Reply to "Frozen head bolt / engine reconditioning"

...This is what I would try, not everyone is going to have this equipment at their disposal:

1. With a Hand-Held Sledge Hammer...say 4 Pounds, hammer on the Head of the Bolt 'Down' in an Action as you would to 'drive a Nail into wood'. This can loosen the threads, or even Snap the Head Off. THEN try the Impact Wrench.
You can hit the bolt head 'Side-ways', but you take the chance of hitting and damaging the Head.
'Shock' is a 'Tool' Often used in the Machine Shop.

2. If you have access to a Acetylene Torch, Heat the Head of the Bolt Up, RED HOT! and let it air cool down, this will loosen the threads up. I would NOT try to Melt the Head off using a 'Cutting Torch', you'll Damage the Head!
And be advised that when the Bolt Head is Heated Up to Red Hot, and cooled, you have annealed (softened) the Hex in the Head, making it subject to being 'Stripped' when next using the Impact with a 'Allen' Socket. BUT, annealing makes the Steel Easier to Drill.

...a 'Sharp HSS Drill'(High Speed Steel) will Not cut a Grade 10 Allen Head Bolt! The 'Core' of the Bolt is Harder than HSS! It will take a Carbide Drill(very expensive). And Not By-Hand!, you will not be able to put enough force behind it! And Not with the Engine in the Vehicle! You'll need to mount the Engine Block on a Mill Table, and use a Carbide End-Mill to cut it, with Enough Down-Force.

Good-Luck with It!

P.S. I once 'tore' a Frozen 3/4" Grade 10 Allen Bolt, 8 Inches Long, In Half, using a 1" Drive 800 Lb/Ft Impact Wrench, after 'Hammering' On It (with the Impact Wrench), Continuously for Nearly an Hour! We Did NOT Heat the Bolt before-hand. I believe we Did Hammer on it, before using the Impact Wrench.