Just went back and forth between several engines on my GP Ultimate last night. I was pretty pumped on putting in an OS 61 FX until I noticed that it was just barely too big for the cowl even without the glow plug. [] If you don't want to cut the cowl out, be prepared in case your cowl may be a bit too small. I had to settle for a Saito .72 4strk....I wanted my 61 or my 91 4s, but my 91 4s found a home. Oh well, hopefully I'll only be denying myself the hovering.