In order to accurately set both the internal cam timing and the external magneto timing on a radial engine two tools are necessary: 1) a protractor that will allow us to read valve openings and closings as well as magneto timing, and 2) as accurate way to determine top dead center for cylinder #1. As the crankshaft turns and the #1piston approaches TDC, there is a point at which the crank is still turning but not much movement can be detected at the top of the piston. This phenomenon occurs because as the piston reaches TDC and reverses direction there are a few thousandths of an inch of play between the crankpin and the master rod bearing, and the piston and the piston pin that allow the piston to remain stationary while the crankshaft continues to turn.

Because of this it is important to have a method of both locating TDC and also of taking up this “play” as the piston reverses direction. The Time Rite tool is useful asa TDC indicator, but is not really very accurate for setting cams and magnetos. In this article we will look at a TDC indicator that can be made for a few dollars and then at a method of accurately determining TDC.

Parts List

One BG style spark plug that can be disassembled
One 1/8” x 1/8” x 6” key stock
One 3/16” x 3/16” x 6” key stock
One ignition terminal spring and stud
Two pieces of .050 steel, (1) 3/8” x ¾” & (1) 3/8” x 2 ¼”
Two 10-32 x ¾” cone point set screws and jamb nuts

Modification Instructions

1. Disassemble the BG spark plug & discard everything except that which
   screws into the cylinder.
2. Machine, grind, or cut all but five threads from the nose of the spark plug
3. Drill and tap two 10-32 holes as shown 180 degrees apart on the spark
   plug nose.
4. Bend the 1/8” key stock as shown, center punch it on both sides as shown, and weld the ignition stud as shown. Measuring from one end of the 1/8” key stock, the stud should be at 3” from the end, the punch marks at 3 5/16” from the end, and the bend at 4 3/8” from the same end. The bend angle is not critical but should be roughly as shown.
5. Weld a 3 ½” section of the 3/16” key stock to the spark plug nose and indicator scale as shown.
6. Assemble the spring on the stud and the indicator arm into the spark plug nose, adjusting the set screws to allow smooth movement of the arm.

Using the indicator

1. Remove all the front spark plugs.
2. With the #1 piston low in the cylinder (not near TDC) install the indicator into the front spark plug hole with the arm pointed down toward the piston.
3. Install the appropriate timing protractor on the propeller cylinder, prop nut, or crankshaft.
4. Turn the crankshaft in the normal direction of rotation until the TDC indicator reaches its approximate maximum travel (roughly TDC).
5. Now set the protractor to zero (TDC).
6. Turn the crank in the direction opposite to normal rotation until the piston is no longer contacting the indicator arm.
7. Now turn in the normal direction of rotation until the indicator arm just begins to move.
8. With a piece of tape, mark the indicator scale where the arm now sits, and take note of the reading on the crankshaft protractor.
9. Continue rotating the crank in the normal direction of rotation past TDC until the indicator arm stops moving.
10. Now carefully move the crank opposite to the normal direction of rotation until the indicator arm reaches the piece of tape that you previously installed, and again read the protractor. The difference between the two protractor readings, divided by 2, is the number of degrees the protractor needs to be moved to zero it. For example, if your first reading was 38 degrees BTDC and your second reading was 42 degrees ATDC, the difference is 4 degrees. Half of that is 2 degrees, so if you move the protractor (not the crankshaft) to read40 degrees, it will now be zeroed. After making that correction you can rotate the crankshaft back to the other side to double check yourself. In our example, it should read40 degrees each time the tape mark is reached.