wears the cylinder wall more at the top of the cylinder. In  most cases, the taper ends with a ridge, that must be removed  during overhaul.  [Figure 10-6] Where cylinders are built with an intentional choke,  measurement of taper becomes more complicated. Cylinder  choke is where the top of the cylinder has been made with  the very top diameter of the cylinder smaller, to compensate  for wear and expansion during operation. It is necessary to  know exactly how the size indicates wear or taper. Taper  can be measured in any cylinder by a cylinder dial gauge  as long as there is not a sharp step. The dial gauge tends to  ride up on the step and causes inaccurate readings at the top  of the cylinder. The measurement for out-of-roundness is usually taken at the  top of the cylinder. However, a reading should also be taken  at the skirt of the cylinder to detect dents or bends caused by  careless handling. A step, or ridge, is formed in the cylinder  by the wearing action of the piston rings.  [Figure 10-6]  The  greatest wear is at the top of the ring travel limit. The ridge  that results is likely to cause damage to the rings or piston. If  the step exceeds tolerances, it should be removed by grinding  the cylinder oversize, or it should be blended by hand-stoning  to break the sharp edge. A step also may be found where the  bottom ring reaches the lowest travel. This step is rarely found  to be excessive, but it should be checked. Check the cylinder  flange for warpage by placing the cylinder on a suitable  jig. Check to see that the flange contacts the jig all the way  around. The amount of warpage can be checked by using a  thickness gauge.  [Figure 10-2]  A cylinder whose flange is  warped beyond the limits should be rejected. The locations for checking runout and edge thickness of the  valves are shown in  Figure 10-7 . Measure the edge thickness  of valve heads. If, after re-facing, the edge thickness is less  than the limit specified by the manufacturer, the valve must  not be re-used. The edge thickness can be measured with  sufficient accuracy by a dial indicator and a surface plate.  Out-of-roundness is usually caused by a stuck valve. If a  valve sticks, the rocker shaft tends to work up and down when  the valve offers excessive resistance to opening. Inspect for  out-of-roundness and oversize using a telescopic gauge and  a micrometer. Inspect the valve for stretch and wear using a micrometer or  a valve radius’ gauge.  [Figure 10-8]  If a micrometer is used,  stretch is found as a smaller diameter of the valve stem near  the neck of the valve. Measure the diameter of the valve stem  and check the fit of the valve in its guide. Compression is tested with a valve spring compression tester.  [Figure 10-9]  The spring is compressed until its total height  is that specified by the manufacturer. The dial on the tester  should indicate the pressure, in pounds, required to compress  the spring to the specified height. This must be within the  pressure limits established by the manufacturer. Valves and Valve Springs Ridge removed by grinding Ridge worn at top of ring travel Original cylinder wall Ridge removed by hand-stoning Figure 10-6.  Ridge or step formed in an engine cylinder.  10-9