The following instructions are for solidbody guitars with 25-1/2" (Fender) scale length. Template diagrams are for traditional or floating Fender Strat tremolo and for Wilkinson/Gotoh vibrato.
Template #1, for the guitar top, has two sets of countersunk holes. Refer to the diagram at right to identify the appropriate holes for your tremolo. Place the template on a flat surface, countersunk side down (the countersunk side is up in the diagrams). On the surface of the template, draw or scribe a line (labeled ‘F' for Fender Strat tremolo, or ‘W' for Wilkinson/Gotoh vibrato in the diagrams) which bisects the centers of the countersunk holes. At a right angle to this line, draw or scribe a centerline (C) which is equidistant from the two countersunk holes; this will correspond to the centerline of the guitar neck and body.
Locate the template on the guitar top, countersunk side upward, with line (C) on the guitar's centerline, and with line (F) or (W) located 25-1/4" from the front of the string nut. The front of the nut is the edge adjacent to the upper end of the fingerboard.
Using a pen, pencil or centerpunch, transfer the positions of the two countersunk holes on line (F) or (W) to a piece of tape on the guitar top. These positions correspond to the outer two pivot screws of a Fender tremolo, or the pivot posts of a Wilkinson/Gotoh vibrato, respectively. Draw line (F) on the tape to help locate the other four Fender pivot screws if needed. With pen or scriber, trace the template's inside contour onto the guitar.
Measure 1/4" from each edge of the top right-hand corner of the inside of the template, and mark this location with a centerpunch. Using a drill press and a brad point bit for accuracy, drill a 1/2"-diameter hole here, completely through the guitar body. This hole will serve as a locating index to align the position of the rear template.
Use screws or double-stick tape to attach the top template onto the guitar body. Using a 1/2" router bit with guide bearing, rout the body inside the template to a depth of 1-1/8", in approximately 1/4" depth increments. The final depths will be routed with the template removed from the guitar; adjust the cutter bit so that its guide bearing will follow the cavity walls.
Turn the guitar body over. Place the rear template (#2) on the guitar, countersunk side upward. Locate the corner of the template over the pre-drilled index hole, as diagrammed. Square up the rear template along the guitar's centerline. Drill mounting screw pilot holes or use double-stick tape to attach the template.
Rout the guitar body inside the template to a depth of 5/8". The rear cavity will meet the front cavity. At this point, the "spring cavity" portion of the template is complete. Continue routing the "block cavity" portion to a depth of 1-1/2". The rear template can now be removed.
Center the tremolo spring mounting claw against the front wall of the rear cavity (the wall closest to the guitar neck). Position the mounting claw on the wall so that its mounting screw holes are 7/16" from the back of the guitar. Transfer the hole positions to the cavity wall and drill these with a long 7/64"-diameter "aircraft" bit. Install the spring mounting claw.
Return to the front of the guitar. Use the correct size bit to drill the mounting holes for the two outer pivot screws (Fender) or the pivot posts (Wilkinson/Gotoh). Locate and drill the four inner screw holes if you're mounting a traditional Strat tremolo.
Complete the tremolo installation by attaching the bridge/block assembly and the springs. Solder a control circuit grounding wire to the spring mounting claw, and screw a plastic cover plate over the guitar's rear cavity.