I have seen TGO members posts on this board saying that you can't put 10 bolt parts into a 9 bolt rear end, however, no one has explained why not? Aside from the parts are from different rear ends, is it just a dimensional problem, ie 9 bolt carrier is 8 in wide, vs 10 bolt carrier is 7 in wide?? If something like this could be acheived with minimal difficulties, then for all the people that have a worn out 9 bolt, they could "fix it" with an Eaton posi carrier for the 7.625 and an appropriate gear set, and then they'd be "off to the races" so to speak!!!

Someone with rear end expertise should be able to explain this better than just saying no you can't do that!!!

Isn't the word "no", just a challenge to find a way to do it????

 

It exactly the same as trying to use Ford 7.5" parts in a GM 7.5" diff or trying to put ford 5.0L heads on a GM 5.0L block. They are 2 completely different makes and parts are not compatible.

The only similarity between the 10 bolt and the 9 bolt is the external mounts which allows them to be installed into a third gen.

 

Gotcha!! I understand about different makes, and non-compatability, particularly with cylinder heads, but what prevents this particular swap/build, is it just a dimension difference, or is there something physically preventing this from happening? Parts can be machined, and adapted, albeit at astronomical prices, but maybe there is a "magic" combination of parts from Ford, GM, Chrysler, or even the Toyota, or Honda guys, that will make this possible. Granted it might take a year or more of researching this, but it would be well worth it, not to have to scrap my 9 bolt rear due to lack of factory replacement parts!!

What do auto restorers do when there is no part available for their 1909 RollsRoyce Silver Ghost??? Simple, they make the part, or adapt something else to fit/work!!!

That's what I'm trying to figure out here, If this swap/build is possible!!

 

This is going to take a while.

To use a 10 bolt Eaton Posi in the 9 bolt would require machining a case or ring gear to fit the 9 bolt ring gear to the Eaton Posi. The 9 bolt actually has 8 bolts in the ring gear. The 10 bolt has 10. I don't know what the ID is on the 9 bolt and 10 bolt gears, but I'm sure that it isn't the same. You can't put 10 bolt gears into the 9 bolt housing becasue of differences in the pinion shaft length, bearing placement, pinion offset from the ring gear center line... so adapting the 9 bolt ring gear to the Eaton would probably be the best way to go. If you could talk a gear maker into making a 9 bolt gear with the correct thickness, ID and bolt pattern to fit the Eaton then that would be the way to go. This would cost more than buying a truck load of Moser 12 bolt rear ends, which are much stronger than a 9 bolt anyway. I don't know what the differences are in the over all width and ring gear flange offset in the 9 bolt and 10 bolt differentials, but I know that the ring gear sits much closer to the center of the differential on a 3 series 9 bolt differential than a 3 series 10 bolt differential, so a thicker ring gear would be required. If the 10 bolt differential would fit in the 9 bolt housing then that would be fine, but if it is too wide or narrow then some machine work and fabrication would be required for that. Then you get to the axles. The 9 bolt and 10 bolt have differenti axle splines. That's not a problem because Moser can make a set of axles with any splines on them that you want. That's the easiest thing to fix about this whole problem.

So what it comes down to is why spend huge amounts of money trying to adapt parts into a weak rear end when you can get a much stronger rear end for less money? The Moser 12 bolt for the 3rd gen starts at around $2250. You couldn't put an Eaton Posi into the 9 bolt for that money.

 

Thanks Big Gear Head!!!

The jist of this is, yes it can be done, for huge amounts of money!!!

That's what I was trying to get the answer to, saying no was just too simple for me!!!

BTW, I agree that the easy way is to just buy another new rear end, and swap it out for something much stronger, for less $$$$, as you have said above.

Thanks again!!

 

With enough time and money, you can make any part work with something it's not designed for. Most of the time, time is never an issue but money is. For what it would cost to adapt and modify a 10 bolt posi to fit into a 9 bolt case, it would be cheaper and easier to just buy an aftermarket 9", 12 bolt or Dana 60 which would all be better than trying to use highly modified weaker components.

As for the restorers, even trying to restore a vintage aircraft where replacement parts are non existent, the cost to fabricate a new component is worth the effort in the long run. It's not worth the effort to modify and fabricate something new when a better replacement or option is available. There are times when even modern day components are just not good enough. A local has a blown SBC in the back of a Fiero. He has a custom built geartrain transfer case because none of the factory style transaxle systems could handle the power and there is no aftermarket. His custom made gears are worth $10,000.

I had to work on a Chinese forklift once. Every dataplate on it was in Chinese. There were zero parts available for it in the country and it needed a starter. I adapted and modified to make a GM starter work. It was easier than trying to find an obsolete starter from China and the customer was very happy that his old forklift was fixed.

Just because the 9 bolt, 10 bolt and a Dana 44 were available for the third gen doesn't mean anything internally is even close to the same. Design ideas change drastically between manufacturers. Even with the aftermarket 9", 12 bolt or Dana 60 diffs, even though they're designed to be a direct fit into a third gen, none of these diffs share any common parts internally.

 

If you want to see adaptive ingenuity at work, go walk through Comp Eliminator pits. Someone may be running an inline 6 engine that has no aftermarket performance parts. They'll take two V8 heads, cut a cylinder off both, weld them together then remachine proper head bolt holes through the head to attach it to the block.

I have a drag race magazine from a few years ago where a racer is running a BBC 4 cylinder engine. He had a custom made block cast based on half a BBC engine. It all depends how much money you're willing to spent to get what you want.

 

Thanks to all for the quick replies!!!

Sometimes the outside appearance is worth the money!!! In this case, my IROC is just a "run of the mill, nothing special" IROC. Priceless I tell you, priceless!!!!

It's nice knowing that it "could be done", as opposed to "impossible, never going to happen"!!!

Thanks again!!!

 

Hey don't give up.

Even GM does pointless stuff like that.

Iam a proud owner of a 8.75 MP posi.

Which has a 10 bolt cover, but stuffed inside is a 12bolt carrier.

And neither 10 bolt or 12bolt stuff sold today fits it.

The 10bolt cover is larger then a reg 10bolt so no fancy covers for me,
or even a cover gasket made anymore.

And of coure the 12bolt stuff is all wrong too.

Which means its like finding a 4leaf clover to get gears to sawp in.

Hence why its still a 2:73 ratio.

I have 2 of these with 2:73 gears, 1 peg leg, 1 posi.

More info n pics here, was use up till the 80s still by GM, avoid at all cost unless you want a strong one with 2:73 gears.
https://www.thirdgen.org/forums/tran...lly-knows.html

 

Yea, I've got an Eaton Posi for one of those. Some had 31 splines and some had 28. The Eaton that I have is 31 spline.

 

I never give up, I may employ different tactics, but I never give up!!!

Persistence is the key!!!!

Thanks Gumby!!

 

would the disc brakes from the 9 bolt fit on tha 10 bolt?? get at me fast i need to know!!!

 

The calipers and rotors are exactly the same between the two. If you wish to swap on PBR's (89-92) to an early year, you'll need 10-bolt backing plates.

JamesC