I've flown the Gladiator extensively for fun and competitively. All engines weigh in differently. Ihave handfuls of different engines here all appearing to look alike with differences greater than 1/2 oz. Off hand Ican't say as I don't recall the weight of the AP. Ido know however it will work. Ihad a piece of steel bolted to my mounts which was 3/16" x 3/4" by 1 3/4" long to balance the Gladiator using a AMEengine. In order to run the Profi, Ihad to have a custom beam made extending the engine an additional 1" out beyond the engine block. In the case of the AP, Imade a aluminum adapter that fit snugly into the AP case utilizing the rubber o-ring. Iran a tap through the existing case holes and used the set screws from Dubro wheel collars to lock my adapter in. The center of my adapter has been drilled and tapped 1/4 - 32 so that a TDventuri can be screwed directly into it. Iuse the Kirn Kraft style needle valve with the 128 tpi. The Galbreath Nelson adapter does fit the AP engine. I was using a APC 4.5 x 3.5 with good results and the APC 4.6 x 3 as well. Ibelieve and its been told to me the real reasoning for the overcontrolled tail heavy performance of the Gladiator is the short control horn that's provided with the plane. Ican't confirm that a larger one helps as I've never tried it. Ijust added the nose weight which by the way was quite a lot. Good luck, Ken