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I am doing some upgrades to my 66 Mustang coupe and one of them is to fit a Trak Loc post unit as the original has a clunk when selecting gears. I know some people rate them and some do not but all I want to know is what size would I need? The car is Stock 66 289 Mustang which is getting a 302 EFI engine so is the rear end an 8" or an 8.8"?
Thanks for any help.
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it's an 8"........the clunk could well be a worn U joint......best to check that first as it's far cheaper to fix than changing out the third member
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Definitely an 8" rear. In the '60s the rears were 7.25", 8" and 9" The 7.25" was used only on six cylinder cars. Most other V8 cars got the 8", unless it was something especially performance oriented. You can tell an 8" from a 9" by whether or not you can get a socket and ratchet on the lowest two bolts that hold the carrier to the housing. You can do it on an 8", but not a 9"
The 8.8" became Ford's go to rear in the '80s, finding its way under every V8 Mustang built starting in '86 and running through the present day. Its a popular swap into the older cars due to its easy availability, low cost, and cheap upgrade parts.
Definitely check your u-joins before tearing into the rear. Its unlikely that a rear end issue would cause a clunk, except when going from say reverse to forward. A rear end clunk is usually caused by excessive backlash in the gearset, which wouldn't show up on a gear change, because the gears are loaded on the drive side; they don't come out of engagement (separate) to cause a clunk. In rare cases a broken spider gear in the differential can also cause a clunk, but again not typically on a gear change IME, only going forward to reverse or vice versa.
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Thanks for the responses and all the info. Very interesting. The first thing I checked were the u-joints in the hope that it was them but I'm sure the play is in the planet gears as there is no noise from the rear end at all.
It s just the annoying "Clunk" into forward or reverse on gear selection. It gets a bit embarrassing when lots of people are admiring the car and then you put it into gear!!
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You sure that car does not have a locker in it put in it by a previous owner?
With a Detroit locker it is perfectly normal to have a pretty healthy clunk when going from forward to reverse or vise versa. Being able to rotate the driveshaft 1/6-1/4 turn with parking brake on in neutral is normal with a Detroit.
Not saying you don’t have something wrong, just saying you may have nothing wrong...
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Did you remove the driveshaft to verify that the u-joints were good, or try to do it while it was still in the car? The reason I ask is that I've had bad u-joints before that defied my attempts to detect slop in them in the vehicle. Once the shaft was removed the slop became apparent. I'd start there. If the u-joints are good you had to remove the driveshaft to remove the carrier anyway.
The next step would be to remove the carrier You'll have to pull the axles to do that. With the carrier on the bench it will be easy to check backlash with a dial indicator. If you don't have one you can get a reasonably cheap one at Harbor Freight. Get the magnetic base too. Backlash should be 0.010-0.015". Over time as the bearings wear out its not uncommon to see the backlash grow to 0.040", which will absolutely cause an audible clunk when going into gear.
Here's a pretty good article about an 8" rebuild:
I'd advise using a solid spacer with shims instead of the crush collar. A little trickier to set up, but can be used repeatedly instead of once like a crush collar. It also adds considerable strength to the rear by preventing an oscillation based failure common to 8" and 9" Ford rears.
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