I put another bolt inside of the plug and then put them in. Use some high heat loctite on them also if they didnt already come with some red stuff on the threads.
Just go to the hardware store.
It's a 5/8" dia. coarse thread. Only needs to be 1 or 1 1/2" long. 2 of those with a pair of lockwashers and pipe sealant and the thermactor's plugged!
should only cost you five bucks at the most. Real easy.