Conversion cost from 2.75 to 3.5 ratio

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jhawk635

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I read through a lot of threads here until I was dizzy trying to figure out what it takes to convert from my 2.75 ratio to 3.5. Is this something that a do it yourself-er can easily tackle or better left up to the pros? Ballpark cost to get it converted? Any help is greatly appreciated.

Thanks,

John

 
Not really a DIY job. You will need new ring and pinion gears in 3.5:1.

You should also get new bearings and seals. Jegs and Summit racing both sell kits with all the stuff you will need.

And if you want to change to a locking differential, you will need a new carrier and some limited slip additive.

 
I would say if you buy a complete pumpkin it could be done. I found these guys here:

www.justrearends.com

They have everything you need to do the swap.

If you want to keep your housing and only change the innards you might need some expert for the setup.

 
If you can pull the axles and 3rd, you can do it. Just find a used 3.50 geared 3rd on craigslist, or pull one from a 67-72 f-100. They were 28 spline like our cars. I paid $50 that last time I bought one.

If you bought gears ($150-200ish) paid a shop for setup ($500ish).. Well, it would be new.

 
Yes, if you have a 9" rear end I would just watch for a 3.25 or 3.50 3rd member for sale. 28 spline. You can swap it out yourself, they are HEAVY so a jack or a friend named Jack to help out is a really really good idea. It's messy and not a lot of fun but it can be done.

 
Last edited by a moderator:
You dont want that ebay item. That is called a spool which means the left and right axles are locked together. This unit is for straight line drag racing or circle track only. You need an open or limited slip or locking differential. NOT a spool. But to answer your question... Yes that unit would bolt in to your 9" housing.

 
Last edited by a moderator:
You dont want that ebay item. That is called a spool which means the left and right axles are locked together. This unit is for straight line drag racing only. You need an open or limited slip or locking differential. NOT a spool. But to answer your question... Yes that unit would bolt in to your 9" housing.
Click to expand...
Thanks...still in learning mode! No drag racing here. Just looking for a little quickness!

 
You did notice that they say that one is for oval track racing only, didn't you. It really isn't that bad. Just heavy and tedious. You'll need a couple of specialized tools, a shop press, and having a friend around helps a lot. I use the crush sleeve eliminator kits instead of a crush sleeve, and it make things a lot easier. Also, change out your bearings and sleeves while your in there. It's not something you want to get into often.

I swapped out the open diff in the 8.8 rear end on my F150 for an Eaton posi, and it wasn't as bad as I feared. I reused the 3.55 ring and pinion, but went with new bearings and seals. Mark you diff caps, before removing and keep everything organized, and it's not that bad. The nearby autoshop wanted $60 to press off the pinion bearing, another $60 to press the carrier bearings on, and another $60 to press on the pinion bearing. I went to Harbor Frieght and now have my own shop press. Cost $200 and now can use it whenever I want. The 12T press would have worked fine for what I did, but I got the 20T, cuz it was on sale. A lot of shops will do it for 10 bucks or so, but I think the one I used to visit, just didn't want to bother.

Anyway, it's not that difficult and I disagree with those who say a normal DIYer can't do this. Just don't rush, take your time, and if you run into problems post up some pictures and someone here should be able to give you some pointers.

Good luck!

 
You did notice that they say that one is for oval track racing only, didn't you. It really isn't that bad. Just heavy and tedious. You'll need a couple of specialized tools, a shop press, and having a friend around helps a lot. I use the crush sleeve eliminator kits instead of a crush sleeve, and it make things a lot easier. Also, change out your bearings and sleeves while your in there. It's not something you want to get into often.

I swapped out the open diff in the 8.8 rear end on my F150 for an Eaton posi, and it wasn't as bad as I feared. I reused the 3.55 ring and pinion, but went with new bearings and seals. Mark you diff caps, before removing and keep everything organized, and it's not that bad. The nearby autoshop wanted $60 to press off the pinion bearing, another $60 to press the carrier bearings on, and another $60 to press on the pinion bearing. I went to Harbor Frieght and now have my own shop press. Cost $200 and now can use it whenever I want. The 12T press would have worked fine for what I did, but I got the 20T, cuz it was on sale. A lot of shops will do it for 10 bucks or so, but I think the one I used to visit, just didn't want to bother.

Anyway, it's not that difficult and I disagree with those who say a normal DIYer can't do this. Just don't rush, take your time, and if you run into problems post up some pictures and someone here should be able to give you some pointers.

Good luck!
Click to expand...
Thanks Ron, good info. They also have 3.7 and 3.89 options. I'm going to get watching and reading. Really getting my feet wet before I jump in. My real goal is to have some quickness when I want to have a little fun.

 
I've done a few gear swaps in the driveway and it is very doable if you have access to a dial indicator to measure backlash on the ring and pinion and an/pound torque wrench to set pinion bearing preload. About the hardest part of the job is compressing the crush sleeve.

 
Depends upon your wrenching skills. I swapped 2.75 pumpkin for a brand new 3.50 pumpkin and had the guys at the shop do it for a couple hundred bucks, if I remember correctly. Swapping pumpkins is doable by yourself.

 
In my opinion, you should probably either have someone setup the new gear set for you or purchase one ready made. I have rebuilt several differentials thru the years and in addition to the other posters comments, one thing that has not been addressed is the pinion depth setting when installing new gears. Small variances in manufacturing processes require a pinion depth check to be made to ensure quiet, long lasting operation. Just getting the new gears and a bearing/seal kit and setting backlash is asking for a problem down the road. The one thing to remember about differentials is you know its right when the paint pattern comes out good.

tech17-ja01illa.jpg

 
When I rebuild a diff, I press off the pinion bearing, then use a cartridge roll to slightly enlarge the opening so it will slide off and on. I can use it for setup, and if I need to change shims, I can slide the bearing off to make that easier. Once I know the setup is correct, I pull the old bearing and press on the new bearing.

I also like to use a crush sleeve eliminator kit. It makes things SOooo much easier.

Also, when checking the paint pattern, install the axles and have some resistance applied to the axles when you turn the pinion. It will allow the paint to be squeezed out and make a distinct pattern.

 
One of the best things I did to my Mustang was to replace the 8" open-differential 2.75 gears for 9" with Auburn differential and 3.5 gears. I would have preferred a Traction-Lok. I bought the 9" at a salvage yard, bought new gears and differential and had the gears setup by an expert. It was easy to swap out -- everything bolted up fine.

 
I converted years ago from a 2.79 open to a 3.50 posi. I bought the new pumpkin with the new gears and installed. You will need your axle gear numbers when ordering. It was a simple swap. You definitely want to check your wheel bearings when you have the axles out.

 

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