The Bosch cap is made for a 14mm spark plug and is shielded..Won't fit the 10mm plug in the G26 without a C&H mod..The NGK cap is made for the CM6 spark plug and is not shielded...
Must be something else, almost all G26 engines work just fine without any shield at all....
Try a new plug, Champion RZ7C or NGK CMR6....
WWW.sparkplugs.com sells both....