RE: Top Flite AT6 ARF
I'm sorry but I have to give you a complicated answer to a simple question. The firewall is supported virtually all the way around with hardwood spars which extend out to what I'll call a 'cowl ring'. The spars limit the firewall diameter to 5 7/8". However the cowl ring, which is approx. 3/4" forward of the firewall, further limit the usable diameter to 5 1/8". This figure is essentially the same as the inner cowling diameter.
Hope that helps.