Sorry to be the bearer of bad news ..
If the threads are that bad to the point the spark plug will not unscrew.
I wouldn’t place any bets that epoxy will be strong enough to hold a socket on it enough to apply enough upwards force to allow it to unscrew from the head.
In all cases both surfaces need to be very clean ( free of oil, grease etc, ) for the epoxy to have a decent chance to hold.
In your case you have all that plus silicone working against your plan.
Unfortunately ( but most likely ) your best course of action would be to remove the head and service it.
I know …. not the news you were looking to hear but it will save you hours of frustration in attempting to get the plug removed and then come to realize after the hours of yelling and cussing the head will have to come off.
Also don’t be shocked if you’ll need to install a spark plug threaded insert, if the treads are that far gone in the head there will not be enough material to re-thread the hole
- This reply was modified 12 months ago by Richard Kirshy.