too bad you had to helicoil those holes. they should have been a course thread to start with.
if that had happened to me, i would have either helicoiled to a course thread on all the holes, or just drilled and tapped the case to either 5/16-18 or 3/8-16 thread. provided there was enough meat around the thread bosses.
I looked at the specs because I thought the same thing about changing them to 5/16-18 but the drill bit for a 5/16-18 is smaller than the required 8mm hole.
Might be better to helicoil them to 5/16-18 ? I've done that on a trans I know would have the pan on and off a lot.
5/16-18 tap drill size is "F" drill [.257]
M8x1.25 tap drill size is "H" drill [.266]
3/8-16 tap drill size is 5/16 [.312]
the required hole size for a 5/16-18 helicoil tap is "Q" drill [.332]
the required hole size for a M8x1.25 helicoil is 21/64 drill [.328]
lots of possibilities here.
just be sure there is enough meat around the hole so the case won't be weakened by drilling the tap drill size required for the hole repair.
