The proper functioning of the spark plugs is essential to the proper running of the engine and efficient operation of the vehicle. At intervals of 6000 miles (10000 km) the plugs should be removed, examined, cleaned, and renewed if much worn, since their condition gives an indication of the general condition of the engine. A clean and white insulator nose is a sign of a weak mixture or hot plug, and hard black deposits suggest a rich mixture. When the plug is black and oily, this could be a result of engine wear with a rich mixture. Light tan to greyish brown deposits would be an indication of correct mixture and would likely be of good engine condition. Long brown tapering stains on the white portion of the plug indicate a faulty joint thus requiring renewal. Cleaning of plugs using a sand blasting machine is more effective than cleaning by hand any plug that does not spark at the recommended pressure should be replaced. The spark plug gap is very important, it is recommended to be from 0.028 - 0.032 in (0.7 - 0.8 mm). To adjust, measure with a feeler gauge and bend the outer electrode, being careful not to bend the center electrode so as to avoid cracking the insulation. When replacing plugs, use new washers and be sure to connect the leads from the distributor cap to the plugs in the correct firing order. 1, 3, 4, 2, with No. 1 cylinder closest to the fan. The plug leads do not require much maintenance, but every 6000 miles, each of the plug leads should be removed and inspected for water corrosion, which should be cleaned off, and suppressor connections should be examined for damage and renewed if necessary.