Been using these for a year or two with very good results. Same tool is sold as Ace and Sears brand Stripped Bolt remover sockets.

Don't have to pund 'em on, a simple TAP with the backside of the ratchet does it and the harder you torque, the deeper they bite. The outside is fitted for a Crescent or open end wrench as well.

Set of 10 covers 19mm to 5mm and fractional equivalents.

Trick with internal hex is the use a good penetrant and break the lock between the bolt head undreside and the surface. Also, using a socket hex driver after cleaning out the hole and tap the end with a small hammer should loosen the bond.

I have also had decent luck with a 12v electric impact drill with the clutch set to break away at the torque rating of the fastener location (you normnally torque it to 22 ft lbs or #%& newtons so set your clutch to break at 20 ft lbs-use a reverse fitting into your torque wrench with the wrench held in a vise to set the clutch-quick and dirty load cell idea). Usually works but access is a ***** sometimes and no body I know makes a 90 deg impact drill in 12V - the Dewalt is a plain jane drill but not impact/hammer.

Strinking trick is good but can soften the metal as well. Same goes for the Blue Tip Wench-Aluminum changes composition easily and lets not talk about toasting the steel/cast iron/alloy around the hole. Impagine torquing the new fastener up and then hearing the "crack" as the metal cracks at the heat affected zone border....

Internal hex can be pulled using a left handed reamer (BF Easy Out Made of Tool Steel) but again its easy to eff it up. Do not buy cheap tools for this- get B&S or Greenlee or Klein and a good tap handle to hold the bit.

I am sure this is not what you want to hear right now.

Good luck/