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A few winter Jobs

GTR1999

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Looks like a 70's posi. Yes it is better then your DANA.

I would take it apart, I can see it has the slotted " snowflake" clutches in it, appears to have 10-17 spiders as well. My recommendation- polish and tune it. Once apart spray it down with brake cleaner and likely blow it off with air. Any cracks will show up still wet. If you find any cracks the case is all done. The marks are from the pinion, those can be polished out no problem. I would install solid steels, a new cross shaft, and mill the pad 120" deeper- that is how I do all mine.

If you are going to reuse the 308's radius the edge of the teeth, an old hot rodders trick and helps with mesh.
 

Antz81

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936
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1981 4 speed
Looks like a 70's posi. Yes it is better then your DANA.

I would take it apart, I can see it has the slotted " snowflake" clutches in it, appears to have 10-17 spiders as well. My recommendation- polish and tune it. Once apart spray it down with brake cleaner and likely blow it off with air. Any cracks will show up still wet. If you find any cracks the case is all done. The marks are from the pinion, those can be polished out no problem. I would install solid steels, a new cross shaft, and mill the pad 120" deeper- that is how I do all mine.

If you are going to reuse the 308's radius the edge of the teeth, an old hot rodders trick and helps with mesh.

As much as I want to just put in in so I can drive the car again I do think you are right.
IIRC you have never taken a diff with snowflake clutches without finding at least one that has broken.
When you say mill the pad 0.120" deeper what exactly are you referring to?
 

GTR1999

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As much as I want to just put in in so I can drive the car again I do think you are right.
IIRC you have never taken a diff with snowflake clutches without finding at least one that has broken.
When you say mill the pad 0.120" deeper what exactly are you referring to?

No, I have taken apart 100's without finding broken clutches but I have also found plenty with broken clutches. You won't know until you do check them. Some rebuilders are still selling & s them but I won't use them. You also want to get it apart to check it for cracks, polish it good inside and out, an radius the windows as shown on my threads. You will want to check the cross shaft holes for a step in them. I ever reuse the springs or plates either.

With the cross shaft bolt out, lay it up against the case where it would be inside and you can see that it is shallow into the bottom of the case, through the shaft. Look on Bman's thread on CF now, I just built him that cryo'd super10 and show this.
 

Antz81

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1981 4 speed
No, I have taken apart 100's without finding broken clutches but I have also found plenty with broken clutches. You won't know until you do check them. Some rebuilders are still selling & s them but I won't use them. You also want to get it apart to check it for cracks, polish it good inside and out, an radius the windows as shown on my threads. You will want to check the cross shaft holes for a step in them. I ever reuse the springs or plates either.

With the cross shaft bolt out, lay it up against the case where it would be inside and you can see that it is shallow into the bottom of the case, through the shaft. Look on Bman's thread on CF now, I just built him that cryo'd super10 and show this.

That thread sure is interesting.

Never heard of POR10 before but I think I would like to use it on mine. Was the top coat one of their products too?
 

GTR1999

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That thread sure is interesting.

Never heard of POR10 before but I think I would like to use it on mine. Was the top coat one of their products too?


No I use Seymours cast blast for diff top coats. I like the way it looks and sprays. Blast, Etch, Rinse, Flame dry, POR15, Cast blast. Tap the drain hole too.
 

Antz81

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Had another look over things yesterday before removing the carrier. found something strange. It seems like something is binding on every ninth rotation of the pinion. Have you every come across anything like that before? If it was every third rotation it would make sense, but why nine?
there did seem to be a strange mark on the pinion gear, but I'll need to take my camera into work on Monday to get a good photo of it.
 

GTR1999

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There should be no binding. Since this diff has been apart before I would suspect there may be some concerns to look over. You know the pinion already hit the case, it might had caused a burr. I have found burrs on both new and used gears and they caused binding and rough rotation. Dress the edges of both the ring and pinion, like I show on the CF thread by Bman.
 

Antz81

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There should be no binding. Since this diff has been apart before I would suspect there may be some concerns to look over. You know the pinion already hit the case, it might had caused a burr. I have found burrs on both new and used gears and they caused binding and rough rotation. Dress the edges of both the ring and pinion, like I show on the CF thread by Bman.

I'm planning on a proper rebuild anyway, which will include a polish and posi tune, and dressing the edges. Just thought you might have come across this before.
I do have a theory that there is a problem with the pinion bearings which causes the pinion to move into a pition that only causes binding at certain places based on where the carrier is in its rotation.
The bearings in the pinion don't seem to rotate smoothly with the carrier out anyway.
I'll pm you later about what parts i will need for this.
 

Antz81

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Odd contact pattern on pinion?

Took my camera into work today to get the photos of the pinion. Once the pinion gear was removed I only found a shim under the bearing on the pinion, I assume this is correct, but please tell me if there should be one somewhere else. This is some of the odd marks I found on the pinion gear. Not every tooth has the odd contact pattern on them, but the ones that do are all in the same area. Are any of these reasons to need a new gear set? I will hopefully get a chance to check for cracks on the carrier tomorrow.

IMG_5667.JPGIMG_5676.JPG
 

GTR1999

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12 37 stamped on the face = 308 pinion. Look at the LH sidewall inside the housing for any gouging or hit marks. If you see any then the rg bolts backed out and sheared in the past. If a snap ring or broken metal chip ended up in the gear mesh it would either lock it up or grind it down and that would damage the teeth. I would get back assembled and make a handle to rotate the posi and gear. Use an axle and make a metal plate to bolt to it. Then weld a nut on the center and come out with a bolt with a large head, like a 3/4" hex. Put your axle into the LH side and use a ratchet to rotate it. That is how I do final pattern checks and listen for gear mesh. You will be able to feel any roughness and hear it. If you see a burr or shiny spot on the gears, dress them with a dremel and recheck OR just buy a new set of Tom's gears and start fresh.
 

Antz81

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1981 4 speed
12 37 stamped on the face = 308 pinion. Look at the LH sidewall inside the housing for any gouging or hit marks. If you see any then the rg bolts backed out and sheared in the past. If a snap ring or broken metal chip ended up in the gear mesh it would either lock it up or grind it down and that would damage the teeth. I would get back assembled and make a handle to rotate the posi and gear. Use an axle and make a metal plate to bolt to it. Then weld a nut on the center and come out with a bolt with a large head, like a 3/4" hex. Put your axle into the LH side and use a ratchet to rotate it. That is how I do final pattern checks and listen for gear mesh. You will be able to feel any roughness and hear it. If you see a burr or shiny spot on the gears, dress them with a dremel and recheck OR just buy a new set of Tom's gears and start fresh.

Housing looks fine. No signs of broken ring gear bolts, or anything else to be concerned about.
Removed all burrs from the gears, and reassembled. Still binds up as it did before. It's binding for about three teeth every ninth rotation of the pinion. (if that makes sense, doesn't matter if i'm using the side yoke or the pinion) Until this point it feels and sounds fine, and returns to normal afterwards. Although it's not consistent, sometimes it will spin freely for a while and then will start again, but it's never less than nine rotations before it binds again. It feels like the teeth are catching on something or not meshing properly, but when looking at them there's no obvious reason why. I've checked for them catching on the carrier and that's all clear so I think it's a meshing problem. I'm starting to think new gears might be easier.
 

GTR1999

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Good sign that no obvious impact spots but again there should be no binding. Sometimes I'll try to reach in and paint the mesh spot of binding and then check the teeth closely when removed. I use setup pinion bearings on job after job, they would be no good for actual use because one they are undersize and two they have been preloaded too many times, but even with these I don't get any gear binding from the bearings. I agree I think I would be looking at a new Tom's gear set, then you know what you have to start and should be good to go another 100,000 miles.
 

Peer81

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'81 Black
Also remember that the original alu 81 diff only came with a 2.87 ratio. The 3.08 you have are an aftermarket set already and the question is what quality or previous life did they have.

Greetings Peter
 

Antz81

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Also remember that the original alu 81 diff only came with a 2.87 ratio. The 3.08 you have are an aftermarket set already and the question is what quality or previous life did they have.

Greetings Peter

3.08 thats in there seems to be the original gm set. Date coded 6 72. At least i think thats s date code. But yest i have no idea what life they have had.
 

Antz81

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Started to get things apart again today after work, and found something interesting (purely by chance) When the carrier was out I placed it on the bench with the ring gear bearing in the race it was in when installed and spun it. I then noticed that it wasn't spinning true, it looked like it was wobbling, but with a pattern to it. I then swapped it to the other side and used the other race, and it looked normal. swapped it back (ring gear end bearing and race) and the wobble was back. I then swapped thing around for every combination of race and bearing, and found it only had the wobble with the race from the ring gear side of the carrier. I can only assume that the race is warped or worn in such a way to cause this.
After this I spun the pinion gear in the lathe to see what it looked like spinning. I was watching the wear pattern on it and noticed that for one or two teeth the pattern seems to move. Reviewing the video I got of it, it looks like it isn't spinning true, but that could be the camera movement or the lathe, or the fact I was on'y gripping it at one end.
 

GTR1999

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Did the diff have the cast iron shims in both sides or the kit steel stackable shims? How was the lash before you took it apart, in the 6-8 range? You can dial in a posi in a lathe but it was vary a little bit on the bearings in the housing. I am doing a 12 bolt conversion now for a 64 vette, the posi case is machined when ordered but I always check them in the housing I am going to build. I have a couple of slip fit case bearings I use to check them. If you suspect the ring gear is running out, witness mark it to the case and remove it. Get a couple of new case bearings and polish them in your lathe, if you have a toolpost grinder that would be much faster but emery will work too. Clean the case and polish the flange in the lathe, install the setup bearings and shim them snug but not binding. Mount an indicator against the flange and very slowly rotate the case and see what you have for flange runout. If you are 002 or under you're good, you might get by at 003" but anything over you have to correct by machining or replacing the posi case. Once you get to checking the lash it also should be 002" max or under, again 003 might work but more then you will have to correct.
 

Antz81

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1981 4 speed
Did the diff have the cast iron shims in both sides or the kit steel stackable shims? How was the lash before you took it apart, in the 6-8 range? You can dial in a posi in a lathe but it was vary a little bit on the bearings in the housing. I am doing a 12 bolt conversion now for a 64 vette, the posi case is machined when ordered but I always check them in the housing I am going to build. I have a couple of slip fit case bearings I use to check them. If you suspect the ring gear is running out, witness mark it to the case and remove it. Get a couple of new case bearings and polish them in your lathe, if you have a toolpost grinder that would be much faster but emery will work too. Clean the case and polish the flange in the lathe, install the setup bearings and shim them snug but not binding. Mount an indicator against the flange and very slowly rotate the case and see what you have for flange runout. If you are 002 or under you're good, you might get by at 003" but anything over you have to correct by machining or replacing the posi case. Once you get to checking the lash it also should be 002" max or under, again 003 might work but more then you will have to correct.

It had the cast iron shims. Lash before I took it apart was fine although I only checked it in a couple of places, but where it was binding there would have been zero lash. If there was any runout of the ring gear I believe it would have been the bearings causing it.
I am concerned about the wear pattern in the pinion, it moves by about 0.08". I think I will go with a new gears, I don't think these ones are going to be very nice to setup.
 

GTR1999

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you're this deep into it now, I agree go with new gears. Word of advice use only TOM's gears and buy them direct from Tom. I would still check the case runout as well.
 

Antz81

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you're this deep into it now, I agree go with new gears. Word of advice use only TOM's gears and buy them direct from Tom. I would still check the case runout as well.

Thinking I'll go to 3.36 gears. Should give me better acceleration but a lower top speed. Engine will be at about 2700 at 65mph, up from 2200rpm that i had with the 2.72, so will use more fuel (but who ever bought a vette because they want fuel economy?)
 

GTR1999

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1969,1972,1975
What trans do you have? I don't recall if I saw it but then I didn't go back through the info. 336's are good street gear. I moved up from 308 to 336's with my 325hp 350 with th400= little rpm difference in that jump. You will feel it more going from 273 to 336. If automatic it won't be a problem, with M20 Muncie or ST10 it will be good, if you should have a M21 CR it won't be too good. 5 Speed will be good although OD gear may be low.
 

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