The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2001-205706 filed on Jul. 6, 2001 the entire contents thereof is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a cylinder head of an overhead camshaft (OHC) four-stoke engine with valve lifters in which at least one of a plurality of valve guides for guiding intake and exhaust poppet valves in an opened or closed direction can have a slide fit of a different depth as compared to the other valve guides. More particularly, the present invention relates to a cylinder head in which a valve pause mechanism is provided to an intake poppet valve or an exhaust poppet valve that is fitted into a shallow valve guide in the depth of a fitted fitting hole.
2. Description of Background Art
As shown in FIG. 13, in an OHC four-stroke internal combustion engine provided with a valve lifter 00, the lower ends X1, X2 of valve lifter springs 03, 04 always press intake and exhaust poppet valves 01, 02 in a closed direction. The valve lifter springs 03, 04 and are in different positions of the valve lifter, depending on whether or not the valve lifter is provided with a valve pause mechanism 05. The valve lifter spring 03 associated with the intake or exhaust poppet valve 01 is provided with the valve pause mechanism 05 that is positioned lower by the height of the valve pause mechanism 05 attached to the valve lifter 00 of the intake or exhaust poppet valve 01. Thus, valve guide 06 permits the intake or exhaust poppet valve 01 to slide a shorter distance as compared with a valve guide 07 on the side provided without a valve pause mechanism 05. For example, refer to Japanese Published Unexamined Patent Application No. 2000-205038.
Therefore, the longer valve guide 07 for guiding the intake or exhaust poppet valve 02 without a valve pause mechanism 05 must be press-fitted deeper into a fitting hole 09. Thus, the press fitting resistance for valve guide 07 without a valve pause mechanism is remarkably larger as compared with the press fitting resistance for the valve guide 06 of the intake or exhaust poppet valve 01 that is provided with the valve pause mechanism 05. As a result, the setting of the press fit time of each valve guide 06, 07 is different, and the management of the manufacturing process is complex.
The invention relates to an improvement of a cylinder head of a four-stroke internal combustion engine in which such a problem is solved. The present invention is based upon a cylinder head of a four-stroke internal combustion engine in which each of a plurality of valve guides for guiding and supporting intake and exhaust poppet valves in an opened or closed direction so that they can slide. The poppet valves have substantially a uniform thickness in the longitudinal direction of the valve guide. A depth of at least one of the plurality of valve guides fitted in the cylinder head is different from the depth of the other valve guides that are fitted in the cylinder head. An idle fitting hole having a depth that is acquired by subtracting the depth that is substantially equal to the depth of the shallower fitted hole from the depth of the deeper fitted hole is formed from the end face of a fitted hole on the side of a valve spring toward an inlet port or an exhaust port in the cylinder head. The inner diameter of the idle fitting hole is larger than the inner diameter of the deeper fitted hole.
The present invention is configured as described above with all the valve guides press-fitted at a uniform press fit depth in the cylinder head independent of the length of a specific valve guide, even if one specific valve guide out of the plurality of valve guides is longer than the other valve guides.
Therefore, the setting of the press fit time is simplified, and the press fitting force required to press-fit each of the valve guides is substantially equal. The control required to press-fit is facilitated, the quality of press fitting is stabilized, and the productivity is enhanced.
Also, according to the present invention, a long valve guide for guiding an intake or exhaust poppet valve not having a valve pause mechanism can be press-fitted with the same force. Therefore, the same time is required forpress fitting either a long or a short valve guide.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.