I'm replacing the glow plugs in my 84 300sd. The old ones came out easily but I can't get the new ones to start threading. I applied anti seize lubricant but the threads just won't start. I assume it's because of carbon deposits. I've seen some high prices for reamers and wonder if you could use the correct size tap to clean the bore. If that is a good idea, does anyone know what the proper size tap would be? The plug is 10mm in diameter on the part that screws into the engine. Thanks for your advice.