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Old 09-18-2013, 08:43 AM
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flex

how much clearance should be allowed for crankshaft, rod, piston flex etc. on a sbc 350 400hp build?

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Old 09-18-2013, 10:14 AM
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Quote:
Originally Posted by atc529r View Post
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 winnerís 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.

Bogie
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Old 09-18-2013, 10:36 AM
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Quote:
Originally Posted by oldbogie View Post
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.

Bogie

piston to head clearance is what I was referencing. Everyone says .040, but I've read that's pushing it. This engine is balanced with zero deck and I am looking @ ordering head gaskets. Redline will be 6k, which it won't see that often...but I want to take any flex or piston wobble into account. I was thinking .045 on the gasket to give me a little wiggle room and maybe meet in the middle of .040 and .050 which seems to be what I have read is safe.

and thanks for your reply.
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Old 09-18-2013, 11:22 AM
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My 1991 4.3L V6 engine was built with a total of 0.042" piston-head clearance which includes 0.030" crushed gasket thickness and the block was milled 0.010". Using forged Speed Pro L2256 + .030" OS pistons, pressed fit pins and 0.0035" skirt clearance on stock 5.700" pressed-forged PM rods.

A 350 CI V8 is the same.

No problems in more than 6,500 miles.
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