This is a Special Thread Tap for 1916-1928 Cylinder Head Bolts. Use to clean out threads in block before installing head bolts. This is not a commonly available tap and is the correct size for all early 4-cylinder models. TORQUE SPECIFICATIONS FOR EARLY VEHICLES Prior to the late 1940's there were no torque specifications. When working on early cars & trucks there is nothing in the shop manual to show you how much to tighten down vital components such as head bolts, main bearing bolts, flywheel bolts, etc. Years ago we found a universal chart of Torque Values. This chart shows torque values in foot pounds based upon the type of bolt and the diameter of the bolt. It has been extremely helpful when working on vehicles prior to 1949. Of course, it is recommended that bolt threads be cleaned with a wire brush and oiled before being torqued (do not oil wheel lug bolts). Running a tap to clean out the bore is also recommended along with a good blast of compressed air for final cleaning. Add a few drops of oil and you will be ready to torque your bolts. "Technically there is a difference between the torque given to a fine thread bolt and that given to a coarse thread bolt. But taking the entire chart shown here, the variation will not exceed 1%. So for increased convenience the chart has been simplified and practical shop values given. Actually automobile manufacturers give a tolerance. For example, when 75 foot pounds is desired, the value will be given as, perhaps, 70-80 foot pounds. And this is so much more accurate than even the most skilled mechanic can manage with the old rod type torque wrench." Original bolts on early Chevys are plain steel - no hardened bolts or alloys. For your old Chevy, use ONLY the plain steel torque value. TORQUE VALUES for Fine or Coarse Thread Bolts (Torque figures are in foot pounds) Bolt Diameter Plain in Inches Steel 1/4.............8 5/16...........16 3/8.............28 7/16...........44 1/2.............67 9/16...........95 5/8.............131 3/4.............228 7/8.............361 1"...............524 When bolts are torqued, it stretches the bolt. Occasionally the original bolts need to be replaced. We offer new head bolts for 1916-1928 Chevy 4-Cylinder engines.