re-threading spark plug holes

[AmadeusCharger500]

Can anyone tell me what size thread these holes are.
I have a 906?, head in which one of the spark plug holes will not allow the plug to be threaded in. I tried matching my tap and die set to the hole and come up with 9/16 nf18. I tapped the hole. Spark plug still will not start or starts crooked. A bolt of the same size starts fine though. Do I have the wrong size tap? I tried many spark plugs and although they are used I cannot believe they are all bad, and the threads look good.
Thanks

[chargerbr549]

If I remember correctly it should be 14mmx1.25 by 3/8 reach. I hope this helps.

Kevin

[dodge freak]

Auto stores sell thread cleaners for the plug holes, its has both sizes one on each end. And it has open spaces on the sides for the junk to fell out, get one of those not a tap. Maybe use some light oil on it. All it does is clean the threads , does not cut like a tap.

[firefighter3931]

  Get a sparkplug thread chaser from your local parts house.

Ron

[AmadeusCharger500]

Well, Am I in trouble?
Now that I tapped it with the 9/16, Is the hole doomed for a helicoil or will the thread chaser reallign things?

[dodge freak]

Sorry but if I was you I pull that head off, you could try a helicoil but with the head on some of the steel will fell in the hole and the piston and rings are going to cut the walls up. I hope you have not crank the engine at all and its somewhere that you can work on it. I would pull the battery cable right now. Make sure you drain the block good and just pull that head. Then try to vacuum all the metal out. Then blow it clean, cover the other holes so they stay clean. I am not sure but maybe oil the walls and then turn the engine over and wipe the walls clean. I would try my best to get all the metal out. A engine shop might be you best bet to repair that thread. While you are at it if the engine has 30,000 miles or more from the last valve job you could pull the other head and have a valve job done on both heads. Hey then it wont be a total waste.

[AmadeusCharger500]

Heads were just worked on, valve job and all. Engine is on a stand. I have been very careful with the tap. Used a vacuum cleaner every half turn on the whole and the tap. I am not opposed to taking the head back off. It would be pretty easy to do.