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Old 11-03-2013, 04:24 PM
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mininum piston to head clearance

all new parts drag only sbc stroker crank 6" rods je pistons, 12 to 1 flat top vortec headed steel rods. how too close is .020?

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Old 11-03-2013, 04:50 PM
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Optimum is .040. Back in the day many were setup at .020 and closer. But pistons get real close to the head, from piston rock, if the pistons are set up old school.
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Old 11-03-2013, 06:14 PM
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Take into consideration the clearance on mains,rods,wristpin,piston rock & add them all up,then subtract that from .020.Then whatever that # comes up to,take a feeler guage blade of that size & look @ it good.Then think of Dirty Harry when he asks the guy,"Do you feel lucky today???" that's with everything brand new & tite befor any wear or carbon build up on the piston & head.Have you also ckd piston to valve clearance with cam of choice.They say .035 to .045 is optimum.For me .035 would be too tite.LOL.When you say Vortec head,are you talking L31 cast iron heads with 12:1 CR? I'm somewhat conservative,but,i'd say you are definitely an optimist of highest degree,or,have grapefruits for cahooona's !!! Or however you spell it !!! Honestly tho,.027 is about the tightest I've realisticly heard of.
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Old 11-03-2013, 06:32 PM
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vortec heads and drag racing?
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Old 11-03-2013, 09:55 PM
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I've seen .032" tap the head. You are way past the danger zone with .020". When the engine heats up, you'll have even less....
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Old 11-03-2013, 10:18 PM
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cahooona's !!! Or however you spell it !!!
Urban Dictionary: cojones
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Old 11-04-2013, 12:22 PM
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all new parts drag only sbc stroker crank 6" rods je pistons, 12 to 1 flat top vortec headed steel rods. how too close is .020?
If you're a big bucks competitive racer that uses engines to win and throws them away when they don't then .020 on a steel rod is acceptable practice.

If you're not one of those nor expect this engine isn't going to make you one, then .035 to .040 is a lot safer. It takes a lot of care and measurement to run .020, it can be and is done but there's no space for error youre assemby thechniques must be pristine and accurate to the gnat's.

The issue is not only clearance stack up from the main though the rod bearings to journals, but also pin, but the forgotten one is piston rock as it rolls over the pin on the thrust moments that take up the skirt to wall clearance. The looser the motor is set up the higher the risk of running out of piston to head space once you get to and certainly under oh-thirtyfive.

Don't forget to check how close the valves get to the piston as well.

Bogie
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Old 11-04-2013, 12:50 PM
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The issue is not only clearance stack up from the main though the rod bearings to journals, but also pin, but the forgotten one is piston rock as it rolls over the pin on the thrust moments that take up the skirt to wall clearance. The looser the motor is set up the higher the risk of running out of piston to head space once you get to and certainly under oh-thirtyfive.
Bogie
Bogie, I have seen this argument many times and every time I wonder if there really is any piston rock in the direction that would make the squish tighter. As the piston comes up the bore, there is (theoretically) the same pressure across the entire crown of the piston. This would seem to stabilize the piston in relation to rock. Now, as the crown comes closer to the head, the squish space begins to minimize and we have a higher pressure over the squish area, which theoretically should push down on the crown on the side opposite the chamber and increase the clearance between the crown and the underside of the head. Am I making any sense here?
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Old 11-05-2013, 11:40 AM
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Bump to top....
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Old 11-06-2013, 04:25 PM
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I've built quite a few SBC 358 oval racing engines that have rules mandating flat top pistons and 62cc heads, so getting all the compression there is to get is important. I shoot for 0.025 for piston to head clearance and have run it that tight many times. The pistons do kiss the heads as they stay completely clean in the quench area. These engines are turned about 7800RPM, use Crower cranks, Oliver rods and JE pistons. If I were you, I would mill off about 0.015 in the areas of the piston of concern.
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Old 11-06-2013, 05:06 PM
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Tightest I've been is .034", it just polishes the carbon off the piston tops but no actual metal contact. Engine is 383 Chevy,11.4 to 1 compression, Scat 3.75" stroke cast crank, 5.7" Eagle SIR rods (stock look) pressed pins, TRW forged flat top pistons at .0035" wall clearance, .0025" rod bearing clearance, run to 7600 rpm drag/street/strip.

Generally consider .040" as plenty tight for this type of use. You are way too tight at .020".
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Old 11-06-2013, 06:00 PM
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Originally Posted by techinspector1 View Post
Bogie, I have seen this argument many times and every time I wonder if there really is any piston rock in the direction that would make the squish tighter. As the piston comes up the bore, there is (theoretically) the same pressure across the entire crown of the piston. This would seem to stabilize the piston in relation to rock. Now, as the crown comes closer to the head, the squish space begins to minimize and we have a higher pressure over the squish area, which theoretically should push down on the crown on the side opposite the chamber and increase the clearance between the crown and the underside of the head. Am I making any sense here?
You know I've seen collisions when the clearance is really tight but exactly what the cause is in the running engine I can't say I know. I do, when checking during assembly, push the outboard side of the piston down with a dial indicator mounted on squish/quench side to see how much angular rise it gets to be when the skirt clearance is taken up. But that's a cold engine where the clearances are greater than at any other time. I add that, allbeit small amount, to the the sum of the bearing clearances of mains, rod and pin though the pin is more theoretical than a real measure. Plus you can;t even get to load deflections of the block and crank. But I'm trying to get to worse case situation at least dimensonally. But we all know that what's going on in the running engine is going to be all different not only from the cold dimension up in the shop but from moment to moment with all the movements the entire assembly makes and how the block is supported and with solid mounts how it reacts to the forces put into it by the frame. So doing what I can in the shop with the understanting that it doesn't reflecct the real world. Everytime I see slow motion photography of a running engine reacting to all these strains, I'm amazed they work at all let alone work well.
In the end all this fretting over tiny details may just be a waste of time, I just can't convince myself of that yet.

Bogie
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Old 11-09-2013, 06:20 PM
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if nothing else this will bumpertothetop.. 383 motor runs 6300 tops through the lights. parts got a season on them eagle rods and scat crank, je pistons. not sure what the vortec and drag racing comments were about but i bought it that way and car goes 6.80`s @ 98. anyways what prompted to thread was i had some .015 gaskets around and i checked the distance in the hole and it varied between .006 and .010. actually it varied on some of the same pistons measured on the outside edge vs inside. realistically i figured on the verdict here and i`m gonna shoot for .032 on the tighter ones. theres a wealth of knowledge on this site and i know dealing with mechanical stuff can always go wrong but everybodys input was welcome.
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Old 11-11-2013, 09:31 PM
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post a drag video,would love to see it go.
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