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#1
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Differential question
Hi
I have a corporate gm differential with 390 posi gears. I've jacked it up and removed the wheels and drums. Both my drums had some warpage but the pads still had 6/32 left on them so about 60% left. I decided to get some new drums. So question for you, I put the new drums on and pads aren't touching them. Which is fine, I know how to run the star wheel till slight drag but what has me concerned is: When I turn the drum on the axle it's easy going for 80% of the revolution then gets very difficult to turn. Then you move through that 20% more and it gets easier. Same thing if you keep turning it. Isn't it suppose to just be free all the way around? Is that normal? This rear end is a c-lip rear end. E-brake is off, vehicle in park. The u-joint is not moving as i spin the drum manually. Hopefully this is normal and it's just me?
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#2
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Go in the car and press the brakes once to center up the shoes. Bolt the brake drums up tight with wheels on, or lugs first. See if that makes a diff for ya. HTH
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#3
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If you jack up both sides of the axle and turn the wheel "Forward travel" on each side, maybe you can pick out which side is causing the 20% binding condition. With a Chebby style clutch type rear you might be able to identify the issue more easily.
Tom Vaught
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#4
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The parts store only gave me one drum at the moment. So I'm only turning the passenger side drum assembly. I will relocate the drum to the other side adjust pads out a bit and try turning it, see if it suffers the same difficulty turning.
The shoes aren't part of the equation as to why the differential is difficult to turn the 20% of the revolution, so hitting the brakes I don't think will make a difference. I'll go try the drum on other side and report back Tom. Thanks everyone.
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#5
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Same thing other side, both directions difficult to turn 20% of the revolution. I guess next step pull dif cover and inspect. Hmmmmmmm
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#6
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Quote:
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#7
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Maybe put 2 lug nuts on to be sure the drum is square to the axle flange. I hope the drums are "round"
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1968 Firebird 400 RAII M21, 3.31 12 bolt, Mayfair Maize. 1977 Trans Am W72 400, TH350, 3.23 T Top Everyone you will ever meet knows something you don't. Bill Nye. |
#8
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I put it into neutral. Spun it forward, difficult to move, requires more strength but the tension all the way around is the same BUT after several revolutions or so... It spins very freely almost as if it's slipping, like where did the tension go? (I notice u-joint not moving when this happens ) - then it catches after a couple more revolutions again and it's very hard to move. This is on the drivers side.
I tried the same procedure in reverse direction and it was identical. the one drum for sure is perfectly round, spun up on machine, brand new.
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#9
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"The shoes aren't part of the equation as to why the differential is difficult to turn the 20% of the revolution, so hitting the brakes I don't think will make a difference."
If the problem seems to be inside the differential, why put the drum on at all? |
#10
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From your first post: [COLOR="Red"]"I have a corporate gm differential with 390 posi gears." [/COLOR=Red"]When the posi is connecting, you are turning both axles, if for some reason (worn out) it loses tension, then your posi is going bad. Adjust your brakes when you get the rest of your parts, put the tires back on, and do a burnout! If only one tire smokes, you have identified the culprit!
Last edited by arcitech; 05-01-2017 at 08:54 PM. Reason: colors |
#11
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#12
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STEELCITYFIREBIRD -> The task at hand was to inspect brakes, and part of adjusting brakes (I'm learning brakes in school / we haven't taken differentials and I know zero about them aside of 2 tire burnouts and one tire burnouts ) is to rotate the drum and feel for shoe drag. While doing that is when I noticed weird differential drag.
I know that my differential isn't a pontiac differential but I haven't gotten 100% confirmation as to which one it is. I posted on her some pics and the consensus was I believe that it was a corporate bop rear end, could be oldsmobile, I bought the car with it in there. A friend told me several years ago after I did a burn out that I left only one tire mark, I always assumed it was because of old tires or I was on an angle. It could very well be worn out. I'll call him , he says it was just one tire moving ... but I know for sure mechanics have said it is a posi unit. So must be worn.
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#13
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Unfortunately the odds are and just like Murphy's law , your Posi is toast!
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#14
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Based on the latest info you have provided, I would agree that the Posi unit needs to come out of the vehicle.
A Factory Cone style Limited Slim Differential by its design TRIES to turn both wheels evenly every time. A Clutch type unit that shows tire rubber on one side of the car is wounded if not failing. A Friend with a 65 GTO had a Eaton friction plate Posi-unit that failed after one weekend at the track with good tires. You might want to read this article as it gives a couple of good tips and some diagnostic advice. http://www.hotrod.com/articles/10-an...on-improvment/ Tom V. I am assuming the 20% tight deal is a spot where the discs bind together and do restrict motion somewhat keep the wheels from moving. Not working as a Posi, just binding.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#15
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Thanks everyone for your help. Now may be the time to drop down to 331 or 342 or 355 gears if at all possible and get posi working again. check all axle bearings and some new fluid and away we go! Now just to find out what differential shop can assist me. Maybe I'll take it on under their guidance if I'm lucky enough.
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#16
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put up a couple good pics of the center of the rearend & I can ID it, have racks & racks & a big pile of original GM muscle era rearend housings & all of them are getting moved to their new home this next week.
Of the original late '60's muscle era A-body rearends, only the PONTIAC 8.2 10 bolt & the '68-70 BUICK 8.2 10 bolt originally used a cone type posi carrier. If the rear is a corporate 8.5 A-body rear, I can ID all three versions. There is a possibility, if that is the case, & it's a '71, that the posi carrier is a Warner Motive 2 pinion cone type. Have one of those as well...
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Buzzards gotta eat... same as worms. |
#17
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Quote:
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Some Fear, Because if you've never been afraid- just a little. You're obviously not going as fast as you could be. |
#18
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A fellow named Keith helped me a long time ago, he wrote:
>>Yes, your rearend is an Olds rearend. Well, it's actually a >>corporate >>rearend, in that it was used on Oldsmobiles, Pontiacs and Buicks >>of the >>same era. It's the 8.2 10-bolt. I've got the same one in my '67 >>that >>came from a '67 442 with a 4-speed. >> >>Keith >> >> > > > With this rearend, there is no 'Pontiac' or 'Olds'...it's GM corporate....that means the exact same rearend was used on various GM makes and models, unlike other rearends that were specific to a certain make. Take a look at the following link for more info: http://www.oldsmobility.com/old/reardiffs.htm http://wildaboutcarsonline.com/cgi-b...=9990382924295 http://www.442.com/oldsfaq/ofdif.htm...ential%20Codes http://forums.maxperformanceinc.com/...d.php?t=795793 Hope this helps. Keith MORE INFO ON MY DIF: Rear drivers side of DIF 12345 67890 Rear passenger SIDE Horizontal 2 FRONT DRIVERS 68D Front PASSENGER SIDE 1385724-F
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#19
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Hi, just lost my message, stupid website.
So in 1st gear, slight brake stand, two tires spin, let off brake and go, one tire burnout. In reverse, two tire burn out when i quickly stab the throttle for a quick second to back up is there upgrades for the posi unit? can i just replace it ? I was thinking of buying the kit and with the help of a red seal mechanic friend, try and fix it? Any videos? Rebuild kits you know of? Or potential R&R as an option?
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
#20
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I've had 2 of these in 2 different cars and no problems out of them at all. If you're just running a car on the street, this fits the bill perfectly. http://www.ronsmachiningservice.net/...line-19674-10/
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"I know just enough to keep me here, but not enough to get me out" |
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