6. Working on the cylinder at TDC you will next check the opener and closer clearances. At TDC both valves should be closed meaning that there should be no pressure on the opening rocker, and so there should be a gap between the rocker and the shim, as well as the cam having pressed the closer rocker shut which should leave virtually no gap between the rocker and the shim. There is more than one way to check clearances but here we will check them using the unloaded and loaded gap.
The unloaded gap is when measurements are taken with no pressure on the closer rocker and the loaded gap is where measurements are taken with pressure is applied to the closer rocker as follows.
7. Unloaded gap.
With the valve in the closed position, at TDC, leaves no pressure on the opening rocker, the unloaded gap between the opener shim and the rocker is measured as the opener clearance and the values noted and compared to the clearance requirements.
The right feeler gauge should slide all the way through the gap without force and has been likened to sliding the feeler gauge through a thick book. If the feeler is too big then there is too much pressure required, the gauge will snag and will need to be forced through.
8. Loaded gap.
Still with the valves in the closed position, at TDC, the gap between the cam and the closer rocker, and then the rocker and the closer shim, should be at a minimum. As wear and tear increases these gaps the slack is taken up by the closer spring to fully seal the valve. If the closer rocker is pressed down with a screwdriver you can feel the rocker move through the gap that has grown. To measure the closer clearance, remeasure the opener gap, as in the previous step, but with a screwdriver pressing down on the closer rocker and note the value. This gap will now be bigger than before. To find the closer clearance simply subtract the value previously found for the opener clearance from the loaded gap measurement. The figure that’s left is your closer clearance.
9. Note down all measurements, opener and closer clearances for the inlet and exhaust valves on the horizontal and vertical cylinder heads, so 8 measurements in all. Check these against the clearance spec’s you wanted at the start and decide which shims, if any, you need to adjust to bring the clearances back into spec’. To work out the new shims needed you need to measure the old ones and add or subtract the amount the clearances were out of spec’.