with MAPP gas for optimum results. Table 3-2 is a good guide to follow when you are using acetylene tips for MAPP-gas welding. MAPP  is  not  sensitive  to  shock,  and  it  is nonflammable in the absence of oxygen. There is no chance of an explosion if a cylinder is bumped, jarred, or  dropped.  The  cylinders  can  be  stored  for transporting in any position without danger of an explosive air pocket being formed. The characteristic odor, while harmless, gives warnings of fuel leaks in the equipment long before danger. MAPP gas is not restricted to a maximum working pressure of 15 psig as is acetylene. In jobs requiring higher  pressure  and  gas  flow,  MAPP  at  the  full- cylinder pressure of 95 psig at 70°F (21°C) can be used safely. WELDING AND BRAZING.—With   MAPP gas, this requires some differences in equipment and technique. Prepare steel to be welded with MAPP the same as for welding with acetylene. For metals thinner than 1/8 inch (3.1 mm), clean the metal to be welded well, either with a wire brush or a grinding wheel. When metal is thicker  than  1/8  inch,  bevel  it  first  and  clean  it thoroughly. Position the steel with a root opening equal to the material thickness when the work is thinner than 1/8 inch. If the metal is thicker than 1/8 inch, leave a gap equal to the diameter of the welding rod to be used. Set  the  gauge  pressure  with  the  torch  valves closed; if the metal to be welded is 3/16 inch (4.2 mm) or less, set the gas (MAPP) pressure at 2-3 psig and the oxygen pressure at 10-15 psig. When working with 1/4 Figure 3-49.—Portable oxygas welding and cutting outfit. cutting, heating, and brazing because of its superior heat transfer characteristics. Acetylene regulators, hoses, torches, and welding tips may be used without any significant disparity. Very often the same-size tip can be used for welding with MAPP gas as with acetylene. However, it is recommended that a tip one or two sizes larger be used Table 3-2.—Substituting Acetylene Tips for MAPP Gas Use DRILL SIZE OF TIP 72-70 65-60 INNER FLAME LENGTH INCHES 1/4 7/16 REGULATOR MAPP GAS USE METAL PRESSURE  RANGE WIDTH MAPP GAS OXYGEN CFH INCHES 1-2 5-6 1-3 1/32 1-3 5-6 2-4 l/32-1/16 59-54 5/8 1-5 6-8 3-8 l/16-1/8 49-48 1 2-8 8-10 5-18 1/8-3/16 43-40 1 1/8 3-9 10-12 6-30 3/16-1/4 36 1 1/4 5-10 10-15 6-35 1/4  -3/8 3-37