Side gapping spark plugs has been use in racing for years to increase horsepower by unshrouding the spark thus allowing the flame created to propagate faster resulting in a more complete burning of the fuel/air mix. Many manufacturers offer plugs deemed racing plugs that are side gapped but at inflated costs. By doing it yourself you can use a cheap .99 cent plug to save money and increase horsepower.
Step 1: Cuttting Ground Electrode
Using a dremel with a cutoff wheel or a hacksaw you will need to cut the ground electrode even with the edge of the center electrode. You need to be careful not to hit the center electrode or the porcelain.
Step 2: Removing Burrs
Next you will need to use a file to remove any burrs left from cutting the ground electrode. If this step is not done the burrs could heat up until they glow red hot causing detonation. After the burrs are removed you will want to use some alcohol or starting fluid to thoroughly clean the plug to make sure there are no metal filings left.
Step 3: Setting the Gap
After all the burrs are removed you must set the gap. To do this you will need to find the manufactures setting for your vehicle and reduce that by 0.010". Example: manufacture setting 0.035" - 0.010" = 0.025". Then using a feeler gauge of the correct thickness insert it at a 45* angle to check and adjust as necessary until the gap is correctly set.
Step 4: Before & After
Here are before and after photos of a non side gapped vs. sided gapped spark plug. With the side gapped plug the spark is now un-shrouded and is more exposed to the fuel/air mix. the flame created will now be able to spread faster resulting in a more efficient burn before the exhaust valve opens.