The Tower muffler for the 40-46 will not fit your Thunder Tiger GP42. That engine's exhaust matches the OS 40 FP and I believe LA series engines. It also matches all the 25 through 39 small case engines that have a helicopter version also. ( OS 28-32 Webra 32-36 Irvine 39 Thunder Tiger 36-39 Etc.) One of the best values for your engine is actually the stock muffler from Thunder Tiger.