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3.91's. Poor set up or poor manufacturing?

Stewsta

Stewsta

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Im think of buying a set of NOS 3.91's for a 741 case and Im curious if the notorious 3.91 growl is strictly from the factory not setting up the gears correctly or if its also a manufacturing problem. Thanks Stew
 
I used to have a factory set up 3.91 and now a 4.10 in my RR. Never heard any growl or noise but neither set was ever apart. Dr Diff can give you a great explanation on differences between high quality machined new gears and the lesser machining step Richmond etc gears. It's all in the amount of machining that goes into them is what I was told and have read.
 
My 3.91s are howlers, but I bought the center section on Craigslist so who knows what the history is, I've heard lots of the factory 3.91s in circulation ended coming up from old A100 Vans and pickups.
 
I'm running 3.91's in my Road Runner and have never heard any growling.
 
if they growl it is either set up poorly or straiter cut gears.could be either.straiter cut gears do not happen with factory gears.if you are sure the gears are stock then something is wrong.
 
OK. Im gonna rephrase what I was going for. My craigslist 3.91 third member growled once I installed it. I searched the forums for days and there was an over whelming number of people that posted that their 3.91's were noisy and that it was more commom with that gear set than with others. Maybe thats not the case and I read too much into what others were saying.
 
I have an NOS 3.91 ring and pinion for a 742 case, put them 2 years ago, (my buddy set them up), quiet as a mouse.
 
Gear noise is from poor set up or bad gear set
Run a pattern on you gears if there is not a wear pattern you still can adjust to make quieter
and if this is a worn set adjust to the old wear pattern and try

good luck
 
Not all growl is gear noise though, it could also be a sign of bad bearings.
 
Every set of 3.91s I have had were some what noisy. Some worse than others.
 
Mine also! Checked the pattern so many times while I was putting it together, everything was good, and that bastard still makes noise. After about 800 miles, tore into it again, and pattern was good, so my conclusion is some sets of 3.91's make noise.
 
You sound as if you have a bearing issue.

A set up issue is when the gear is set wrong from the inability to understand the pattern or how important .0015 is either way.

If you have a gear that makes a noise a between say 55 and 65, but it's quiet everywhere else and only at cruise, not at accel and decel then you have a gear manufacturing issue.
If you have a gear that makes noise at a speed longer than a 10 mph area, or does it on accel only or decel only, that is because it's so easy to set up gears and the person that did it thought so also.

.
 
With mine it's almost a rumble in the car at all speeds. Louder faster. Jacked up the rear end, put it in gear and crawled under the car. You could almost hear every tooth engage and you could feel it on the housing as we'll. Its like it was grinding sand between the gears.
 
FWIW My 3.91 third member came out of a pickup at a "Pick'n'Pull" and it runs quiet.
 
I would pull it and look at it and see whats going on, thats not right if you can feel the teeth moving onto eachother.

Just hope there isn't something else going on.
 
I've never heard of a particular gear ratio being a problem and I've been doing rear end work for a long time but have never done it for a living. I've also never had any problems with Richmond gears and that brand has been the majority of of my work but have heard others complain about that brand. Anyways, gears have a distinctive sound of their own and as they get worse, the noise does change and gets louder but there's usually no mistake between bearing noise and gear noise. The noise can be from a high pitch whine to a lower pitch howl (growl?) and this changes with speed. Generally, a good set of gears will usually be noisy on acceleration if the pinion is set too shallow....and noisy on deceleration if set too deep. Noise all the time? Junk em. Sometimes a lightly used set that's been set up wrong can be redone but it's a crap shoot. I've done a few used sets and have had good results but I will also bead blast them real good before putting them together.
 
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