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You are probably going to use KE280A springs. The free length is 4.0625". The load at 2.25" is from 58 to 65 pounds. This is referred to as the closed length. At the open length of 2.5" you should have a force of 85 to 90 pounds. As for heads; I am a little hesisatant to state my opinions on an open forum. Please take this as my opinion, and not what you should do. The basic rule of thumb, for gasoline engines, is that for every percentage increase in compression, an equal percentage in RPMs must be added to gain any output. Just raising compression means more burned fuel with little more than ping to show for the work. To me, stock means a tractor that still can perform as a tractor. Can you raise the governed load RPM and get away with it. Higher compression also will kill low RPM lug. Read the rules before opening your wallet. Paying attention to spark voltage, plug heat range, and timing will yield far more power than reworking heads. Throwing money at a Moline engine yields little. Pay attention to the little things on a good stock engine and you will be far more satisfied with the results. I run a 'UTU' with tight stock GE30M heads, a stock five inch crank, and enough bore to clean it up for my aluminum pistons. I run points (no solid state) at 12.6 volts on thirty degree dwell and a well set TSX67 carb on an M5 manifold. The boys that throw money at their tractors have no more power than me. I time the engine for the octane in the tank. I keep the plugs two ranges cooler than stock and fire across a 0.035" gap. My 'pulling' tractor can also run full throttle, of 1300 RPM, on a dyno for an hour, or wide open in fourth gear all day. A simple twist of the high speed screw is all I need to tailor the unit for power. Build a good strong tractor. Drop in a six inch crank and have fun, if you want to. A stock 403 engine will bust up a lot of challengers, if she is tuned properly. A hot, high compression 403, engine that is not tuned properly will not even keep up with a 283 in shape. Valves, guides, seats, and such are available. Good luck and have fun doing it.
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