1965 MF 135 with Z145 gas engine: Timing mark question

klancf51 · Aug 5, 2017

Jerry/MT said:
Why would there be a difference in the location of the stop point? Back lash? Most of these engines are not initially timed at TDC. They are usually times at 5ｰ to 7ｰ BTDC so you would have to know the flywheel diameter, calculate the arc length from TDC to the initial timing point and mark off the distance to the initial timing point.

The angular difference between the stop points is due to the stop bolt extending beyond TDC:

Angular difference = 360 x (1 - (S-D)/S)

where

S = Piston Stroke

D = Distance the stop bolt extends beyond TCD

The Continental Z-145 ignition timing specification is:

6 degrees BTDC at idle (or static) advancing to 30 +/- 2 degrees at 2000 RPM.

I suppose you could locate 6 and 30 degrees BTDC using the flywheel diameter, but most mechanics I know would simply use a degree wheel (on the crankshaft pulley). However all of this is beside the point. The fact is that locating the TDC mark is independent of which stroke in the operating cycle the engine happens to be on. That someone might mistime an engine because they don't understand what they are doing doesn't change this fact. That's my story and I'm sticking to it.

Cedarsnapp · Aug 24, 2017

Thanks everyone for a very informative discussion about the timing marks. Would it also work to find TDC by looking at the rotor in relation to the #1 spark plug? Would that be approx. TDC so I could then be close enough to find it by slightly rotating the crankshaft with a wrench?

Also, it sounds like there is just a single timing mark on the crankshaft, representing TDC. If true, do I just estimate where 6 degrees BTDC would be (at idle using a timing light)? Or is there a separate mark showing 6 degrees BTDC?

Thanks, Cedarsnapp

klancf51 · Aug 28, 2017

Cedarsnapp said:
Thanks everyone for a very informative discussion about the timing marks. Would it also work to find TDC by looking at the rotor in relation to the #1 spark plug? Would that be approx. TDC so I could then be close enough to find it by slightly rotating the crankshaft with a wrench?

Also, it sounds like there is just a single timing mark on the crankshaft, representing TDC. If true, do I just estimate where 6 degrees BTDC would be (at idle using a timing light)? Or is there a separate mark showing 6 degrees BTDC?

Thanks, Cedarsnapp

No - the flywheel should be stamped with individual marks that start at "30" degrees BTDC and proceed to "DC". "5" is stamped at 5 degrees BTDC. You should be able to see the marks and quite accurately time the ignition.

Cedarsnapp · Aug 28, 2017

Thanks, Klancf51. That will, of course make it much more accurate than if I had to estimate it.

Will rotating the crankshaft until the rotor points at the #1 spark plug wire get me close to the timing marks, so I can mark them? In other words, does the rotor pass by #1 plug wire at times other than when it fires for #1? I don't think it could, but am not sure.

Thanks, again. Cedarsnapp

klancf51 · Aug 29, 2017

Cedarsnapp said:
Thanks, Klancf51. That will, of course make it much more accurate than if I had to estimate it.

Will rotating the crankshaft until the rotor points at the #1 spark plug wire get me close to the timing marks, so I can mark them? In other words, does the rotor pass by #1 plug wire at times other than when it fires for #1? I don't think it could, but am not sure.

Thanks, again. Cedarsnapp

With the transmission in neutral, use a ratchet with a 1 1/4" socket on the crankshaft pulley bolt to slowly rotate the crankshaft clockwise. Using a flashlight, you should be able to pretty quickly locate the flywheel timing marks in the inspection hole.
Yes - rotating the crankshaft until the rotor points at the #1 spark plug wire should get you close to the timing marks. But you shouldn't have to do that.