lower projections 25 and 26 may be formed to be aligned vertically on the inside surface of the snubber base 21 as shown in fig7 for inserting the snubber base spring 24 therebetween or to straddle on the other for inserting the snubber base spring 24 however the upper and lower projections 25 and 26 are not restricted to the aforementioned individual configuration because objects of the present invention can be achieved even if the upper and lower projections 25 and 26 are not formed individually namely each of the upper and lower projections 25 and 26 may be formed as a single unit however this configuration of the upper and lower projections 25 and 26 may cause problems in that it may not be favorable in view of mass production because processing of an undercut portion with injection molding is difficult furthermore fitting or taking out the snubber base spring 24 in or from the upper and lower projections 25 and 26 is cumbersome of the plurality of upper and lower projections 25 and 26 formed on inside surface of the snubber base 21 the upper projections 25 may be formed vertical and the lower projections 26 may be formed vertically below the upper projections 25 to reinforce the strength of the outside circumference of the snubber base i e the sliding surface a thickness t of the sliding surface 21b at a portion between the upper and lower projections 25 and 26 is set to be in a range of 0 1 u02dc 0 7 mm because if the thickness t of the sliding surface 21b is set less than 0 1 mm the thickness is too thin to have a good injection moldability and will result in many defects if the thickness.