Camshaft sprocket markings (again)

I am not trying to restart a discussion on the difference in the inlet and exhaust sprocket - let?s all agree that the exhaust marking is one tooth further clockwise than the marking on the inlet.

I was wondering about the mathematics of this.

The exhaust MOP (lobe centre) is 110 degrees BTDC and the inlet?s is 110 degrees ATDC. Therefore the inlet is 220 (crankshaft) degrees behind the exhaust - or 110 camshaft degrees.

For the next bit, let?s assume that you were to using one of the new exhaust sprocket that has both inlet and exhaust markings (one tooth apart). Let?s also not worry about valves clashing.

If you were to put both timing sprockets at the same timing point (say, with the inlet timing mark level with the head but both marks to the right of the sprocket) then you would then need to rotate the exhaust sprocket clockwise to get it to the right place. First by the 110 degrees we calculated above and then by a further 54 degrees to take into account the different angle of the inlet and exhaust valves. So, a total of 164 degrees.

The exhaust sprocket would now be correctly timed.

The exhaust sprockets started with its inlet marking level with the head and to the right of the sprocket. It?s has rotated by 164 degrees so would now be just below the level of the head but now on the left of the sprocket. In fact the inlet marking would be 16 degrees below (180-164) and the exhaust marking just under 6 degrees down (as one tooth equals 10.5 degrees). Or about half a tooth.

So why isn?t the exhaust mark 1.5 teeth clockwise (rather than only one tooth?

Or have I gone wrong somewhere?

Richard

I have found that sprocket markings as a rule are seldom reliable.

The markings are relative, so in/ex valve angles should not be a factor.

So I set up the timing exactly as desired with offset dowels (or vernier sprockets, more expensive but so easy to set up) and then mark off the sprockets.

If you change the cams they will need to be re-checked. :slight_smile: