Dummy cross threaded his glow plug.
What size tap to fix?

Most are 1/4 x 28.

NO.

1/4 x 32

cfaw10 is 110% correct, glow plug threads are 1/4" x 32. Forget your neighborhood hardware store, they are difficult to find. Try: rutlandtool.com, No. 2144-0171 @ $11.75 or use-enco.com, No. 891-4924 @ $4.87. My catalogs are old and those prices have probably increased.
Unless you have a better class engine with an insert in the head, the chances are that cross threading has removed enough material and that just re-tapping will not be adequate. And you will probably never use the tap again, maybe a new head is a better way to go. OR- C.F. Lee Co. can install a device called a "heli-coil" which is actually an insert with the proper threads. Price ranges from $12.50 to $15 depending on engine size. (818)352-3766.

You can a Higley tap or die at https://www.quicktechhobby.com/Airpl...b%20higley.htm

if all else fails send the head to clearance lee charge used to be $10 for a 2 stroke.

1/4-32 !!!!!!!!!!!!!!!!!!!! !/4-28 IS A NO-NO!!!!!!!

harvee

Just get CL to install a helicoil, once those threads strip there's not enough left to tap.