This tool can properly hold the HOWIN L6-210 spark plug with 3/16-40 inch thread. The exterior diameter 9.85mm but the spark plug recess diameter on my HOWIN L6-210 cylinder's head is only 8.89mm, therefore this tool cannot be insert into the cylinder head. I have to use my lathe turning down the socket exterior diameter to about 8.6mm (smaller than 9.85mm) for ease of insertion during spark plug installation.
Stirlingkit need to check with engine manufacturer and tool supplier on this dimension matching and improve it to provide a correct tool to customer without further machining process
