Hi I was replacing my Toro 1438HXL Wheel horse spark plug and I took the old spark plug out and let the engine sitting in the garage for 3 days before I find time to do the replacement, Finally when I had time last night and try with the new one its not threading in. Also the old one doesn't thread in. So I thought that cold (49 degrees here in No. VA) had made the aluminum head shrunk and waited give it a try on morning, and still its not thread in this morning. Is this a common problem, should I heat the engine head with a propane torch or so to get it to above 60 degrees? I am sure the thread has not stripped off while removing the plug, I can see clearly no any visible mar or stripping. Advise please.