1970 GMC C2500 rear axle hub diameter?

[tow rig]

I have a 1970 GMC C2500 (2WD) that I have bought aftermarket wheels and tires for. The wheels are 8 X 6.5 bolt pattern. The center hole in the wheel is just under 131mm. If I bolt a wheel onto the hub of the Dana 60 full-floating rear axle, the wheel has a significant gap between the center hole of the wheel and the hub of the axle. The axle hub is difficult for me to measure despite having a digital caliper and 4-5" micrometer. The studs get in the way of my making an accurate measurement. Does anyone know the diameter of the hub at the wheel mounting surface? (I will need to order some rings to fill the gap between the hub and the inside of the rims center hole if I want my rims to be hub-centric.)

[jerry moss]

your tapered lug nuts center the wheel to the mounting flange of the hub. you must be thinking of a Ford truck that uses hub centric wheels.

[tow rig]

Nearly all factory installed wheels are hub-centric. The 1970 GMC had steel wheels that mated tightly to the hub. I know that many aftermarket wheels are designed to be more universal, and center off of the lugs, in order to have a single wheel casting be useable with several applications. My opinion is that lug centering wheels may be less than ideal, especially for a HD truck that will be carrying heavy loads, driven in rough terrain, abused with dragstrip launches, and/or towing a trailer. Anyway, it looks like my question is turning into something that I haven't asked. Are hub-centric rings needed for aftermarket wheels?(This is a simple google search that I have already done. The jury seems to be about evenly divided on the subject.) Thank you for sharing your experience.

[Stocker]

FWIW, I have owned my K20 for 47 years and during that time it has had four different sets of wheels.

1) The stock 16" lock-ring wheels.
2) Aftermarket 15x8" white wagon spokes.
3) Aftermarket 16.5x9.75" chrome wagon spokes.
4) Ford (Accuride?) 16x7" steelies.

I couldn't say if the stock wheels were hub-centric, but none of the others were. The truck has been heavily loaded more times than I can count (up to a verified 3 tons in the bed), with no wheel-related problems ever.
I agree with Jerry Moss, hub-centric rings are absolutely not needed in your configuration.

[tow rig]

Thank you for weighing in, Mike. You and Jerry supplied just the kind of advice/reassurance that I was needing. I will install the wheels without hub-centric rings and feel confident that there won't be any wheel related issues. I came to the right place.

Thanks Again

Brad

[PbFut]

Just one thing to consider when not using the hub to center a wheel. It will be all the more important to sneak up on tightening those lug nuts allowing the wheel to slip some as the nuts center the wheel before too tight for movement. Rotating the wheel when doing so may also be advisable, in a way centering the weight during the tightening process. This might be overkill, but I've had wheels bind up at the track while mounting. You think everything is fine till that 165MPH section of track comes up. It about blinds you.

[mr48chev]

Originally Posted by PbFut

Just one thing to consider when not using the hub to center a wheel. It will be all the more important to sneak up on tightening those lug nuts allowing the wheel to slip some as the nuts center the wheel before too tight for movement. Rotating the wheel when doing so may also be advisable, in a way centering the weight during the tightening process. This might be overkill, but I've had wheels bind up at the track while mounting. You think everything is fine till that 165MPH section of track comes up. It about blinds you.

Good point there. It's better to cinch them down a little at a time and spin the wheel a bit when you do it to get it centered. My 71 GMC 3/4 ton has Ultra 50 8 lug wheels that are not hub centric and I have to do that. In truth you should do it on all wheels even rusty stock steel wheels as they can get cocked a bit if you run one lug all the way down before working the wheel into the mounting surface.

[Sheepdip]

In this era GM rear hubs are 4.56"+/- Ford rear hubs are 4.77"+/-

[Bob B.]

Originally Posted by tow rig

.....Are hub-centric rings needed for aftermarket wheels?........

Rule of thumb is if the aftermarket wheels use a tapered lug nut, then they center off the lugs. If the wheels use a flanged nut that is flat on the bottom, the wheels center off the hubs (hub-centric). If you have any doubts, call the wheel manufacturer. If your wheels center off the lugs I would not be too concerned as for many years heavy trucks and buses used 'Budd' wheels which used tapered lug nuts (and studs for the inner wheels). They never came loose if properly torqued! Here is a picture of my '67, tapered nuts and a little gap around the hub:

[Richard]

In my experience I have not recall having seen 8 lug hub centric wheels. The older stuff was not AFAIK. Honestly, in the newer vehicles I have worked on I never even thought about it. The temp wheels on my 72 truck are not hub centric as well as the final US Mags wheels I am using. Have used wheel spacers on the rear of my 94 1/2 ton for many years, not hub centric. I have worked on semi trucks and the Alcoa wheels were not hub centric. Nothing to worry about IMO.