The trigonometry of timing

The Chummy has no visible timing marks unless you remove the “bacon slicer” starter motor that lives in the passenger compartment on top of the bellhousing, it’s heavy and it’s dark down there so not easy to do a quick timing check. Luckily the head has the spark plug holes in the right place to allow a dial gauge to check top dead centre (TDC) so if I knew what 8 degrees of crank rotation was in piston travel it would be good.

The App

I was a terrible student, what I did retain was lost soon after the exams but I have been saved by technology, firstly spelling and now trigonometry as they say “There’s an App for that” and on my Android phone I use a free app called aptly “Right Angle Triangle”

To cut a long story short, literally working on the back of an envelope there are 2 triangles in this calculation, the first one has a hypotenuse of half the stroke and the angle is 8 degrees, using my trusty triangle app I get the adjacent and opposite measurements. The second triangle is stacked on top with an hypotenuse of the distance between centres of the big and little ends on the conrod and the opposite side is the same as the first triangle’s opposite side from that we get the adjacent measurement. Lastly add the two adjacent measurements together and subtract them from the sum of distance between centres of the big and little ends on the conrod and half the stroke and bingo we have 0.46 mm.

The Envelope

So I zeroed the dial gauge at TDC disconnected the low tension lead to the distributor and connected my multimeter across the distributors LT terminal and earth, then cranked the motor (with the handle) and on the compression stroke the buzzing stopped at 0.45mm before TDC.

A bit of a letdown really as my original guess was  just about dead on so no easy jump in horsepower so back it tracking down the oil leaks of which there are many.

As an aside this is the internet where everyone is an expert so feel free to shoot down my mathematics.

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7 Responses to The trigonometry of timing

  1. Bob King says:


    Surely easier to measure the diameter of the flywheel; convert to circumference and do an 8/360th sum. Anyway, I would try more advance if you want to be surprised.
    (I know a tubby guy called ‘Sir Cumference’).

    • Mark says:

      The reason I did this was that the flywheel was too hard to access and with 2 main bearings supporting a bent piece of wire I’m not sure how much more advance it will take, maybe I will reconnect the steering wheel control to dial it in while driving. Please send my best regards to Sir Cumference.

  2. Warwick says:

    If you give me a few dimensions, I can draw it quite quickly in AutoCAD, and then you can ‘see’ the piston, conrod and flywheel.

  3. Warwick says:


    Computer says Yes. Well, almost.
    The simulation gets 0.4631 mm. That’s a pretty good dial gauge you have there.

  4. Darrell Jeffries says:

    Be thankful your spark plug holes are directly above the bores.
    During lockdown I rebuilt a Triumph motorcycle engine. The engine has no timing marks but the 1964 workshop manual tells one to “set the timing when the piston is 0.016″ before TDC.” This needs to be measured before the cylinder head is fitted and the engine is installed in the frame.
    How “pommish” is that?

    • Mark says:

      Did you read to the end before putting the head on? Sometimes you wonder what they were thinking. I always loved that the Japanese put a window in their carby bowls so you could check the float level as the complete opposite to what the Pomms were doing at the time.

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