Question about center bore sizes

[Rayyy]

I bought a set of wheels from friend of a friend a while back, which he told me were “GM bolt pattern” 6 x 5.5

I installed them and they seemed to fit fine. Recently, I noticed my neighbor’s Dodge Ram truck… He has the same wheels… I realized the rims on my truck are from a Dodge Ram!

The bolt pattern is the same… However, the listed bore sizes are different:

GM 78.3
Ram 77.8

How is it possible that a rim with slightly smaller bore size fit perfectly onto the GM hub??

Is this safe to drive?

(already drove 10,000 miles like this!)

 

[Rayyy]

If it was the other way around (larger GM bore size wheel onto smaller Ram hub), what would pose more of an issue…

But, I’m thinking the smaller bore size of the Ram wheels was enlarged when the rims were torqued down into the GM hub.

Given that they are extra tight… Is it okay to keep driving like this?

 

[Rayyy]

Weird… When I check the BORA wheel spacer site, it shows the center bore size for GM as 77.8 (same as Dodge Ram),

However, when I check everywhere else… It shows up as 78.3.

Which is it?

 

[tom3]

If the wheel/tires run smooth and the lug nuts are still on torque spec I'd say they are fine.

 

[swathdiver]

All my information for GM wheels was a 78.1 mm bore diameter! LOL

Whatever it is, you want hub centric wheels, wheels that mount securely to your hub, this is what keeps them in place, not the lug nuts. Guys who run lug centric wheels usually pick them up off the side of the road at some point.

 

[Rayyy]

All my information for GM wheels was a 78.1 mm bore diameter! LOL

Whatever it is, you want hub centric wheels, wheels that mount securely to your hub, this is what keeps them in place, not the lug nuts. Guys who run lug centric wheels usually pick them up off the side of the road at some point.

Indeed, I see many sources specify either 78.1 or 78.3 center bore size for the NBS 1500 trucks… but, nobody else lists it as 77.8

I’m rather surprised at the level of discrepancy here, usually center bore size specs for wheels seem universally accurate.

 

[Rayyy]

All my information for GM wheels was a 78.1 mm bore diameter! LOL

Whatever it is, you want hub centric wheels, wheels that mount securely to your hub, this is what keeps them in place, not the lug nuts. Guys who run lug centric wheels usually pick them up off the side of the road at some point.

Hey btw, just spoke to Lenny from Motorsport-tech.com (they make Bora spacers for 30yrs and measure everything precisely). He said that although the specs all over the Internet do say 78.1 or 78.3, if you use a micrometer to measure the actual GM hubs… They are exactly 77.8!

Apparently most online sites simply upload data sets when making these databases, which creates and echo chamber, sometimes resulting in proliferation of inaccuracy.

This all makes sense since my blasphemous Ram wheels are literally spec’d at 77.8, and fit my GM hubs like a glove… Perfectly tight. And come to think of it, likely wouldn’t even fit at all if the hub was indeed .5mm too big.

Either way, I’m shocked… I’m going to measure the center bore on some GM rims next and see what that comes to.

 

[swathdiver]

Just measured the hub on an axle in the garage, it's kind of rusty, 78.3! LOL Only measured from one direction and had to go and get the dogs.

 

[Fless]

Just measured the hub on an axle in the garage, it's kind of rusty, 78.3! LOL Only measured from one direction and had to go and get the dogs.

What diameter did the dogs measure?

 

[swathdiver]

What diameter did the dogs measure?

Almost .177! A racoon or something was rooting around on the other side of the fence and the hound was howling something fierce. On second thought, it was probably an opossum, they don't care about dogs barking usually.

 

[tom3]

I have seen that on older Chevys the hub is a bit bigger than later models. Say up though early/mid 90s larger, then in the 2000s it was smaller.

 

[Alex_M]

Whatever it is, you want hub centric wheels, wheels that mount securely to your hub, this is what keeps them in place, not the lug nuts. Guys who run lug centric wheels usually pick them up off the side of the road at some point.

That's more than a little bit of an exaggeration. There's nothing wrong with lug centric wheels as long as no wheel spacers are used, your studs and lug nuts are in good shape, and as long as you are staying within the load rating of the studs.

 

[Alex_M]

Hey btw, just spoke to Lenny from Motorsport-tech.com (they make Bora spacers for 30yrs and measure everything precisely). He said that although the specs all over the Internet do say 78.1 or 78.3, if you use a micrometer to measure the actual GM hubs… They are exactly 77.8!

Apparently most online sites simply upload data sets when making these databases, which creates and echo chamber, sometimes resulting in proliferation of inaccuracy.

This all makes sense since my blasphemous Ram wheels are literally spec’d at 77.8, and fit my GM hubs like a glove… Perfectly tight. And come to think of it, likely wouldn’t even fit at all if the hub was indeed .5mm too big.

Either way, I’m shocked… I’m going to measure the center bore on some GM rims next and see what that comes to.

You're talking about a .013" difference between measurements in standard imperial measurements. I'd guess that the wheels are probably machined .005" larger than the hubs to avoid stacking tolerances making a press fit, and a used set may have a bit of wear from install/removal. That spec was probably taken from a measurement someone took from a used set. May have also been measured with a bore gauge that wasn't straight in the hub bore, or slightly out of spec.

I'd be curious to see a picture of the Mopar wheels on your GM.

 

[Rayyy]

I haven’t measured the center bore of these rims yet, but will do so when I take them off to install the spacers I ordered (which is what sent me down this rabbit hole to begin with). I’ll measure my center bore of the hub too.

I would prefer everything sits extra tight, especially if adding spacers.

In any case, here are the wheels in case you are wondering which ones they are.