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So there is lots of debate on whether or not a 120mm bolt pattern wheel will fit on a 4.75" bolt pattern hub, and vice versa. For us with an F-body, 4.75" has a limited selection of wheels compared to 120mm. Some people say they will fit just fine. Others say that the difference will bow the wheel studs, break them off, send you flying into a wall at 199 mph, and cause Armageddon. I decided to do the measuring and math on my own.
So, 4.75" is equal to 120.65mm. 120mm is equal to 4.72441". That is a difference of 0.65mm or 0.02559" of overall diameter. That is OVERALL. Which means it is half of that on either side of the diameter. Half is 0.325 mm or 0.012795". Let's take a look at that difference.
A mechanical pencil has lead (not actually lead, but you get it) refills in .5 and .7 mm. Look.
The one on the left is a .5mm and a .7 mm on the right. .65mm is somewhere in between, closer to the .7 obviously. So that's how much size difference there is between 4.75" and 120mm.
Now I made an exact template of my 5x4.75 bolt pattern hub, and my stock, 1985 Camaro 15" 5x4.75 wheel. I also drew an exact 4.75" diameter circle.
The large black circle is the 4.75" diameter. The 5 large black circles are the bolt pattern of my wheel. The 5 small black circles are the wheel studs. The inner red circle is a 120mm diameter circle. As you can see, it touches the black diameter circle and passes perfectly through the center of the wheel studs. They were right up against eachother.
Here it is at 4.75" diameter.
Here it is at 120mm or 12cm.
So there you have it. The difference is BEYOND miniscule. 120mm will fit almost perfectly on a 5 x 4.75". There is not nearly enough of a difference to remotely bow wheel studs of any kind. Now then, you have a far more varied selection of wheels to choose from. Have fun!
Last edited by 85projectZ28; 10-29-2016 at 10:23 PM.
Reason: Typo
There is a similar controversy with 115 mm vs. 4.5" bolt circles for Chrysler products (0.027" difference). Lots of warnings and reports of broken studs. Some say that hub centric wheels are better for centering the wheel and distributing the difference among all the studs, but if the hole was tight enough to do that, there would be problems getting the wheels on and off. Whichever lug nut is tightened first will pull the wheel over. This is theory. I'd like to see actual reports from people who have used metric pattern wheels. Something else to consider, vendors selling to GM customers will describe the pattern as 4.75 while those selling to Beemer owners will call it 120. Actually measuring, even with a digital caliper, isn't easy.
It makes no difference how big the base circle. All that matters is the allowable tolerance of the hole location. Wheel studs don't like to bend and a small bit of deflection causes a massive amount of stress. For what it is worth, Weld Wheels sells both 120 mm and 4-3/4" bolt circle.
And you're right, the selection of affordable and good looking wheels for older cars is terrible.
It isn't 1994 when it comes to 3rd gen wheel selection, when every manufacturer made direct fit wheels for the cars, but it is improving. There are choices from Hawks, CTW Motorsports, and even a reproduction of the Ronal wheels they put on the Firehawk. With the amount of owners willing to upgrade their 14 to 16" wheels very small (even with millions of cars still on the road) - the aftermarket is very cautious about making affordable direct fit wheels. That is a function of how businesses are putting out products these days.
Originally Posted by QwkTrip
It makes no difference how big the base circle. All that matters is the allowable tolerance of the hole location. Wheel studs don't like to bend and a small bit of deflection causes a massive amount of stress. For what it is worth, Weld Wheels sells both 120 mm and 4-3/4" bolt circle.
And you're right, the selection of affordable and good looking wheels for older cars is terrible.
I made a visualiser that may be easier to see the actual difference. In the first picture, you can barely see the difference between the blue and red lines. The blue is 4.75" (120.65mm) and the red is 120mm. The bolt diameters are 12mm.
Zoomed in you can see how small the gap is, 0.013" which is less than 1/64". I think wobble nuts have about 1mm of play, so that's well within the range here.
He's my position on it. I am a machinist. I would not intentionally lock a taper into a seat knowing it's of center. As mentioned, that puts a lot of stress on the studs. Would I consider taking axles and having the flanges drilled for the 120mm pattern? Maybe so!
He's my position on it. I am a machinist. I would not intentionally lock a taper into a seat knowing it's of center. As mentioned, that puts a lot of stress on the studs. Would I consider taking axles and having the flanges drilled for the 120mm pattern? Maybe so!
I used to be a machinist as well, but less than 1/64" is a pretty small amount and it's a perfect situation to use wobble nuts.
If you're willing to go through the effort of redrillng your hubs, you might as well get a more popular bolt pattern like 5x115.3 (5x4.5). You could also get 20mm wheel adapters to change your bolt pattern to whatever you want.
Regardless of that, countless GM guys run BMW wheels and vice versa. I sold a set of GTA cross lace wheels to a guy with an E39 and they looked awesome. I've never heard of one wheel failure when using 5x120 wheels and until I do, I see no issue with using them.
I used to be a machinist as well, but less than 1/64" is a pretty small amount and it's a perfect situation to use wobble nuts.
If you're willing to go through the effort of redrillng your hubs, you might as well get a more popular bolt pattern like 5x115.3 (5x4.5). You could also get 20mm wheel adapters to change your bolt pattern to whatever you want.
Regardless of that, countless GM guys run BMW wheels and vice versa. I sold a set of GTA cross lace wheels to a guy with an E39 and they looked awesome. I've never heard of one wheel failure when using 5x120 wheels and until I do, I see no issue with using them.
I've never used wobble nuts. That may work fine, I don't know. Just giving my thoughts on the subject
Wobble nuts work, but the wheel must be hubcentric (or using hubcentric rings) to work. This is a much better idea than just bolting the wheels on a hoping for the best.
As for the difference between the two patterns, I had refrained from responding to this thread years back, but the difference is massive in the machinist world, and is not anywhere close to acceptable tolerances for much of anything except maybe fine woodworking.
For the OP and his pictures, I appreciate the effort but have you ever put a 5x120 wheel on a 5x4.75 hub, and hand tightened the lugs so you can see the gap left because of the misalignment? I have and it's quite noticable.
People will ultimately do what they want, but this practice is not correct, especially if your not using a hubcentric wheel since there is nothing to center the wheel up as the lugs are being tightened.
This is Sesame Street engineering. Draw two circles and talk about how similar they look.
The wheels studs have a nominal location with tolerance. Can't have a useful talk about this until tolerance is known. Perhaps there is an SAE guideline for wheel bolt patterns. That would be the first place to begin if somebody knows & has access to that document.
This is Sesame Street engineering. Draw two circles and talk about how similar they look.
The wheels studs have a nominal location with tolerance. Can't have a useful talk about this until tolerance is known. Perhaps there is an SAE guideline for wheel bolt patterns. That would be the first place to begin if somebody knows & has access to that document.
Ditto. The image was drawn at perfect nominal which doesn't exist. After you get the positional and relational tolerance of both bolt patterns you can do your dimensional stack-up. You'll find that your worst case misalignment will certainly cause interference with the studs. Not to mention, lots of guys run 1/2" studs. Further exacerbating the issue. That is the genesis behind hubcentric design. Simplifies the design capability of a wheel onto a hub.
FYI - I spoke to a certain BMW wheel manufacturer and the lug hole position tolerance was 0.1 mm. (not to be confused with the tolerance of the size of the hole) The only reason I am looking in to this is because I'm sick and tired of shopping wheels at $5K.
Originally Posted by QwkTrip
Perhaps there is an SAE guideline for wheel bolt patterns.
I suspect the first step to unlocking the mystery is getting a copy of ISO 3911.
10-13-2016, 01:10 PM
