Bicycle Mechanics - Non-standard rear der mount bolt hole?

I just finished building up an inexpensive frame I acquired. I built it up initially as a single speed to check the fit before deciding whether it is worth spending money for a good paint job. Before I decided to go SS for the time being I did try to mount a standard off-the-shelf Dura Ace rear der that was on my Bianchi. The mounting bolt hole on the new frame seems a smidge too small. Are there different size mounting bolts?
Thanks,
Raymond

I believe there have been a few different thread and diameter standards for derailleur hangers. My usual solution is to mount the desired derailleur using whatever hanging bolt from my parts bin threads properly into my frame (e.g. a SunTour derailleur on a Simplex dropout, with a Simplex bolt instead of a SunTour).

Yes, Rainman. the mounting holes are frequently different.

I have been creative with this type of problem and had great success.

As long as the derailure is stable on the frame and has room to do it's job moving the chain, you should be able to drill, tap, or even use hose clamps + bolts to attach the derailure.

And, because people crossthread, and ruin holes by overtorquing :D , they make threaded inserts so that you can drill out the old one, tap it, and screw in the insert - with a standard hole/thread size in it - into the dropout. Could be a better way to go.

Cheers...Gary

OK, turns out it was just a matter of chasing the threads. LBS dude with a 10x1 mm tap took care of the problem in about 90 seconds flat.
Thanks for the replies.
Regards,
Raymond