Originally Posted by atc529r
how much clearance should be allowed for crankshaft, rod, piston flex etc. on a sbc 350 400hp build?
Exactly what clearances are you speaking of?
If this is piston to head for the street the minimum is around .038 to .040 inch. This is not so much flex as with a steel rod there really isn't enough to be concerned about but the piston rocks about the pin taking up skirt clearance which raises one side of the piston by a trig function of the angle formed when the piston leans into the clearance. A competition engine will be tighter in piston to head clearance but that is risk take to make big bucks in the winner’s circle.
Crankshaft bearing clearances for mains and rods should stay inside the factory recommendations unless you're an expert engine builder with a few blown motors under your belt and possess an understanding of why they blew that you apply to your craft. Factory clearances will not get you into trouble with part collisions.
The crankshaft is well constrained in the SBC, normal flex does occur but if the engine is well balanced and dampened this is in the millionths of an inch and is not a problem for a 400 horse SBC. Now 6, 7, 8 hundred horse and RPMs well above 6000 we're talking a different animal. Even a 400 horse engine that spins up high, as in many dirt racers, then flex gets to be a problem even though the power output is in the 400 hp zone. In this case it's gotten to be a different technique than a similar output street engine by using limited induction (carb sizing) and lots of cam versus a street 400 hp SBC that uses a big carb with a more limited cam that produces the power under 6000 RPM. In the high RPM racing cases, the engine is built with better parts that have more strength to resist bending and flexing along with even better balancing and highly effective, read expensive, dampers so the end result of unwanted motions remains constrained and does not affect clearances.