On my 4th rear end please help

574 Views 5 Replies 5 Participants Last post by Grimbrand, 2 mo ago

80SNAKE

Discussion Starter · 2 mo ago

Im on my 4th rear end. Every time it starts with a hum coming from the back then a boom followed by clunking, like gears binding up and every time the diff is pulled busted metal. Now the first three were 7.5 but this last was a8.8 from an 89gt completely rebuilt with carbon clutches the whole nine yards. Drove it gently for a while and eventually started to hammer down. Now I hear a faint hum.im man dam my luck. 1980 cobra 302 60 over carb,cam,intake and long tube headers. T5. Thanks

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crjackson

· #2 · 2 mo ago

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5LHO

· #3 · 2 mo ago

You may wish to prove the case and tubes are actually straight. These things when got from wrecking yards have, often, been in a wreck and thus, are tweaked. I'm not going to go into setting up these rears. Factory, it's slap to torque and they go a zillion miles. Redone, more attention needs to be paid to the pattern, depth, pinion nut torque and crush sleeve dimensions. Don't waste time with the frangible 7.5s, unless it's a gentle, low powered cruiser.

When you say diff....busted metl, do you mean the differential is breaking or the main gears?

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Grimbrand

· #4 · 2 mo ago

The 7.5 is inadequate. Enough said.

However the 8.8 should be fine for anything a 302 can throw at it! Did you buy new gears? What kind of gear oil did you decide to run? Did you follow the manufacturer's break-in procedure, and change the gear oil out at the appropriate interval? These things have to wear the teeth to match, and there is a very specific process to follow! If you did not use the recommended fluid, and just drove it a hundred miles or so before flogging it, I am not surprised you may have some noise.

This may not be the news you wanted to hear, but people that put rear ends together ALWAYS include detailed instructions on how to break them in. I've never met one that didn't, because failure to follow procedure usually ends in catastrophe.

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Atilla

· #5 · 2 mo ago

I had another brand with that problem, a Triumph Spitfire. Previous owner had failure after failure and gave me the car. I bought a new differential gear set and followed the break in procedure. First time i took it on the freeway, found it wouldn't do more than 55 MPH at WOT. I had installed the wrong set of gears, Trying to push for a few more MPH, blew the gears again. Be sure you get the proper ratio based on your engine, transmission gearing ratio and the final in your Diff. Finally found the right ratio and sold (gave) my toy to my younger sister.

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Grimbrand

· #6 · 2 mo ago

Atilla said:
I had another brand with that problem, a Triumph Spitfire. Previous owner had failure after failure and gave me the car. I bought a new differential gear set and followed the break in procedure. First time i took it on the freeway, found it wouldn't do more than 55 MPH at WOT. I had installed the wrong set of gears, Trying to push for a few more MPH, blew the gears again. Be sure you get the proper ratio based on your engine, transmission gearing ratio and the final in your Diff. Finally found the right ratio and sold (gave) my toy to my younger sister.

The 8" will handle stupendous abuse, as I can personally attest to. Neutral drops, bootlegger turns, doughnuts, burnouts for distance - it took all of that and more when I was a younger man, and never complained.

I have seen them fail, but that's usually when combined with sticky drag radials on asphalt. If you let out the extra horsepower in the form of smoke, the rear end will last indefinitely. Even in most race scenarios, the 8" works pretty well with a Windsor (once the gears are broken in and have gone through some heat cycles). With enough power from the engine, and enough traction, you'd be as likely to candycane your rear axles as break the diff.

A 9" has more hypoid (gear offset) and better pinion support. That, combined with its beefier ring gear makes it much stronger, but also steals about 4% more power due to frictional losses. It's also a lot heavier. If you're not running more than 400 horsepower, and/or run street tires, not slicks, you would be hard pressed to hurt an 8" in a Mustang.

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