Hi Jim, using 35% nitro fuel all depends upon your application and even which Cox engine

you are using. If you are flying in FF contests, old school TD Combat or Mouse racing;

then yes, 35%-40% is great to use. You will have to pay attention to deck clearance.

The higher the Nitro %, the more clearance you need. .012" is safe for 30%-35% Nitro.

.012" head space(TD .049) is what I used when flying 1/2A Combat decades ago, and

this was safe enough to last at least twenty 24,500rpm(ground rpm) Combat flights; so

the reliability was there.

If you are sport flying, then 25% Nitro fuel makes more sense, just try to watch your

deck clearance and prop load....a 5" X 4" is too great a load, even on TDs.

This brings up an additional point. Most 1/2A competition flyers, started with an engine

that is setup to blueprint specs as much as possible. Foremost is setting the cylinder

height with cylinder shims(NOT PLUG shims), still available from an ebay source. The cylinder is shimmed

up so that the piston top at TDC is exactly even with the plug shelf. then the TD plugs

are selected so that with plug shims, you acheive the desired head space.

This gets tricky because you must include the distance between the bottom of the plug

up to top of the squish band. What makes this weird is that this squish band measurement

varies a BUNCH because of mfg tolerances. Sorry for the Tome..

Tony