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[jaspermaggie]

What oil should I use in my 55 posi. I also have a vipration decelorating at 2,300 rpm and noise after 5,300 to 6,000. It is a 4.11 do you think I have to much preload or bat bearing, oil could also be low will look later.

[LDRollow]

You could go with the tried and true 80-90 wt. dino oil or the similar viscosity synthectic. But for a posi, I would add the GM or similar additive. If I read your post correctly, you are having noise on deceleraton. This is indicative of a problem in the gearset. Perhaps too much free play in the ring and pinion.

[wheezer]

Larry is correct on this response.
The correction is to pull out the third member and have a shop guy or club gearhead examine it.
High audable whine on decleration is a problem, with new gears or old. If you hear it early then its fixable if the gearset is fresh. If the gears are old it's questionable about saving the gears. I'ts my experience the older gears don't have the heat treatment to withstand too much back lash drag as the newer sets with better heat treatment.
If anyone has an interest I have and audio of a toasted 5.38 gearset at the old Denton Dragway/now NorthStar Dragway a few years back. Burnout/Acceleration then the horendus gear whine on shut down. Not a comforting sound. It was a gear set replacement effort after the event for that '55 Chevy and 3 weeks lost racing activity.
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"A Classic Car is Not Aerodynamic"

[jaspermaggie]

Took it out the bearings were bad.