Deck clearance?
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Deck clearance?
How much is enough at 8500 rpm? SBC, 4340 5.85" H-beams, 3.75" Crower 4340 crank, approximately a 440 gram piston and .927" x 2.75" straight wall pins (.150").
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Re: Deck clearance?
Well first off to me the problem is spinning 8500 rpm with a H beam Rod, because to me your walking right on the edge even taking into account that weight piston!
For longevity at that rpm you should be into Bilet or Titainum, or if you can wallet the cost, then run Aluminum rods that you replace every 100 passes!
With steel rods that will work for that rpm you want a piston to Head clearance absolute minimum of whatever your piston to bore clearance is cold+.003, but the basic rule of Thumb for deck clearance is .032” to .037” and then the top ring groove to piston top needed to not have a melt down or picnched ring land is .200” for me.
For longevity at that rpm you should be into Bilet or Titainum, or if you can wallet the cost, then run Aluminum rods that you replace every 100 passes!
With steel rods that will work for that rpm you want a piston to Head clearance absolute minimum of whatever your piston to bore clearance is cold+.003, but the basic rule of Thumb for deck clearance is .032” to .037” and then the top ring groove to piston top needed to not have a melt down or picnched ring land is .200” for me.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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Re: Deck clearance?
Then please provide us with a more complete answer if you will?
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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Re: Deck clearance?
For me, I like .040. mainly because I think that's a good safe clearance with a steel rod and it's close enough to be taking what I would consider as full advantage of the "squish" effect. I don't think going any closer really makes any more difference.
Having said that, the minimum I would ever use is .030 and that would only be if I was trying go get maximum compression ratio..
Having said that, the minimum I would ever use is .030 and that would only be if I was trying go get maximum compression ratio..
Re: Deck clearance?
Most questions assume generic SBC practices, but there's more to it than just .030" or .040". As mentioned, rod construction, piston weight, RPM, piston-to-wall clearance, bore diameter, rod journal diameter and clearance, all come into play in determining a safe piston-to-head clearance.
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Obsolete Engineering
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Obsolete Engineering
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Re: Deck clearance?
mag2555 wrote: ↑Sat May 15, 2021 10:15 am Well first off to me the problem is spinning 8500 rpm with a H beam Rod, because to me your walking right on the edge even taking into account that weight piston!
For longevity at that rpm you should be into Bilet or Titainum, or if you can wallet the cost, then run Aluminum rods that you replace every 100 passes!
With steel rods that will work for that rpm you want a piston to Head clearance absolute minimum of whatever your piston to bore clearance is cold+.003, but the basic rule of Thumb for deck clearance is .032” to .037” and then the top ring groove to piston top needed to not have a melt down or picnched ring land is .200” for me.
https://youtu.be/mBj2a8reih4
These run hundreds and hundreds of laps same stroke. 8500 is a common rev limit, big Boys 9000.
Steel H beams in most, I beams in some.
The valvetrain longevity is a MUCH larger issue.
Re: Deck clearance?
Block material is also a factor. The coefficient of thermal expansion is ~ 6 ppm for iron, ~13 for common aluminum alloys, so (assuming uniform engine temperature) heating it from 70°F. to 170°F., a 9" deck height aluminum block will expand (13-6) x 9" x 100 = 0.0063" more than an iron one.
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.
Happy is he who can discover the cause of things.
Re: Deck clearance?
I knew I didn't provide enough detail, lol.
Engine is a pretty good effort sb2.2. Aluminum block, name brand rods and crank, and .004" skirt clearance. It may occasionally see 9000 rpm, but the shift point should be 8500. It's a drag racer.
.040" has been enough in the past in 12.5:1 bracket racers with Chinese rods and cranks, and factory blocks.
A total guess and some experience tells me .050" should be enough. What have y'all ran in similar engines and not tore up anything?
Engine is a pretty good effort sb2.2. Aluminum block, name brand rods and crank, and .004" skirt clearance. It may occasionally see 9000 rpm, but the shift point should be 8500. It's a drag racer.
.040" has been enough in the past in 12.5:1 bracket racers with Chinese rods and cranks, and factory blocks.
A total guess and some experience tells me .050" should be enough. What have y'all ran in similar engines and not tore up anything?
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Re: Deck clearance?
For got to state
They do hundreds of laps, the engines get freshened and the rods are usually reused.
They do hundreds of laps, the engines get freshened and the rods are usually reused.
Re: Deck clearance?
I think there are guys here who would raise their hand for 0.030" on an iron block SBC. I'd say 0.035", closely verified in each cylinder, is safe for you. 0.040" and up is just giving away power.
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.
Happy is he who can discover the cause of things.
Re: Deck clearance?
I know you're an experienced engine builder, so I do not take your recommendation lightly. Thank you!
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Re: Deck clearance?
Re loosing power.
It's well documented in 2 strokes that too much and too tight squish, while boosting bottom end and mid range kills top end.
On a 4 stroke I heard too tight hurts top end as well.
(I used to strongly disbelieve this)
Most 4 strokes with performance oriented valve sizes can't get anywhere near the quench area a 2t can, but a 2v engine with quench pads may get closer than a 4v.
Now I sometimes wonder....
It's well documented in 2 strokes that too much and too tight squish, while boosting bottom end and mid range kills top end.
On a 4 stroke I heard too tight hurts top end as well.
(I used to strongly disbelieve this)
Most 4 strokes with performance oriented valve sizes can't get anywhere near the quench area a 2t can, but a 2v engine with quench pads may get closer than a 4v.
Now I sometimes wonder....
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