I've been meaning to post this for awhile, in fact I have found 6oz tanks that fit in the engine nacelles with no mods, I waited a few months from tower to get them but there the tanks that come with the Viper 500 ARF, tank part # GPMA2552. there always in limited supply, you have to build the plumbing and fit the clunk, then take it back out and slip the tank in the nacelle neck first and then without the engine or mount on put the filler neck and plumbing in the tank thru the firewall, pretty easy actually, bolt the engine in and off you go, I had FP 40's in mine and flew for quite awhile on 6oz's.