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rear end yoke sizes

6

6T3Vette

Guest
Hopefully you guys can bail me out. I recently bought a 3.55 Posi out of a '79 Vette with an automatic transmission off of ebay. My intent is to swap it out with the 4.11 Posi that is currently in my '63. While I love the take-off power, I'd like to have something in her that is more gas friendly.

Here's the problem. The companion flange yoke on my '63 is 3 1/2" and the flange on the 3.55 is 4 1/2". I thought I could simply put my flange on the 3.55, however, after pulling both flanges off I discovered that my 3 1/2" has 17 splines and the 4 1/2" has 30. Is there a 3 1/2" flange that was made with 30 splines on it or is there some kind of conversion or work-around I should be looking for? Someone also told me I may be able to use the pinion out of my 4.11 on the 3.55. I thought that sounded hokie. Doesn't the pinion and ring gear need to be a matched set?

Thanks,
Terry
 

WayneC

Well-known member
Joined
Jan 5, 2002
Messages
472
Location
Santa Barbara CA
Corvette
63 roadster project
Ring & pinion are a matched set.


I'm not fully certain I understand the flange issue, not having expertise on differentials, but I think you are saying you are going to replace the entire differential (not just the posi unit). Where is the extra inch? Is the problem that the driveshaft won't fit because the newer differential has a longer companion flange (with a different spline that prevents you from using your old companion flange), or is the problem that the bolt pattern fot the U-joints is different than on the driveshaft?. Could you have a driveshaft shop shorten your driveshaft by an inch, or change out the U-joint yoke for a later one, or are there other issues than just the driveshaft?

If you don't get a reply here, it might be worthwhile to call a transmissions and differential vendor, such as Terry Rosebush Motorsports 517-893-8299..
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
'63 and '64 used 17-spline pinion gears with the small flange; '65 and up used 30-spline pinion gears with the large flange. In order to use the 3.55 diff you bought, you'll need to have a shop convert your prop shaft to the large yoke style, or you may be able to find a "conversion U-joint" that adapts two different sizes of yokes (has two different cross lengths on it). Ring and pinion gears only work in matched ratio sets, even with the same ratio and tooth count - they're machined and lapped together as a pair, and can't be separated.
 
6

6T3Vette

Guest
John,

Thanks for the info about the conversion u-joint. I called my local NAPA parts store and they carried it as a regular-stocked item and very reasonably priced...about $18.

Thanks,
Terry
 

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