Do you have fouled damaged threads due to old spark plugs? You need to straighten those threads without dropping engine-killing metal shavings into your cylinders, or without removing the cylinder heads, and no risky thread inserts. You need Powerbuilt’s patented 12 Millimeter Back-Tap Thread Repair Tool to save time and possibly that engine. The Back-Tap's patented collapsible design allows it to engage be placed through the spark plug hole and into the chamber. A mandrel is then drawn back, expanding the tool's threaded area and threading into the clean threads at the bottom of the spark plug hole. The innovative Back tap repairs the damaged thread as it is drawn up through the spark plug hole, bringing debris and metal filings with it. Meets or exceeds ANSI standards. Lifetime warranty.