telling apart the two TDCs with spark plug :unsure:

OK in the process of adjusting valves. my camshafts are off and everything has been going swimmingly. now time to put them back on and timed but as the crankshaft has been moved so I’m not sure which TDC to turn the engine to (there are two TDCs per cylinder every 720 degree cycle - one between compression and power strokes and one between exhaust and intake strokes right?).

There are no obvious markings on camshaft chain for timing btw.

My question is can i connect a spark plug on that cylinder’s lead (rested on top of the engine case). Then turn on the electrics but NOT the ignition and then turn the crankshaft at each TDC until I see a spark? Then I’ll know which TDC is the one between the compression and power strokes right? Would that work?

Appreciate any advice! :slight_smile:

Check what the valves are doing (or visualise what they will be doing) simplified …

Inlet stroke - piston moves from TDC to BDC, inlet valve opens & closes.
Compression stroke - piston moves from BDC to TDC, all valves closed.
Combustion stroke - plug sparks, piston moves from TDC to BDC, all valves closed.
Exhaust Stroke - piston moves from BDC to TDC, exhaust valve opens and closes.

So …

If all valves remain closed from BDC through TDC to BDC you’re where you want to be with the TDC between the Compression and Combustion strokes.

If just before TDC the exhaust valve closes and just after TDC the inlet valve opens you’re at the TDC between the Exhaust and Inlet stroke.


thanks but i know that. The thing is I can’t check by looking at the valves as the camshafts are off.

I’m wondering if this spark plug trick will work or if there’s any other way.

Almost certainly not.

Pretty much all 4-stroke engines run on a wasted spark principle, so there’s a spark every TDC -

Have you not got a manual? It’ll explain the process for setting up the cams after a top end strip in there, it’s usually pretty straight forward enough.


Assuming your 4 stroke twin has the most common spark at every TDC, does it matter which TDC you use for setting the cams? As far as I can think this through, no.

If you don’t have the redundant spark ignition set up, then you can find the ignition TDC using a spark plug but you, obviously, will have to have the ignition turned on. (Make sure both spark plugs are earthed to the engine to spark to avoid a ECU F up.)

Pity you didn’t set it to spark ignition TDC on cylinder 1, lock the crank shaft and mark the cams before you started.

Sorry about the last line. I know no one love a smart arse.

Thanks I didn’t bother checking with the spark plug (as I checked it was wasted spark system) and it started fine. I actually got a bit of a surprise :w00t: when it let the clutch out - felt like it was a bit more torquey.

Haha well I know for next time - which I don’t intend to be doing any time soon…

Good to know theory and practice coincide. Sometimes.

Well done it hasn’t lunched itself then, started Reading this thread thinking it would end in tears…

Little centre punch or paint, I always mark before I take engines to bits…