Read the thread size before buying a repair tool
Write down the thread from the equipment documentation or a verified matching fastener. For a metric thread, the designation gives the nominal diameter and pitch. M8×1.0 and M8×1.25 share the same nominal diameter and require different pitch matches. Inch threads use a different convention. A 1/4-20 designation describes a nominal quarter-inch thread with 20 threads per inch, while 1/4-28 has 28 threads per inch.
A thread pitch gauge helps distinguish nearby pitches, but it needs to be paired with a diameter check and the applicable thread standard. If the damaged threads are hard to read, use an undamaged matching part or the manufacturer's identification. Keep internal and external threads separate when looking up a tool. A kit may cover both or only one.
A multi-size kit still needs to list the size you need. For example, the Heli-Coil 4937-125 and 4937-150 sets both include several smaller metric sizes, but the listed M10 pitch differs. Check the full model number and the contents of the selected listing.
Will the closest pitch work?
Choose the documented thread designation. When the original thread cannot be identified, hold the order until the equipment documentation or a matching part resolves the size. A near match can look plausible in a photo or chart and still be a different thread.