Engine manufacturers use close tolerances in the production of their engines and there are times when cylinder bores may have to be machined oversize to stay within production limits. This is entirely normal, but can confuse mechanics who are not aware of this practice.

All cylinders should be measured for size with a micrometer, or check the ring for size in the cylinder bore before the rings are installed on the pistons.

The ring should be placed in the cylinder BELOW THE RING TRAVEL for accurate checking. Ring gap on cast iron rings should be at least .0035" for each inch of bore size. For instance a 3" ring should have a minimum gap of .012". Ring gap of more than .008" per inch of cylinder bore indicates an oversize cylinder and an oversize ring should be used.

The illustration below shows the results of standard rings that have been used in an oversize bore. Notice darkened portion of rings near gaps.

  (P32)