Don't see why this is a stupid question. Have you tried calling QT? I have found them very helpful in the past. From memory of my own fitting of a QT guard I think it is 1/4 UNC, about 1" long. Try Wrights Auto Supplies; 02476 470377, www.wrightsautosupplies.co.uk or Steve's stainless 01527 831451, stevestainless2@aol.com. Watch the torgue if you use stainless though.