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MG MGB GT V8 Factory Originals Technical - What is the weak part of the rear ends on the MG

Is it the axels, dif, or what?
STD

The tube types are very strong. I've only managed to break one;)
Dauntless

UJs on propshaft
Roger

UJ & Spider gears My spiers only broke when I goofed around tryinng to tighten up the action.
Larry Embrey

Isnt there some issue with the roll pin that holds the spider peg in place? Seem to recall it can split and the peg will walk out of the diff cage until it encounters an immovable object- like the diff housing or the pinion gear.

Simple two bit part and an hour under the car if i read this right
Greg Fast

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R T BONDPACNWBNDMGC@aol.com

Posted 08 June 2001 at 05:27:24 UK time


I finally got around to checking the small clunk in the rearend, as was reccommended in the AMGCR "C-NOTES". When I removed the cover and inspected the pinion pin I found that the roll pin had sheared and the pinion pin was free floating, Why it had not fallen out and totally destroyed the rearend I don't know. I had recently been cruising between 80-90 the results of a failure would have been deadly. The cost of a new roll pin is 55 cents.If you have not checked yours, DO IT NOW !The cost of a new Diff is probably close to 800.00. This way you can find out your gear ratio also. Mine is a 1969 roadster with O/D and the gears are 3.7:1. Believe me don't wait

Greg Fast

One other "weak" point, lack of ratio selections.
Bill Young

yes the pin is also an issue. I heard from some MG shops that the factory team had issues with that.
Larry Embrey

There is a simple, cheap fix for the roll pin problem. I found this in a tech article somewhere 20 years ago & have always done this to MG rears.

This is normally done when replacing the cup washers & phenothalic shims in the rear to eliminate the clunk. After replacing the roll pin with a new one, insert a cotter pin inside the roll pin, chosing a size that makes as tight a fit as possible, & spread the ends in the usual way.

This gives you something close to a solid pin that is much stronger, & if the tension gives up due to heat or whatever, the cotter (split pin) will prevent the roll pin from falling out.

How is that for a $.15 fix!
Jim Stuart

Great idea-

I was wondering why no one had used or suggested a dowel pin rather than a roll pin. It should certainly be more stout, but the diameter would need to be much more accurate than a roll pin.

I just replaced the roll pin- and the cover is still off the diff- it's off to the parts store when they open in 4 hours!
Greg Fast


Greg-

A dowel pin does not provide the "tension" required to stay in place. Roll pins are a slit of spring steel, sometimes called tension pins, with tapered ends which allow them to be driven into a hole in a compressed state. This compression is what is supposed to keep them in place.

After many, many heating & cooling cycles, roll pins eventually loose their temper & no longer have enough tension to keep them in place.

Possibly, you could obtain a dowel with the correct interference fit to stay in place, but a cotter pin is a cheasper solution.
Jim Stuart

OR, we could do what ford does on the 8.8 and thread one side of the carrier and use a screw pin type set-up. that is how they hold thier main dowel in the 8.8 OR, just get a more stout rear end?? I know not feasible for everyone, took me 2years to get my hands on one and now I have to get it narrowed...
Larry Embrey

What we have been doing with the rool pin is a #12 long aviation bolt that I get from by brothers shop, cut it down, take a punch in a vice & swell the end of the shaft, with another punch & hammer flip it over & swell the other end. We have had no problems with this fix. We change the fiber thrust washers & use stainless steel thrust washers on the spider gear & cage. I have broken 2 axels & once got stuck in the snow, burned the oil off the spider gear shaft & broke the spider gears. I have a M.G.C. 3.07 rear with 450,000 miles on it & have pulled a 3,000 lbs U-haul ALL over the east coast (250,000 miles) & it is VERY quiet. I take the flywheel & lighten it to 20-22 lbs. to take the bang off the rear when I am running her hard. If I am putting a 3500 or 3900 together for me I am going to use the B rear.
Glenn Towery

Jim-

Concur that the roll pin w/ cotter is a simpler faster solution, and did this on the car before I had to move it yesterday.

My question is based on failure cause- does the roll pin loose tension, or is there something else happening? I'm asking from a position of ignorance, not challenging anyone, as I've never personally seen the failure. Merely heard the dire warnings.

If the failure is strictly loosing tension and being thrown out, the cotter is not only quick but totally adequate.

I looked at the roll pin I took out of the MGC rear end on Friday- there was no evidence of wear, so maybe there is no real shear loading on the pin- for some reason, I was expecting to see some evidence of fretting or movement near or around the peg/diff cage joint.

Maybe a dowel pin is a more permanent solution, maybe not. If i were to use a dowel, I'd ream the hole to size, install the pin, then peen the ends of the hole to prevent the pin from moving out.

But without evidnce that the roll pin fails due to shear, this may be overkill

Greg Fast

This thread was discussed between 15/10/2003 and 20/10/2003

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