Ignition Timing

When rebuilding Gnasher, I bought one of those TDC gauges that fits down the spark-plug hole, like this:

I quickly realised that because the spark-plug hole is at 35º from vertical, the gauge will read wrong. I checked to make sure that it’s millimetres really are millimetres and they are (if you did make it with a correction, you would have to say what angle it was designed for).

What to do?

So we need a correction factor. The gauge moves more than the piston by an amount determined by the cosine of the spark-plug angle in the head:

Required gauge reading = required BTDC / Cosine of spark plug angle

In my example (Gnasher should fire 7.14 mm BTDC on full-advance):

Required gauge reading = 7.14 / cosine 35º = 7.14*1.22 =8.72mm

When I forgot to do this Gnasher was less lively at the top and and noisier (exhaust noise) at the bottom end, so it does make a practical difference.