To shim or not to shim
09-13-2023, 12:05 AM
#1
To shim or not to shim
I recently scored a 4th gen Camaro rear end with a 3.42 gear and torsen differential to swap into my '89 Camaro 7.5 rear end which had the dreaded 2.73 gear open diff set-up. I'm wondering if it pays to get too technical about this when I do the swap by checking pinion depth, setting backlash, shimming, etc., or would it be a better strategy just to put everything from the 4th gen rear into my 3rd gen rear the way it was set up at the factory, do a paint test and if the gears are meshing properly and the pinion bearing has the right amount of preload on it call it a day. The reason I ask this is because it looks like GM rear ends were never assembled with a great deal of precision to begin with. Neither one of these rear ends has stacks of thin shims on the pinion or ring gear carrier: all they have is a very thick shim on either side of the carrier and a crush sleeve for the pinion. Everything inside both of these rear ends looks good like it was working properly. One thing I am planning to do is to put new bearings in the rear end while I have everything apart, so I don't know if that will change any measurements or not.
09-13-2023, 08:05 AM
#2
Re: To shim or not to shim
I recently scored a 4th gen Camaro rear end with a 3.42 gear and torsen differential to swap into my '89 Camaro 7.5 rear end which had the dreaded 2.73 gear open diff set-up. I'm wondering if it pays to get too technical about this when I do the swap by checking pinion depth, setting backlash, shimming, etc., or would it be a better strategy just to put everything from the 4th gen rear into my 3rd gen rear the way it was set up at the factory, do a paint test and if the gears are meshing properly and the pinion bearing has the right amount of preload on it call it a day. The reason I ask this is because it looks like GM rear ends were never assembled with a great deal of precision to begin with. Neither one of these rear ends has stacks of thin shims on the pinion or ring gear carrier: all they have is a very thick shim on either side of the carrier and a crush sleeve for the pinion. Everything inside both of these rear ends looks good like it was working properly. One thing I am planning to do is to put new bearings in the rear end while I have everything apart, so I don't know if that will change any measurements or not.
09-13-2023, 10:13 AM
#3
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Re: To shim or not to shim
Right: they had bins of single shims in various thicknesses, and just pulled the sizes needed for a particular housing.
Shims go with the housing. The housing is what has all the tolerances in it, that need to be adjusted for. Gears are VERY precise: usually no more than a .001" or 2 between sets.
This means then, that you can imagine a gearset floating around in space somewhere, all perfectly aligned and set up; and your job is to hang this random housing you've got around it and make it fit the gears. This also means that whenever you take a rear apart, then if it was fit together properly to begin with, you can just put the same shim thicknesses back in; because they go with the housing, not the gears. In your case, that means you'll need to take the head bearing off of your existing pinion, measure the shim under it, remove the head bearing from your "new" pinion, and put the same thickness shim under the new bearing you'll be putting on that gear before you install it.
That said, the factory's original carrier shims are something very brittle and prone to wear; maybe even just cast iron. They are RARELY re-useable as-is. They are often broken, or break during disassembly, and virtually always have DEEP grooves worn in them where the bearings ride. This is the primary reason for the backlash opening up in high-mileage rears. They should ALWAYS be replaced during any kind of service, if at all possible. Sometimes you can measure what's left of them, if there's any undamaged portion, and duplicate the thickness with better quality ones; sometimes not.
For whyever, they got the pinion bearing bore MUCH more accurate than the carrier one. The vast majority of 7½" housings will take a .035" pinion shim as a result. No guarantees but that's what's almost always there. The carrier shims are usually anywhere from .230" up to .270" or so. They should be selected to give NO LESS THAN the backlash spec (different sources, for their own reasons, give this as anywhere from .007" to .010", with the high end anywhere from .010" to .013"... more toward the high side of the spec is better... I usually set em up at .010" - .012"), then at least .005" more shim should be added to each side, to preload the carrier bearings, before final installing the carrier. This is not always easy. I use 2 large C-clamps to sqweeeeezze the carrier and bearings including the outer races together so they'll go down between the shims.
Aftermarket shim packs come with 2 thick washers per side, usually .100" each; then you stack up abuncha thin ones in between those, to get the thickness you need.
Re-using bearings is a REAL BAD idea, unless they're very low mileage, and you know they were good to begin with. Best to get a "gear install kit". It will have all the bearings, seals, shims, etc. to do the job right.
I HIGHLY recommend using a pinion crush sleeve eliminator. https://www.summitracing.com/parts/rat-4111 That way, you don't have to stop tightening the pinion nut when the right pinion bearing preload is reached; instead, you can keep on tightening it until it SCREAMS for mercy. Furthermore, that keeps the nut from backing off in the future when the crush sleeve crushes some more. I also HIGHLY recommend getting new axles, as stock ones are CRAP, and are almost always bad. The axle itself is the inner race of the bearing; if it's bad, then the bearing as a whole is bad by definition, even if you replace the part that's called the "bearing".
As relatively cheeeeeeep as gears are, this is onehelluvalotta maze to go through without maximizing the cheeeeeze you end up with. If 3.42 isn't y our "ideal" gear ratio then it'd be PROFOUNDLY STUUUUPID to put yourself through all this work and expense and risk, and yet leave something in there you don't really want.
Shims go with the housing. The housing is what has all the tolerances in it, that need to be adjusted for. Gears are VERY precise: usually no more than a .001" or 2 between sets.
This means then, that you can imagine a gearset floating around in space somewhere, all perfectly aligned and set up; and your job is to hang this random housing you've got around it and make it fit the gears. This also means that whenever you take a rear apart, then if it was fit together properly to begin with, you can just put the same shim thicknesses back in; because they go with the housing, not the gears. In your case, that means you'll need to take the head bearing off of your existing pinion, measure the shim under it, remove the head bearing from your "new" pinion, and put the same thickness shim under the new bearing you'll be putting on that gear before you install it.
That said, the factory's original carrier shims are something very brittle and prone to wear; maybe even just cast iron. They are RARELY re-useable as-is. They are often broken, or break during disassembly, and virtually always have DEEP grooves worn in them where the bearings ride. This is the primary reason for the backlash opening up in high-mileage rears. They should ALWAYS be replaced during any kind of service, if at all possible. Sometimes you can measure what's left of them, if there's any undamaged portion, and duplicate the thickness with better quality ones; sometimes not.
For whyever, they got the pinion bearing bore MUCH more accurate than the carrier one. The vast majority of 7½" housings will take a .035" pinion shim as a result. No guarantees but that's what's almost always there. The carrier shims are usually anywhere from .230" up to .270" or so. They should be selected to give NO LESS THAN the backlash spec (different sources, for their own reasons, give this as anywhere from .007" to .010", with the high end anywhere from .010" to .013"... more toward the high side of the spec is better... I usually set em up at .010" - .012"), then at least .005" more shim should be added to each side, to preload the carrier bearings, before final installing the carrier. This is not always easy. I use 2 large C-clamps to sqweeeeezze the carrier and bearings including the outer races together so they'll go down between the shims.
Aftermarket shim packs come with 2 thick washers per side, usually .100" each; then you stack up abuncha thin ones in between those, to get the thickness you need.
Re-using bearings is a REAL BAD idea, unless they're very low mileage, and you know they were good to begin with. Best to get a "gear install kit". It will have all the bearings, seals, shims, etc. to do the job right.
I HIGHLY recommend using a pinion crush sleeve eliminator. https://www.summitracing.com/parts/rat-4111 That way, you don't have to stop tightening the pinion nut when the right pinion bearing preload is reached; instead, you can keep on tightening it until it SCREAMS for mercy. Furthermore, that keeps the nut from backing off in the future when the crush sleeve crushes some more. I also HIGHLY recommend getting new axles, as stock ones are CRAP, and are almost always bad. The axle itself is the inner race of the bearing; if it's bad, then the bearing as a whole is bad by definition, even if you replace the part that's called the "bearing".
As relatively cheeeeeeep as gears are, this is onehelluvalotta maze to go through without maximizing the cheeeeeze you end up with. If 3.42 isn't y our "ideal" gear ratio then it'd be PROFOUNDLY STUUUUPID to put yourself through all this work and expense and risk, and yet leave something in there you don't really want.
09-13-2023, 10:53 AM
#4
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Re: To shim or not to shim
I should add also:
If this isn't THE IDEAL housing that you want for your "forever" rear axle, it'd be equally profoundly stuuuupid to do all this work to it. That means, you should be doing this to a core with PBR discs. Not drums, not D-M discs.
If this isn't THE IDEAL housing that you want for your "forever" rear axle, it'd be equally profoundly stuuuupid to do all this work to it. That means, you should be doing this to a core with PBR discs. Not drums, not D-M discs.
09-13-2023, 11:02 AM
#5
Re: To shim or not to shim
Thanks for the advice. Yes, I did order a ring and pinion install kit and new axles. I'll look into the crush sleeve eliminator.
I also get concerned about having bearing rollers riding directly on the ends of the axles where they just get splash lubrication.
A 3.42 gear was what I was looking for to go with the T5 I swapped in for the car's original 700R-4 earlier this summer.
I also get concerned about having bearing rollers riding directly on the ends of the axles where they just get splash lubrication.
A 3.42 gear was what I was looking for to go with the T5 I swapped in for the car's original 700R-4 earlier this summer.
09-13-2023, 11:36 AM
#6
Re: To shim or not to shim
I’ve had this same theory working in my head for some time now, I think I’m going to take the plunge. My question is when going over to a crush sleeve eliminator do I just measure the removed crush sleeve and replace with matching eliminator and shims? You mentioned that the crush sleeve has likely crushed more since initial installation, so I’m sure the measurement would be somewhat off. Is there a “usual” amount they crush more or should the pinion be assembled and measured for preload?
09-13-2023, 12:06 PM
#7
Re: To shim or not to shim
Excuse my ignorance, but what are PBR discs and D-M discs?
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09-13-2023, 05:01 PM
#8
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Re: To shim or not to shim
One at a time guys. 
First, the crush sleeve eliminator:
There's absolutely no way to know how much a crush sleeve is going to crush. It's a property of the difference between however long it starts out, and however much space there needs to be between the head & tail bearings to get the right preload. No way to predict it. If there was, you wouldn't need a crush sleeve in the first place: you could just use some standard length solid spacer in EVERY rear, and not have to deal with that. BUTT, the housing isn't machined sufficiently accurately for that; not by a long shot. So, one way or another, you have to have something adjustable.
Consider a front wheel bearing. Keep in mind that the pinion bearings are the EXACT SAME THING, except at the front wheel the outer piece spins and the inner sits still, while it's the other way round on the pinion. So, when you're putting a FWB together you tighten the bearing to achieve the desired preload and put in a cotter pin to hold the nut, right? Well, believe it or don't, pinions used to be the same way. "Classic" Chevy trucks still used that as late as the early 60s. Problem was, at the speeds and power levels that pinions operate at in a modern V8 powered car, a cotter pin is too easy to shear. Gotta have some MUCH more secure way to hold the nut in place. A Nylock won't cut it; besides not having enough retention on its own even when new, all it takes is getting good and hot one time, and it's toast. The "solution" (I use the word very loosely) the factories came up with is the crush sleeve. It gives the nut something to tighten against so it doesn't just back off. Problem is, if the crush sleeve ever crushes any more after its original installation, there's no tension on the nut anymore; and that happens ALL THE TIME. The act of dropping the car into reverse does the EXACT SAME THING to the crush sleeve as the nut does; and sooner or later, if it's done violently enough, the crush sleeve crushes some more, and voilą! the nut is now loose, and you have vibes, whine, clunking in reverse and metal shed into the fluid as the pinion gets sucked into the case and chews up the carrier, etc. etc. etc. All the usual "nut backed off" problems.
The crush sleeve eliminator has shims that you stack up (or more accurately, UNstack) to set the bearing preload. First, before working on it, you have to arrive at the right pinion shim, which is beyond the scope of this discussion, if you don't already know what it should be by way of the one that the factory put in that housing. You final-install the head bearing with that much shim behind it; put a tail bearing that's honed out to slip over the pinion without too much trouble into the hsg with no seal; slip the solid spacer and too many shims onto the pinion, and install it into the hsg; tighten the nut tight enough to be sure everything in there is metal-to-metal; measure the play with a dial indicator; and take out that much shim. Repeat as necessary. Then once you get all the play out, you keep taking out more shim, in very small increments, until you get the preload you want (say, 24 in-lbs to turn the pinion in the bearings). Then you take it back apart one more time, install the seal, set your new unmodified bearing in the pumpkin snout, install the seal, install the pinion, and tighten THE LIVING BEJESUS out of the nut. Now, there's NO WAY the spacer is ever going to crush; the nut is WWWWWWAAAAAAAAYYYYYYY tighter than it could ever be if you have to stop tightening when the crush sleeve crushes enough to get the preload you want, to begin with; and the nut NEVER backs off. EVER.
Brakes:
PBR = 89-92 aluminum calipers with 12" rotors (made by PBR in Australia)
D-M = 82-88 cast-iron GARBAGE calipers with 10½" rotors (made by GM, D-M stands for Delco-Moraine)
First, the crush sleeve eliminator:
There's absolutely no way to know how much a crush sleeve is going to crush. It's a property of the difference between however long it starts out, and however much space there needs to be between the head & tail bearings to get the right preload. No way to predict it. If there was, you wouldn't need a crush sleeve in the first place: you could just use some standard length solid spacer in EVERY rear, and not have to deal with that. BUTT, the housing isn't machined sufficiently accurately for that; not by a long shot. So, one way or another, you have to have something adjustable.
Consider a front wheel bearing. Keep in mind that the pinion bearings are the EXACT SAME THING, except at the front wheel the outer piece spins and the inner sits still, while it's the other way round on the pinion. So, when you're putting a FWB together you tighten the bearing to achieve the desired preload and put in a cotter pin to hold the nut, right? Well, believe it or don't, pinions used to be the same way. "Classic" Chevy trucks still used that as late as the early 60s. Problem was, at the speeds and power levels that pinions operate at in a modern V8 powered car, a cotter pin is too easy to shear. Gotta have some MUCH more secure way to hold the nut in place. A Nylock won't cut it; besides not having enough retention on its own even when new, all it takes is getting good and hot one time, and it's toast. The "solution" (I use the word very loosely) the factories came up with is the crush sleeve. It gives the nut something to tighten against so it doesn't just back off. Problem is, if the crush sleeve ever crushes any more after its original installation, there's no tension on the nut anymore; and that happens ALL THE TIME. The act of dropping the car into reverse does the EXACT SAME THING to the crush sleeve as the nut does; and sooner or later, if it's done violently enough, the crush sleeve crushes some more, and voilą! the nut is now loose, and you have vibes, whine, clunking in reverse and metal shed into the fluid as the pinion gets sucked into the case and chews up the carrier, etc. etc. etc. All the usual "nut backed off" problems.
The crush sleeve eliminator has shims that you stack up (or more accurately, UNstack) to set the bearing preload. First, before working on it, you have to arrive at the right pinion shim, which is beyond the scope of this discussion, if you don't already know what it should be by way of the one that the factory put in that housing. You final-install the head bearing with that much shim behind it; put a tail bearing that's honed out to slip over the pinion without too much trouble into the hsg with no seal; slip the solid spacer and too many shims onto the pinion, and install it into the hsg; tighten the nut tight enough to be sure everything in there is metal-to-metal; measure the play with a dial indicator; and take out that much shim. Repeat as necessary. Then once you get all the play out, you keep taking out more shim, in very small increments, until you get the preload you want (say, 24 in-lbs to turn the pinion in the bearings). Then you take it back apart one more time, install the seal, set your new unmodified bearing in the pumpkin snout, install the seal, install the pinion, and tighten THE LIVING BEJESUS out of the nut. Now, there's NO WAY the spacer is ever going to crush; the nut is WWWWWWAAAAAAAAYYYYYYY tighter than it could ever be if you have to stop tightening when the crush sleeve crushes enough to get the preload you want, to begin with; and the nut NEVER backs off. EVER.
Brakes:
PBR = 89-92 aluminum calipers with 12" rotors (made by PBR in Australia)
D-M = 82-88 cast-iron GARBAGE calipers with 10½" rotors (made by GM, D-M stands for Delco-Moraine)
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Gordon G E (09-14-2023)
09-14-2023, 12:29 PM
#9
Re: To shim or not to shim
Somehow I ended up on another thread with my comment about using the brakes that came with my 4th gen rear end on the 3rd gen rear end I'm rebuilding.
I got the idea to do it from this thread on the forum: https://www.thirdgen.org/ls1reardisc/ . What I'm seeing is the 3rd gen caliper mounting flange is a bit narrower where the bolts for the 4th gen backing plates need to go through - and it's cut out a bit on one of the bottom corners - than the 4th gen flange (and it's a bit thicker which doesn't matter), so three of the bolt holes you end up having to drill in the flange for the new backing plate end up being very close to the edge (which may not matter) and one bolt hole can't be drilled at all unless you add some metal to it with a welder.
Probably the proper way to deal with this is to just cut the old drum brake flanges off of the old rear end, fabricate some new brackets like the 4th gen caliper brackets, and then weld them in place. A lot of work but, it's probably worth it.
I got the idea to do it from this thread on the forum: https://www.thirdgen.org/ls1reardisc/ . What I'm seeing is the 3rd gen caliper mounting flange is a bit narrower where the bolts for the 4th gen backing plates need to go through - and it's cut out a bit on one of the bottom corners - than the 4th gen flange (and it's a bit thicker which doesn't matter), so three of the bolt holes you end up having to drill in the flange for the new backing plate end up being very close to the edge (which may not matter) and one bolt hole can't be drilled at all unless you add some metal to it with a welder.
Probably the proper way to deal with this is to just cut the old drum brake flanges off of the old rear end, fabricate some new brackets like the 4th gen caliper brackets, and then weld them in place. A lot of work but, it's probably worth it.
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