Yes, Casey, it's a slip fit. Pop the 20's shell off and pop the 18's on - a simpler swap can't be had. It does sit a fraction high and the tank is wrong for a GP18 but, if this doesn't worry you, you are off and running. Gary.
I've had good luck putting the GP-18 on an atlas gp-7/9 frame with very little work.The fuel tank will be correct and it will run with the rest of your high hoods....If you do want the GP-20 to run faster,put atlas four axle trucks in place of the LL ones.They only cost $2.50 each so it's a good cheap modification. ------------------ THE B&O ROCKS!!!! ROYAL BLUE
Maybe I have a use for my GP18 shell now. IF i didn't see this post it would have been scrapped! Thank you, Dane Nicholson ------------------ BCRailKing@Canada.com Teen Association of Model Railroaders- for information, contact Dane N. at TAMR2860@Canada.com http://www.tamr.org "I was DC, and went DCC!"