Well, I'll do my best to explain the three engine types currently available in the US. 40mm stroke PK80 type, 38mm stroke GT5 type and 38mm stroke "Halfbreed" type. Although there is no consensus on the nomenclature for these engines, these terms are the best ones that I could come up with. If you shop in my store, these are the terms that I use for describing the three different engine geometry types. GT5 type engines can be purchased from Rose 326a (ebay), Gasbike/Kingsmotorbikes or Piston Bikes. PK80 engines can be purchased from DAX, Neil Senior (facebook) or Best Engine Kits. PK80 motors with ZL rods can be purchased from Bikeberry. Half Breed engines can be purchased from Piston Bikes (silver/black jet), Boy Goes Fast (ebay) or bestmotorkit (wide mount, 8mm stud). There are quite a few other vendors out there, but I'm not familiar with them. Optimal engines can be purchased from crmachine (piston port design) or Arrow Motorized Cycles (reed valve type).