Its all in the timing

With time on my hands before we head south for the winter, I decided to test the timing marks on the flywheel/block to a dial indicator on #6 (front) piston and level of piston #'s 2 & 3. With 2 & 3 measuring equal depth w/ dial indicator & #6 measuring height also w/ dial indicator, this is what I came up with. Can this be right. I had previously tried a degree wheel but it became too cumbersome so I followed the manual and used the timing marks as it described and used that as TDC to set the cams and it ran fine. I should mention the depth & height described above started to change measurably when I used the factory marks meaning the cams would be retarded a bit. And yes I did factor in the transitional rotation of the crank on #6 to determine true TDC.

Is this a reliable test of TDC? Or do the domes of the pistons (and position of the plug holes) make this unreliable?

They certainly result in some error, though whether it is significant is hard to say without more detailed analysis of the geometry and a description of exactly how the measurements were taken. It would also be important to know whether we are talking about a 3.8 or 4.2 engine, since all but #2 and #5 cylinders on the 4.2 are offset from the chambers in the head.

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I should add the measurements were on a 4.2 w/ the head off. Pistons are new A&E 30 over at (IIRC) 8.7 compression ratio. Dial indicator sat on the edge or flat portion of the piston.

Thanks for clarifying. That takes a lot of the opportunities for error out of the equation.

I used to put the dial indicator on # 1, and using degree wheel note 20 deg before and after, then set the mark halfway in between.

Hold your thumb or finger on top of piston as it rises to minimise the float. If you can reach…no reason you can’t use the most forward piston to check TDC/pointer accuracy. Come up smoothly to avoid backlash/slack of timing chain to induce an error.

Timing chain must be under tension.

That’s what I used to do. And after all that for a street car, it didn’t matter much.

PS: checking on the edge piston can induce error as piston can have a slight “rock” to it.

If you haven’t read these articles - in particular page 5, you should. Assembly was all so very approximate.

http://www.jagweb.com/aj6eng/xk-engine/index.php

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