connecting pin 51 is slidably fitted and a female threaded bore 78 into which the tappet screw 43 is threadedly engaged in making the first intake side rocker arm 25 the following steps are used in sequence subjecting steel to a hot forging to produce a rocker body 77 as shown in fig7 a brazing a slipper surface forming piece 41 to the rocker arm body 77 and forming a first aperture 36a for a support hole a second aperture 50a for a first guide hole and a third aperture 78a for a female threaded bore by drilling to produce a rocker arm blank 79 as shown in fig7 b subjecting the first aperture 36a for the support hole and the second aperture 50a for the first guide hole to a reaming to provide a support hole 36 and a first guide hole 50 respectively and subjecting the third aperture 78a for female threaded bore 78 to a tapping to provide the female threaded bore 78 and subjecting the rocker arm blank 79 to a nitriding treatment if strict dimensional accuracy is required for the support hole 36 and the first guide hole 50 they may be subjected to burnishing prior to the nitriding treatment table i shows the composition of the nitrided steel used in each of examples 1 to 3 and comparative examples 1 and 2 table 1__________________________________________________________________________ chemical constituent by weight material c mn cr al si p s cu ni pb bi mo__________________________________________________________________________example 1 0 31 0 99 0 98 1 08 0 24 0 014 0 019 0 04 0 03 0 07 example 2 0 33 0 98 0 98 1 08 0 28 0 016 0 016 0 03 0.