My timming marks
Discussion
Well as I have the odd head off still I thought I'd check out what's TDC.
I borrowed a dial gauge and set it up on a bracket over the cylinder. I chose a reading near the top (0.5 = 0 on the little dial and 5.0 on the big one) and marked out the position of the crank pulley against the pointer in both directions on each side of TDC. Check the two points are consistent, and if so, bisect the two marks gives true TDC.
So - this is what I got:
Real TCD corresponds to the "O" in the "BEFORE"! The 'P' line corresponds to the pulley idea of TDC. The leftmost paint line is where I last set up the timing by guesswork, which turns out at 6Deg BTDC approx. It should be 17Deg BTDC so a bit more to go!
Here's a piccie of the pointer mounting - at first I thought it was bolted in wrong but now it seems there is one bolt dropped out...
I wonder if I could relocate the pointer by shifting the bolt holes, then it would almost be correct....
I borrowed a dial gauge and set it up on a bracket over the cylinder. I chose a reading near the top (0.5 = 0 on the little dial and 5.0 on the big one) and marked out the position of the crank pulley against the pointer in both directions on each side of TDC. Check the two points are consistent, and if so, bisect the two marks gives true TDC.
So - this is what I got:
Real TCD corresponds to the "O" in the "BEFORE"! The 'P' line corresponds to the pulley idea of TDC. The leftmost paint line is where I last set up the timing by guesswork, which turns out at 6Deg BTDC approx. It should be 17Deg BTDC so a bit more to go!
Here's a piccie of the pointer mounting - at first I thought it was bolted in wrong but now it seems there is one bolt dropped out...
I wonder if I could relocate the pointer by shifting the bolt holes, then it would almost be correct....
Edited by adam quantrill on Thursday 16th May 21:50
Well the crank and the piston are linked 1:1 unlike the camshaft which runs at 2:1, but I know what you mean. Also as I recall the crank pulley is keyed to the crankshaft with a key and slot arrangement, so it can only go on one way.
THe timing marks are on the crank of course. The reason for measuring the piston near - but not at - TDC is that you can measure the angular position much more accurately. As the piston gets closer to TDC it slows down and stops moving.
So what you do is measure is coming up to a certain height, then measure it in the other direction to the same height. In fact I did this repeatedly in different directions and came up with the same two marks, so there isn't any backlash. When you have the piston position either side of TDC, then bang in the middle is actual TDC.
I actually reproduced this too with the dial gauge, it really does show the highest point at the same place I made the middle mark on the paper.
Apparently it's not that uncommon for timing marks to be that far out. Maybe it happened when the supercharger drive pulley was added?
Now - here's a twist - take a zoomed-in look at the photo of the timing marks. I can see;
- A faint line through the O of the BEFORE.
- Another BEFORE (faint) and another TDC.
Amazing! Looks like someone has had trouble with this one before...
THe timing marks are on the crank of course. The reason for measuring the piston near - but not at - TDC is that you can measure the angular position much more accurately. As the piston gets closer to TDC it slows down and stops moving.
So what you do is measure is coming up to a certain height, then measure it in the other direction to the same height. In fact I did this repeatedly in different directions and came up with the same two marks, so there isn't any backlash. When you have the piston position either side of TDC, then bang in the middle is actual TDC.
I actually reproduced this too with the dial gauge, it really does show the highest point at the same place I made the middle mark on the paper.
Apparently it's not that uncommon for timing marks to be that far out. Maybe it happened when the supercharger drive pulley was added?
Now - here's a twist - take a zoomed-in look at the photo of the timing marks. I can see;
- A faint line through the O of the BEFORE.
- Another BEFORE (faint) and another TDC.
Amazing! Looks like someone has had trouble with this one before...
Edited by adam quantrill on Friday 17th May 23:07
Mark, don't fret... I've found it.
"Note that the timing markers are not always accurately fitted to the engines. Inaccuracies of up to ten degrees have been observed. Another cause of error in the marker can be that the crankshaft pulley is not properly secured, or has been loose at some time in the past. This causes fretting in the pulley key-way and allows it to move round on the crankshaft "
"Note that the timing markers are not always accurately fitted to the engines. Inaccuracies of up to ten degrees have been observed. Another cause of error in the marker can be that the crankshaft pulley is not properly secured, or has been loose at some time in the past. This causes fretting in the pulley key-way and allows it to move round on the crankshaft "
I made some more marks today outside the two that I made earlier as a double-check, and also I tested my patent electrical device (special sprung pin and a buzzer) which turned out to be accurate to 1 division on the dial gauge - well under a degree of crank angle.
So I shall be making this into a device that you can stick into the spark plug hole, if you want to test your own engine.
So I shall be making this into a device that you can stick into the spark plug hole, if you want to test your own engine.
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