The gap in the ring is designed to provide space for the ring to expand as it heats up. An ideal ring gap will result in an exactly zero gap at normal operating temperatures
Setting the ring gaps incorrectly has one of two impacts.
1 - If the gap is too large, then a gap is still evident at normal operating temperatures and important compression will be lost in the cylinder, hence reducing power.
2 - If the gap is too small then the ring will push outwards on the bore as it expands and the ends but up against each other. This will cause engine ware and increased friction, hence reducing the life or your engine and power.
Tools Required
-- Piston ring grinder or very fine metal file.
-- feeler gauges
Steps
1 - make sure your bores have been machined and/or honed and require no more work
2 - Identify the top and second compression rings. Identify the required gaps, typically the ring manufacturer will specify this. In my case flying miata recommend 0.016 on the second compression ring and 0.015 on the top compression ring.
3 - Put the ring you are working with in the cylinder bore that it will be used in. Do not gap a ring in bore 1 say and then install it in bore 4
4 - The gap in the ring will change due to the orientation of the ring in the bore, so it is important to get it square. If you don't have a tool for this, you can install the oil rings and use the piston inverted as below
5 - Now get a set of feeler gauges, ensure they are clean and measure the gap in the ring. Start small and build up.
6 - The ring will be typically to large. Increase the gap using a ring grinder or file. Always ensure that the direction of the grinder or file is from the outside inwards, so that none of the outer coating of the ring is chipped off. Only take a small amount of metal off. If the gap is increased too far then the ring is scrap and a new one will be needed.
7 - Repeat steps 3,4,5,6 until the desired gap is achieved.
8 - Now fit your rings using a ring expander, ensuring the gaps are directed as per the manufacture recomendations. Typically the gap in the two compression rings face each end of the engine, ie. one towards the fly wheel and the other towards the pulley wheel.
The end.
