Whether to bore and install new pistons versus honing and re-ringing depends upon several factors:
One - Is the clearances and taper in the bores low enough to allow simply honing and re-ringing it? Too much taper and clearances just means excess blow-by (crankcase pressurization) under boost. If you're going to install new bearings in the bottom end (both mains and rods) you should have the crank checked for straightness and polished - at a minimum. Installing a turbo charger ensures increasing power output over natural aspiration under normal street driving, or the same power at higher elevations a NA engine has at sea level.
Two - Do you plan on towing with the vehicle? Especially in the mountains?
Once you install the turbo, at what level of boost do you plan on calling it quits? The temptation is hey, I can boost this bad puppy up. Why stop at 6-7 psi? If you plan on boosting the motor and acknowledge you'll probably jack up the boost, rebore the engine and install an appropriate compression ratio'd set of forged pistons. You don't want to blow up a boosted engine using cast pistions. Neither do I suggest boosting one without new rod bearings.
Never spend good money on a rebuild (bore, pistons, reconditioning rods, crank grind n polish, new bearings, yada yada) without shelling out the extra bucks to have the reciprocating assembly balanced. That's like washing a dirty car and not waxing it....
A normal aspirated 24V engine breathes better than a 12V. It also has the added benefit of placing the spark plug or diesel fuel injector in the center of the combustion chamber. Under boost, you're better off with a 12V arrangement. They work better under boost, and more area around the valve seats means less chance of cracking the head. An engine operating under 12 psi of boost will never know there's only 12V versus 24V in the head. Boost is boost. If you plan on boosting the engine, I'd use the original valves - provided they're good. Increasing valve size is the same theory as increasing the number of valves in a NA engine, you want better air flow. Under boost, you already have the air flow at a steady constant pressure. More and or bigger valves are not as big a factor in a boosted engine as they are in NA conditions.
Bear in mind if you build the engine and boost it up, you're probably gonna want to play with 1UZ-FE injectors, throttle body, the AFM, and a bigger turbo-charger than the CT26....
Your level of participation in any sport is directly porportional to the size of your wallet.
If it was me rebuilding a 5M-GE for a 4x4, I'd keep it NA and determine to rebore on the amount of cylinder to piston clearance and taper. Inline sixes tend to wear less than the v6 and v8. You go around jacking up the power over 190 hp/190 lbs torque, you're gonna start destroying driveline parts.
My personal choice would be a 6M-GE or a 7M-GTE short block with a 5M-GE head. Pistons would be dictated by whether I'm boosting the engine or not. I'd much rather run a roots style blower versus the turbo charger anyways. No turbo lag and instant boost. I want the power in the bottom end, not midrange to top end.
JR
