used 3.91s

-
aaronk785

aaronk785

Well-Known Member
Joined
Apr 21, 2013
Messages
1,038
Reaction score
876
Location
westmoreland ks.
Bought some used 3.91s off ebay a while back. Just installed them and they whine horrible on coast and very light cruise. Any other time their quiet. Backlash at .008 and pinion depth is correct and pattern looks good. New bearings and preload is set correctly. Any ideas. Did I buy a set of junk gears.
 
Sounds like it. Ring and pinions and camshafts are two things I would never buy used.
 
Are they factory or aftermarket? the factory are always quiet, but the aftermarket ones i have had some issues with noise.
 
I used to build those rears. Started with small stuff under the guidance of the line-foreman. Moved up to bigger and bigger stuff.
We were a rebuilder company.
In over 2 years,I never ever saw a new CnP.
I admit tho that bad gears exist, and once they have been run noisy, it seems they will always be noisy. My own 3.55s have been adjusted several times....... and ,they're still a little noisy for like 18 years. I met a guy who wanted to buy them. I took him for a ride and he got to know those gears. They weren't bad really and he liked them enough to have me install them in his 71 E-body. The deal was if he couldn't live with the noise, I would buy them back; but he would pay the RnR. He showed up about a week later, and took me for a ride. Hokedoodle those gears were loud in his E. I yanked them out, and put them back in my car. I could barely hear them in my 68 Barracuda. So IDK. But I'll tell you one thing; You'll never see an E-body in my driveway,lol.
 
Pro gears? I think the “hardened” Pro gears tend to whine a little more than a “street” gear.
 
Factory gears. Took it for a 30 mile cruise tonight. At 30 mph it will damn near drive you out of the car. Very loud. At 60 it disappears. At low speeds you can hear the gears growling. Pretty sure the gears are toast. Thanks for the replies. Forgot to mention I'm running a spool till I rebuild my sure grip.
 
I like used gears for testing - to see if thy might be the right match for a track combo. But for street, and for longevity at the track, you should go new.
In terms of pattern, if the bearings were replaced on a used set of gears, you're better setting up the pattern on the coast side. Less wear there and usually they will stay quiet. If you're looking at the drive side, it doesn't take much to be noisy, so yeah - pictures of the pattern would be nice. I'll add - .008" backlash on a used set of gears (like well used - enough to wear out bearings) is probably not enough backlash... So the gears IMO are not set up properly just based on that.
 
Thanks everyone. Gears are still in the car and will be for a couple weeks. When I get it out I will try to get pics and get my nephew to post them cause I'm computer stupid.
 
I heard an old mechanics tale of sandblasting the R&P and set it up again. The guy said it allows the R&P to 'mate up' again, then you readjust after 200 miles. Could be a load of stinking carrier lube, I dont know.
 
If therefactory gears in a 741 case u will almost never set rid of the noise as they came from the factory with as much as .030 back lash. U have to set them up on the coast side no matter what the drive side looks like. I’ve tried setting up with less bl and they all whine even if they were quiet b4. Kim
 
pishta said:
I heard an old mechanics tale of sandblasting the R&P and set it up again. The guy said it allows the R&P to 'mate up' again, then you readjust after 200 miles. Could be a load of stinking carrier lube, I dont know.
Click to expand...


I used to sandblast ring and pinion and then set them up.
Had a mechanic tell me that sandblasting them changes the hardness of the gears. And I thought about it and I stopped doing the sandblasting.
So I don't know. I don't remember having a whine on the ones that I sandblasted though.
 
Mine still whine but they sure blast off in that little Barracuda . Only problem they were made to drive 55 .... I CANT DRIVE 65. Hagr812. Let Sally Drive. Best Sammy song ever, Outside Montrose
 
I bead blast used gears lightly to remove the gloss. I can't imagine that the surface tempetature is raised enough to modify hardness. Plus, I've never see any detremental effects. I'm sure if you hit 'em with 240 pounds and a hard abrasive things would be different.
 
Least nothing harder than a 3 gear Smoke run to cleaned up. I got mine here from green his dad had them in a bucket since the 70s from His AAR Cuda
 
You must log in or register to reply here.
-

Back
Top