File-Fitting Rings
The end gap of a set of compression rings represents a leakage path for the gasses in the cylinder, and that applies to every phase of a four-cycles engine's operation. Minimizing this leakage is a means of improving the mechanical efficiency of the engine. Typically, file-fit rings are manufactured to a 0.005-inch larger diametrical specification than the bore, allowing the ring end gap to be custom filed to a desired end gap. Adequate gap is required to prevent the ring ends from butting in operation, so the desired specification will vary, depending upon the engine's usage. File-fit ring sets will usually come with an instruction sheet specifying recommended end gaps for varying applications. There are numerous tools and techniques that can be used for file-fitting rings, from simple hand files, to hand-cranked filers, to more elaborate power-driven units such as ProForm filer used here. All will get the job done, it's just a matter of time invested and what you're used to.

Custom gapped rings can represent a measurable increase in power, but a botched job can lead to a power loss or engine damage. Here is how rings are gapped and what to look out for.

Block Prep
The final build-up begins with the block prep. A block may be fully prepped from the machine shop, and ready for assembly. However, in most instances, the builder still has work to do after the major machining is complete--particularly if the engine is being built from a used core. The final pre-assembly block prep should be done after all of the mock-assembly checks are made. Although it isn't absolutely required, the first step is to perform any deburring or grinding operations, such as removing casting flash, radiusing or enlarging oil passages, or smoothing drain-back areas in the lifter valley. With used blocks especially, the threaded holes in the block should be chased with a bottoming tap to clean debris from the threads. Once all of these debris-creating jobs are completed, the block can be cleaned and painted for final assembly.