My dad has this happening to him as well. He compared my cap to his and his is missing threads, bad machining, he is working with amsoil for a replacment.
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05 3500 QC LB NV5600 4X4 SRW
Pricol Boost and EGT
Amsoil everywhere, BMK-11
GDP MK-2+ w/ Cat 2 mic
Smarty Jr Revo