My car wheels have 4 bolts with a 108mm bolt circle (4x108). Problem is that there aren’t many aftermarket wheels for that setup.
Could I take my wheel hubs to a machine shop and have them drilled at the more common 5x100 or 5x112 PCD? Is it safe?
Even if it was possible, a machine shop would probably hesitate doing so. Liability concerns. What kind of car? I’d look first at replacing the hubs, perhaps from a larger model of your manufacturer. Suggest finding a forum for your car. Betcha someone has done it.
I don’t know if hubs can be, but wheels often are. Sometimes it’s also possible to replace the hubs with those from another car with another bolt pattern. I could put Mustang hubs on my Thunderbird, for instance.
Have you looked for aftermarket hubs with a different bolt pattern?
What is your car out of curiosity?
The car is a Citroen C4. I want to put 18 inch wheels of 8 or 9 inch width. All wheels I can find are 7x18 inches, they take up to 215 width tyre. With an 8 inch wheel I can easily put 235-245 tyre.
There aren’t many mods for this car so I doubt there are aftermarket hubs. Also bigger models do have 5 bolt hubs but they also have double wishbone suspension instead of mcpherson so they are not compatible.
It is possible to purchase or if needed, have manufactured, adapters that will change the bolt patterns.
I was going to suggest adapters also, but with the qualifier that adapters do change the effective offset of the rim. You need to take into account the thickness of the spacer when you buy rims.
As for re-drilling, it’s not that unusual in the custom car world to weld up the stud holes and have them re-drilled for a different bolt pattern. It was pretty common in the1990’s Southern California Volkswagen scene. A word of warning though: Everything needs to be done by competent professionals. Check in your area for machine shops that specialize in custom cars and they should be able to do it or recommend somebody that can.
Do the old holes need to be welded shut? It would be nice to have both sets of holes so I can also use my old set of wheels.
It seems to me that having both sets of holes would almost certainly result in some holes overlapping (which would greatly compromise the integrity of their threads), and would at least result in some holes being so close together as to significantly weaken the hub.
Stockton Wheel can build whatever width of rim you may want, if you’re willing to do overseas business and you want steel wheels. You might want to just call them and explain that you’re in Greece and you want wide rims for a Citroen. If they can’t help, then they may know someone who can.
Of course, there may be someone closer to Athens, or someone who would be willing to widen an aluminum or magnesium rim.
Thanks, but what I actually had in mind was putting these wheels: OZ Ultraleggera.
The 8x18 Ultraleggera is almost 3 kilos lighter each than my stock 6.5x17 Resolfen stock wheels 
As Gary T mentioned above, you’ll most likely need the holes welded up. I have seen brake discs and rims drilled for multiple bolt circle patterns, but I don’t know if if your hubs can be done based on the size you have versus the size you want. That’s one advantage to the above mentioned spacers. You can simply unbolt them and toss the stock rims back on.