For proper block-cleaning, every plug should be removed. For stubborn plugs, heat the plug (not the block) with a torch until its cherry red. Then with the torch removed, allow the plug to slightly cool down and squirt engine oil around it. The oil will penetrate the threads, allowing the plug to be removed easily. To install the new one, remember to add liquid thread sealant. Teflon tape should not be used because it can sometimes work loose and travel into the engines oil passages.