the present invention relates to an improved method for the manufacture of aircraft engine cylinder barrels to prevent their premature failure due to hairline cracks or specks thought to be caused by caustic stress corrosion cracking during black oxide treatment . machined aircraft cylinder barrels immersed into a black oxide chemical bath composed of a solution containing about 60 % sodium hydroxide , about 0 % sodium nitrate , and about 40 % sodium nitrite most effectively prevents specks and hairline cracks . since residual stresses from machining also contribute to the probability that specks or hairline cracks will occur during black oxide treatment , the maximum selected number of cylinder barrels essentially free of detectible specks or hairline cracks determines the maximum number of cylinder barrels to be machined on a given set of tool bits .