aka the wallet diet

How the M3 got its groove back…again

I finally had the chance to take out the M out of storage after…2 years?! Well, my kid is now 1.5 years old, minus 9 months…geez long time! She was all dusty and cobweb-ridden. Over the last 6 months I embarked on a little wheel project to keep me busy. She was currently sitting on e38 mesh wheels in 16×8 offset 23 shod with Kumho XS in 215/45/16. They were nice but plain. I thought about painting them a different color. It wasn’t too exciting. Maybe I should get some 17″ e39 style 5′s? Doesn’t everyone have those? Very nice but ubiquitous. Plus I didn’t want to buy more tires, my Kumho’s are brand new.

On my regular trolling of Craigslist I found an ad for some gold 16″ mesh wheels. I really liked the style. They look like a cross between BBS’s and SSR meshes. Originally, they’re out of the Pontiac Trans AM GTA circa late 80′s. After some quick research on offsets and bolt pattern (more on this later), I picked them up that same weekend for less than $300. These are them:

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Specs:

- 16×8 et0 (original front)
- 16×8 et16 (original rear)
- 70.7 mm center bore
- 5×4.75 in bolt pattern (5×120.6)
- One piece, all aluminum construction
- Made in Japan

Now, the first thing you’re going to say a bolt pattern of 5×120.6 is not the same as 5×120. I know it’s not-and I’m not going to say I’m some expert on the minute radial stresses caused by 0.3 mm of difference. I read many discussions on this online between the users and naysayers. Mainly they’re folks trying to fit BMW wheels on GM vehicles. Some smarty guys would argue extensively on these “minute stresses” and the cataclysmic results. I could not find, nor the proponents, find a single case of catastrophic failure. The “smarty” guys are probably right, but my common sense kicks in saying this is only a difference in unit semantics. For all practical purposes 5x120mm=5×4.75 in. DISCLAIMER: This is only my opinion! At worst, the opposing lug really is 0.3 mm farther-and again, my common sense kicks in to say these parts can tolerate that delta. A nice snug hub fit should center the wheel and lug holes appropriately to diminish the chance of a shifted wheel. I’m willing to bet that any failures are probably attributed to mixing of hub-centric and lug-centric applications. Such as installing a BMW wheel with a center bore of 72.5 on a GM with a hub of 70.7. This puts more stress on the wheel since the hub isn’t there to support. Even then it’s probably a torquing issue as the lugs/bolts are just there to provide clamping force. The wheel/hub interaction is what’s holding the car up. When I performed the initial fitment test, I eyeballed the centerline of the wheel lug hole and the hub hole-I did not see an offset. Please post comments on this!

So, more about the wheels. They are a lug-centric application which, in theory, should make the wheel strong. They were made to support a American iron at over 3,400 pounds. The wheels weigh about 17lbs each. Pretty light in my book. They have an oddball original application: the lower offset pair, wider lip, are the fronts for the Pontiac. The et16 is the rear. Very counterintuitive in the euro world. My plan was to swap the pairs onto my M3.

I had two major issues: first, the center bore is too small; second, the fronts hit the caliper. I was bummed but not out of the game. To salvage the situation I needed to take it step by step. First thing to do is get the wheel’s center machined. Once that was done at a wheel shop, I estimated how much spacer was needed to clear the calipers. Using some washers, 10 mm was the magic number. This would bring the offset down to et6.

A 10 mm spacer? That’s easy enough. Wrong! Remember our hub lips extend from the hub about 10 to 12 mm (seems like model years vary). Most 10 mm spacers are hub-centric. Meaning they have a built-in lip for the wheel to sit on. So if you’ve to a car with hub lips greater than 10 mm, you’ll have to get 12 mm hub-centric spacers. It’s not physically possible to make 10 mm hub-centric spacers to fit on hubs >10 mm. My car came in at 12 mm. Great. Just great. I really didn’t want the offset lower by using a 12 mm spacer.

After much brain wave action I found the solution. I used a quality BMW 10 mm aluminum spacer without the hub lip, basically flat. This left me 2 mm on the car’s hub lip…no way enough to safely put a wheel on. The next piece of the puzzle is VAC Motorsports hub extender. This gave me the minimum spacer and hub that I needed. Very trick!

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6 Responses to “How the M3 got its groove back…again”

  1. catchakab Says:
    July 8th, 2011 at 4:29 am

    So all you had to do was bore out the hubs and run hub extenders? I’ve been looking to run these wheels for a while, thanks for the great writeup!!

  2. Basically, yes. Be careful of the center bore machining. Make sure they’re spot on. Take one of the extenders and have them match it snugly.

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