Top Dead Center According To Bruce Lancaster
So I’ve heard the mark on a Flathead crank pulley coordinates with top dead center. I’ve also heard at times that it coordinates with 2-degrees before top dead center and yet other times I’ve heard it coordinates with four degrees before top dead center. I’m fine tuning the timing on the new motor in the ’39 and became curious as to the truth of the matter. I didn’t want to bug Keith yet again, so I figured I would ask someone else that I know… Someone else that seemingly knows EVERYTHING about everything – Bruce Lancaster.
His response was so wonderful I figured I had better do my job and share it with the world.
Mark on ’49-up is at 2 degrees crank… If you have an early pulley with notch, it is homemade and needs to be verified. Original mark SHOULD be checked, as not particularly accurate, and engines generally actually like a bit more – like around 4 as the starting point.
Heads OFF: Bolt a strap across #1, bolt through center to stop piston a ways down the hole. Rotate one way, mark pulley at stopping point, rotate other way, mark. Very accurate TDC is 1/2 way between – mark it.
Heads ON: Slide a large tie-wrap or any sort of lump on a wire through plug hole and against farthest point where it hits the wall. Have apprentice holditthere. Rotate as above… Bump, bump, divide. Repeat a couple times to be sure it is repeatable and your stop isn’t moving. Have apprentice flogged if inaccurate.
Once you have TDC nailed…
Go mug an eighth grader leaving his plane geometry class. Steal his protractor and compass, discard the rest.
Draw a circle on a piece of cardboard matching OD of your pulley. Draw a larger circle around it from same center.
Put protractor on center, draw lines at whatever interval you want – 5 degrees is adequate for estimation, can be as many degree points as you want. Mark off about 30 degrees.
Cut out inner circle, hold your new device over pulley with beginning line at your TDC, mark off rest of degrees. Can be permanent file or chisel marks, or just mark a swath with liquid paper and pencil in the lines.
You now have accurate TDC and permanent or easily re-doable scale for full range checking with a simple timing light.