Went to JDparts catalog and the plug part number is R515195. Went to part detail looked up the part and the following specs where provided.
I second WilliamBos post indicating 10mm
English Metric Specification
Weight 0.09 LBS 0.04 Kg
Thread Diameter 22.000 MM
Thread Pitch 1.500 UNITY
Drive Size 10.000 MM
Length 10.000 MM
Length 13.000 MM
Diameter 30.000 MM
Drive Type SQSCH