If the ring isn't square with the cylinder,your readings will be off. Specialized engine-tool companies like Powerhouse Products sell ring installation tools that makesure the ring sits parallel to the deck of the block (which, if properly machined, will be 90 degrees to the path of the piston). | 
Another option is to use an old piston with an old top ring installed. Put the piston to be gapped in the cylinder just almost at the top of the deck. Use the old piston to press the ring into the cylin-der until the uncompressed ring in the piston stops you. Now the ring should be square to the cylin-der bore. Of course, this only works with flat-top or dished pistons where the ring you are pushing into the bore can make contact all the way around the top of the piston. | 
With the ring in place, use a gap tool to check your gaps. |
Powerhouse Products sells an economical ring grinder that does a much better job than trying to gap your rings with a hand file. Simply lay the ring on top of the apparatus, butt the ends against the locator pins, and squeeze the ring so that both ends make contact with the grinding wheel. Spin the wheel so that it is pushing the ring down and into the locator , and grind away. Be careful not to cut away too much at a time; you can always cut more, but you can't add metal back to a ring. Many engine builders say that if your target end gap is 0.018 inch, you can get away with 0.004 to 0.005 inch of extra gap without harming performance. Don't, however, leave a gap too tight. | 
Hit the ends of the ring with a very fine file. You only want to remove burrs that may have formed when grinding the ring. Keep the edges square to make sure the ring does its job effectively. | 
John Wing of Midland Engine Machine checks the gaps on a new engine. Both the top and the second rings must be done. Notice the ring-squaring tool is within easy reach. Wing used it every time he put a ring into a bore to ensure he wouldn't get a false reading. |
The gap tool uses metal tabs of specific thicknesses. If you can just barely squeeze the gauge into the gap, you've got it right. | 
When installing a ring onto the piston, be careful not to stretch it more than you have to. Start by putting one end of the ring into the ring groove and slowly work the ring around until it's completely seated. Start with the oil ring and work your way up. | 
Getting rings off a piston is a different story. They can be almost impossible to remove by hand, so a ring expander like this one can be a helpful device. |