Subtle, Bob, asked me if I could move the contents of an e- mail to this thread. He has had some difficulty fitting some reproduction TZ wheels to his spider as the center holes are too small to fit over the rear half shaft stubs, the offset is off by about 3 mm, and the wheels inside web touches the finned drums. So here is an attempt!
This has come up a few times with any alloy aftermarket wheels and
Alfa's. Here's the trick and reason. The hole in the center of the
wheel is supposed to center the wheels spider on the lugs. This hole
must be reasonably tight over the axle stub, just as it is with the
steel wheels. When manufactured, the lug holed are drilled, actually
milled, using this hole as the wheel "true center" reference.
If the center hole is simply opened up, it may not act as the wheels
"true center" on the axle or studs. This can cause radial cracks to
develop from the lug holes.
The solution is equally simple. Unfortunately for me, I have to
drive to my friend John's shop near Indianapolis for the fix. He has
a nice big Bridgeport mill, with a rotary table. The wheel is mounted
on the table outside face down, centered on the "true center". Then
with the mill, the hole is opened up to the correct size for a slip
fit on the axle stub. Any included angles on the outside (wheel
inside) of the center hole can also be added with this set up.
FINALLY, if the wheel spider needs to be thinned for offset, or the
inside of the wheel web opened up to clear the drums, NOW is the
time. All light alloy wheels (not necessarily pure magnesium race
wheels) have plenty of "meat" to them for 3 / 5 mm cuts in these
areas. Remember, the light alloy can flex some in use, where as the
Magnesium cast wheel cannot. This, I have found, to be a very safe
modification. I have never had a light alloy break or even crack
after this modification.
Finally, once set up to do one, the mill operator can do 4 or 5 quite
easily, allowing them to be all the same, and clear fender lips!