Wheel lug fitment when changing from 7.5" to 8.5" rear axle

dgmeadows

Greasemonkey
Supporting Member
Mar 29, 2017
166
63
Georgia
My 1979 Cutlass had a 7.5" rear axle. As part of drivetrain upgrade, I acquired an 8.5" rear axle from a 1984-5 Hurst Olds. The '79, 7.5 axle had 7/16" lugs, the newer 8.5" has metric lugs.

I ordered Year One 17" Olds SSII wheels with 7/16" lug nuts. It appears the 7/16" lugs from the 7.5" axle are not a direct swap into the 8.5" axle. The flanges on the 7/16" lugs are smaller than the recess in the 8.5" axle (i.e., the metric lugs have a larger flange.)

So, has anyone encountered this ? Does anyone have a part number or other reference for a 7/16" lug that will have the correct size flange to fit the 8.5" axle ?

Thanks for any insight.
 

565bbchevy

Geezer
Aug 8, 2011
9,200
113
Michigan
I would contact YearOne and see what they offer in the correct metric size to fit those wheels with the 8.5.
 
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dgmeadows

Greasemonkey
Supporting Member
Thread starter
Mar 29, 2017
166
63
Georgia
I would contact YearOne and see what they offer in the correct metric size to fit those wheels with the 8.5.
Yep, that is an option. Year One does have nuts for both the 7/16" and metric applications. We just didn't want to have sae in front and metric in back.
 

565bbchevy

Geezer
Aug 8, 2011
9,200
113
Michigan
Yep, that is an option. Year One does have nuts for both the 7/16" and metric applications. We just didn't want to have sae in front and metric in back.
You could change the rear studs to 7/16" but would have to either match the knurl on the factory studs or go to a larger size and drill the axle for them
 

565bbchevy

Geezer
Aug 8, 2011
9,200
113
Michigan
My 9" came with 1/2" studs and I drilled my front rotors to install 1/2" ARP studs to not only increase strength but also so all the lug nuts were the same.
 

dgmeadows

Greasemonkey
Supporting Member
Thread starter
Mar 29, 2017
166
63
Georgia
Looks like we will have to have sae front and metric rear lug nuts. We cannot find 7/16" studs with the same size head/knurl as the 12MM studs. Not too big a deal, Year One has the metric lug nuts in stock. I am using staggered size wheels, so won't be doing rotations front to rear. I will just have to remember the lug nuts are different.
 

oldsmobile joe

Royal Smart Person
Nov 12, 2015
2,040
113
mpls
Upgrade the front brake rotors to a year with metric lugs. Get a set of 20 metric lug nuts. Done.
 
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