(3639 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
79 MGB, Buick 215

authors avatar
Re: GM 10 bolt rear axle S10
Posted by: MGBV8
Date: May 07, 2016 09:15AM

Very nice!

I was trying to find the weight difference, if any, between the Ford 7.5 & the S10 7.5.


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