[Johnny406]

I'm not even sure this is feasible hence the question, but has anyone ever machined down a 3 series ring gear to fit a 4 series spool? I'm currently running the "thin" Yukon 3.90 gears in my bracket car with a full spool running 275-60-15 drag radials. I generally race the street/foot brake class hence the radials over slicks. My problem is my very stout TH400 transmission has a 2.75 first gear so overpowering any tire isn't too difficult. I know I can go to the thin 4 series 3.73 but I'd like to get into the 3.42 to 3.55 range and sadly no one seems to make a 12 series ring in those ratios for a 4 series carrier.

[firechickenman]

Is a 3 series spool not available for your application?

[Johnny406]

Originally Posted by firechickenman

Is a 3 series spool not available for your application?

Believe it or not, nobody makes a 3 series full spool for a 12 bolt.

[firechickenman]

Strange has them in 33 and 35 spline. I didn't see one for 30 spline.

[Johnny406]

Originally Posted by firechickenman

Strange has them in 33 and 35 spline. I didn't see one for 30 spline.

You're right, I forgot to mention that I have aftermarket 30 spline axles and C clip eliminators so I'd rather not replace those again. My best bet is to probably go with a Eaton posi and be done with it.

[ponjohn]

3.73 is it for a 4 series spool.

That is what is so attractive about a ford 9, spool in any ratio.

Even a Dana is limited to 3.54.

[77 TRASHCAN]

Your gear could be machined, a real machinist would need to perform the job, though.

[Scarebird]

not so sure of that - those gears are hard as hell.

[Skip Fix]

I have a thin 3.73 in the IA/Camaro's 12 bolt Moser spool 33 spline.

[77 TRASHCAN]

Originally Posted by Scarebird

not so sure of that - those gears are hard as hell.

Most machinist's would not tackle this job. I have been blessed to have a friend that's a real deal machinist. He likely would do such a job for me, not many others...just because of the degree of difficulty...

[Jack Gifford]

Originally Posted by Scarebird

... not so sure of that - those gears are hard as hell...

Not that hard where it needs to be cut. No problem with carbide mills.

Having said that; I can't speak about the overall gear design, regarding the resulting strength of the gear.

Also, if the result is too little bolt thread engagement in the gear, that's a problem. I imagine that the gear is too hard to drive a bottoming tap another 1/4" or so into the gear.

[Half-Inch Stud]

My 2-Series and 3-series 12-bolt posi chunks sold like hot cakes awhile ago. Got no more, just good used guts.

Got a ring gear spacer of a correct thickness for some sort of series/gear swap. Probably have the longer bolts..

[Half-Inch Stud]

Such a hard-steel gear would be grinded. right?

Probably at equal cost of a custom spool from Strange.

[firechickenman]

I had the cover off my 12-bolt the other day and didn't remember how thin the 3.73 (11/41) for a 4 series carrier looks. I'm not sure if a 3.42 (12/42) or 3.55 (11/39) was machined for a 4 series carrier that there would be enough material to support the ring gear teeth. It wold be interesting to see what the difference is in the pinion size of the three gear sets.

.310" is the difference between a 3 series and 4 series carrier.