haulinkraut

Scored some 996 GT3cup centerlock E29 wheels. I have always liked the solid spoke wheels over the scalloped spoke BBS motorsport wheels so after a bit of research I caved and bought some.

So here is the deal. The cups use a different centerlock system than any of the street cars. And for my purpose, centerlock is of no use. And especially with the car also seeing street use. So back in the Grand-Am 996 cup days regulation requires 5-lug. So the cups had three different BBS motorsport wheels that came with them and were factory. One was the magnesium E28 which was 5-lug and centerlock compatible. But the magnesium wheels were prone to cracks and failures. The second wheel were the E29 aluminum centerlock only. The third was a magnesium monoblock 5-lug variant that was pretty rare. So many of the cars came with the E29 aluminum wheels. But for the guys that ran Grand-Am these dis not work. But Porsche Motorsport offered a service to machine the factory E29s to 5-lug. And seeing that from the backside the E29 wheels were identical to the other BBS motorsport 5-lug wheels except that the 5-lug holes were drilled all the way through and the frontside was milled for the lug and socket recesses.

So with my machining knowledge, some Porsche race engineer consulting and a drawing from BBS on the lug hole dimensions I went at it. To bore the recess for the socket is easy. But you have to start by locating the 5-lug holes and through drill. It is important to drill straight and center the through holes on the existing and correct drive pin holes. Once drilled through, you use 2 different piloted counterbores which ensure accurate boring of the socket recess. The lug seat is 14mm radius. So this was custom ground piloted counterbore. Here you can see the through drill and the socket counterbore. Following the BBS dimensions ensure you are not weakening the wheel and allow enough material for the lugs to adequately hold. Also that you don't cut through into the other drive pin recesses near by. And with our wheels being hub centric you really have to hack up this process to ruin a wheel or modify and make a unsafe wheel. Anyways, once I have my special ground counterbore for the lug recess then that will be complete and the wheels will get painted BBS gold.

I wouldn't suggest this process to anyone unless you have the data, correct tools and machining experience. Also, the E88 wheels are available in the 5-lug configuration and are lighter than the E29 wheels. So unless you are like me and enjoy a good engineering and machining project, this isn't for you. Also, I really dig the solid spokes over the scalloped spokes personally.

Evil Bunny

Awesome!! - Could this also be done with BBS E88 centre locks?

Would you offer the machining service to others?

haulinkraut

Some centerlocks, but not all. Some E88 have extra lightening holes that prevent that. I wouldn't want to do this for someone else. Too much liability. But, I would be happy to share info and guide anyone looking to do this.

Evil Bunny

Are you able to share the guide on how you achieved this please?

haulinkraut

You will need a 1 1/8" piloted counterbore ground down to 28mm with a 14mm radius custom ground. Then you will need a 1 1/4" piloted counterbore. Measure the drive pin holes. I seem to remember they were 16mm. You can get an SAE pilot that will work well too. Or you can get a pilot ground to the proper diameter. Once you get that, get an E88 drawing from BBS showing the lug depths. Oddly, from the bottom of the radius lug seat to the back side of the factory BBS 5 lug wheels don't have has much depth as you'd expect. I left a little more material as it made me a bit nervous boring as deep as BBS does. Once you have that figured out is is pretty straight forward on a milling machine.