Face grain direction of doublers LONGITUDINAL CRACK Note: 1. Make doublers from plywood for  longitudinal crack repairs on spar face 2. Make doublers from solid wood (same  species as spar) for insert repair of top  or bottom of spar 3A 3A ¼ A A B B /10 (max) No fitting within these limits Local damage Scarf at ends of insert No less than 12 to 1 5 to 1 slope (minimum) Insert block—same species as spar Direction of grain in plywood reinforcement plates to be same as original web 2A 2A 2A 6A 6A 15A ½ B A No fitting within these limits Plywood B A A Plywood Solid wood filler block Solid wood filler block Solid wood filler block Solid wood filler block Figure 6-27.  A method to repair damage to solid spar. Figure 6-28.  Repairs to a built-up I spar. Bolt and Bushing Holes deformation of the wood may cause splitting or unequal load  distribution. All bolts and bushings used in aircraft structures must fit  snugly into the holes. If the bolt or bushing is loose, movement  of the structure allows it to enlarge the hole. In the case of  elongated bolt holes in a spar or cracks in close proximity  to the bolt holes, the repair may require a new section to be  spliced in the spar, or replacement of the entire spar.  All holes drilled in a wood structure to receive bolts or  bushings should be of such size that inserting the bolt or  bushing requires a light tapping with a wood or rawhide  mallet. If the hole is so tight that heavy blows are necessary,  For boring accurate smooth holes, it is recommended that  a drill press be utilized where possible. Holes should be  drilled with sharp bits using slow steady pressure. Standard  twist drills can be used in wood when sharpened to a 60°  angle. However, a better designed drill was developed for  wood boring called a lip and spur or brad point. The center  of the drill has a spur with a sharp point and four sharp  corners to center and cut rather than walk as a conventional  drill sometimes does. It has the outside corner of the cutting  6-19