What techniques have riders used to remove stubborn rounded out rear disc holding bolts on two valve Norge rear wheels please?
I replied to this on FB 
- drill head off, remove stud, new bolt.
Bang an old imperial sized socket onto the metric rounded off bolt. Sometimes a slightly smaller imperial can just about get on. Give the bolt some shock treatment from a hammer beforehand ( carefully of course).
Try grinding a screwdriver slot into the head so that an impact driver with a flat blade screwdriver bit will fit. If that doesn’t do it grind the heads off which is guaranteed to release the disc, then get a decent pair of mole grips on the remaining bolt.
Squirt some penetrating oil on the screws first though.
Lastly drill out the bolts…
Hope that helps
Always make or use a “T” bar so that you are applying a rotational force with no push to one side .
If they are hex head bolts start with a hex socket (NOT 12 pointed).
Even better start with a hex box-spanner as they do not have the rounded relief inside the bottom edge so give more engagement (in my experience i have always been able to find a hex socket to drive the other end of the box-spanner).
If the bolts are allen head try knocking a tight Torx drive into them these seem to be harder and dig into the hex.
If the bolts are made by Aprilia - they are cheese so get a mouse to nibble them away
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Drilled the head off. Made a nut slide over diameter and counter sunk. Welded to stud, wound out.
Thank you to everyone who offered advice, most of which I tried. Thank you.
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Impressive fabricating skills