Changing the ratio of a Ford 7.5" diff

sidecar · Post by **sidecar** » Sun Feb 14, 2010 5:08 pm

Hi There,

I read somewhere that its is possible to change the ratio of a Ford 7.5" LSD diff by changing the crown and pinnion gears.

Anyway I might be able to get hold of a set of new gears meant for a granny diff and I want to fit them into a Cosi 2WD diff, does anyone know whether this is possible?

I want to go from 3.64:1 to 3.36:1

Cheers!

Cobratone · Post by **Cobratone** » Sun Feb 14, 2010 6:02 pm

Pete, have you tried posting on one of the dedicated "fast Ford" websites??

sidecar · Post by **sidecar** » Sun Feb 14, 2010 6:10 pm

Cobratone wrote:Pete, have you tried posting on one of the dedicated "fast Ford" websites??

Cheers Tone, I'll do that if I get no joy elsewhere!

Darkspeed · Post by **Darkspeed** » Sun Feb 14, 2010 6:56 pm

If the 2WD is 7.5" I don't see that it would be a problem

I am running a 7" diff and just found a reasonably priced 3.38 for my car to replace the 3.93 that had been fitted.

Andrew

richardpope50 · Post by **richardpope50** » Sun Feb 14, 2010 7:13 pm

I was told by Stuart at Classic Conversions that the key internals of a 7" and 7.5" diff are the same so no effective difference.

Richard.
(In case you do not know, CC is a well known in the se7en world small engineering company in / near Sheffield but I only have his old contact details and he does not have a web site! He makes all sorts of diff combinations.)

stevieturbo · Post by **stevieturbo** » Sun Feb 14, 2010 9:03 pm

If the parts exist, no real reason why it shouldnt be possible. Although it would be handier to build an LSD into the casing with the 3.36 gears, than build a complete assembly from scratch. Setting pinion depth is critical, and a PITA. Lash is easier, but still a PITA.

Special tools are required for various parts.

sidecar · Post by **sidecar** » Mon Feb 15, 2010 4:54 pm

Thanks for the advice guys, I'll keep you posted.

stevieturbo · Post by **stevieturbo** » Mon Feb 15, 2010 7:28 pm

richardpope50 wrote:I was told by Stuart at Classic Conversions that the key internals of a 7" and 7.5" diff are the same so no effective difference.

Richard.
(In case you do not know, CC is a well known in the se7en world small engineering company in / near Sheffield but I only have his old contact details and he does not have a web site! He makes all sorts of diff combinations.)

Im sure he knows more than me.

But when the dimension relates to the Crownwheel size...I really cannot see how a 7.5" diff unit would ever fit onto the smaller crownwheel, or fit into the smaller casing. The output shafts are larger too, with a larger spline.

I dont see how any if the internals are the same ?

SuperV8 · Post by **SuperV8** » Wed Feb 17, 2010 1:10 pm

Just done the exact same thing.

I had a 7.5" granny cosworth 3.64:1 with LSD but wanted a lower ratio. Got hold of a 7.5" 3.36:1 open diff. I did plan on swapping the gears into my LSD case but was strongly advised against it. The gears and case need to stay together otherwise you will need to set the pinion height, which belive me you don't want to do.

Much easyer (and what I did) to fit your LSD into the case with the 3.36 gears. As it happens the 3.36:1 case I used had thicker/ more webbing so would be stronger than the cossy case.

Have a look at my pictures:
http://forum.daxsportingclub.com/phpBB3 ... =35&t=4305

Still a tough job. Lots of pulling and pressing of bearings (if you re-use your old bearings would be a lot easyer) Still need to make a couple of tools to hold the pinion nut and to undo the output bearing housings.

I have a manual. PM me if you need it.

Tom.

sidecar · Post by **sidecar** » Wed Feb 17, 2010 3:38 pm

SuperV8 wrote:Just done the exact same thing.

I had a 7.5" granny cosworth 3.64:1 with LSD but wanted a lower ratio. Got hold of a 7.5" 3.36:1 open diff. I did plan on swapping the gears into my LSD case but was strongly advised against it. The gears and case need to stay together otherwise you will need to set the pinion height, which belive me you don't want to do.

Much easyer (and what I did) to fit your LSD into the case with the 3.36 gears. As it happens the 3.36:1 case I used had thicker/ more webbing so would be stronger than the cossy case.

Have a look at my pictures:
http://forum.daxsportingclub.com/phpBB3 ... =35&t=4305

Still a tough job. Lots of pulling and pressing of bearings (if you re-use your old bearings would be a lot easyer) Still need to make a couple of tools to hold the pinion nut and to undo the output bearing housings.

I have a manual. PM me if you need it.

Tom.

Hi Tom,

Cheers for the advice,

I was planning to buy the gears off a bloke on ebay but he has not got back to me so I reckon the plan has gone down the pan!

I'll keep an eye open for an open 3.36 diff although they do seem to be very rare. If I get my hands on one I'll be back!

Pete