The specifications originally required machine facing for all these surfaces. The art of machine molding from metal patterns with draw plates, etc., has, however, attained such excellence in certain shops, that in regular practice “foundry faced” cylinder heads and piston faces can be made true and fair, and steam joints can be made, tight under 80 lbs. pressure with a packing of oiled paper only 1⁄100 inch thick.
Under proper assurance that this precision can be obtained in regular practice at the shop in question, foundry finish may be accepted on cylinder heads and piston faces, steam chests and steam-chest covers.
In the case of built-up pistons, of separable form, it must be conclusively shown that the boring and finishing are carried on by such methods as will ensure the faces of pistons being exactly square to the piston rod and exactly parallel to the cylinder head.
In the case of solid pistons the two faces must be machine faced, as proper parallelism cannot well be obtained by foundry methods.
Ordinary foundry finish secured by the old methods and wooden patterns is not acceptable and acceptance of any foundry-finish can only be secured after a special investigation of shop practices.
d. Heads at both ends of cylinder must be beveled off very slightly over a ring about one inch wide, or equivalent means provided to give steam a quick push at piston, should it stand at contact stroke.
9. Bolts and Studs.
a. The stress on bolts or studs in connection with steam cylinders must not exceed 7,500 lbs. per square inch under a test pressure of 80 lbs. steam, disregarding such initial strain as may be due to setting up. (Compute pressure area out to center line of bolts.)
No stud or bolt smaller than 3⁄4 inch should be used to assemble parts subject to the strain of steam pressure as smaller bolts are likely to be twisted off.