I do not recommend going oversize. Far better off to get a helicoil and repair the threads there. Certainly enough meat to do an insert ...

I believe the bolt you are referencing is to the back of the axle and is a 1/2-13. Fairly deep in the axle casting, but it is a blind bottom hole, so use caution to ensure the bottom is clean out (air gun works great) before assembling.

Brian

 

Going up to 9/16 drill size coarse thread be 31/64 the old 1/2 be 27/64 drill bit so up one size should work, next size be 5/8 be 17/32. If decide to go find thread here link to drill bit tap size.

http://www.boltdepot.com/fastener-information/us-tap-drill-size.aspx

 

Here is a tap drill size chart
http://www.newmantools.com/tapdrill.htm Use a good quality american made tap try to borrow a tap they are not cheap