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Discussion Starter · #1 ·
Seems that most consider that an NA combination with a 4'' bore size will be happy enough and safe at 0.039'' + or so deck clearance with steel rods.. The question is assuming the former statement to be about right, As the bore size increases what additional clearance, if any, is required? Further does "piston rock" have any real effect, particularly as size increases... lets say from 4'' to 4.6" or so?
 

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My engines are 4.070 bore size and I run them at .037. These are flat top pistons with .008 piston to wall clearance. RPM at shift is 8000 and trap rpm is 8700- 9000. Nothing is touching. Pistons are light steel Crower rods.
 

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Discussion Starter · #3 ·
Thank you for the real world answer,,, seems the pist to bore is taken up when hot and totally stabilising the piston.. given what I was thinking this ^^ would be a certain candidate for touching at the very least,,, Time to revise any crazy theory I might have lol
 

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There is no correlation between cylinder bore size and quench. Most engine builders set quench measurement at around .040", due to the fact that this allows the fuel mixture to squeeze into the combustion chamber generating swirl and also allows for any growth (center to center) of the connecting rod.
 

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.010-.011" per inch of bore is pretty close for N/A engines with steel rods. Deck clearance is a tuning tool. Deck clearance combined with squish to bore ratio is big determining factor of combustion activity.
Lower rpm engines under 7500 rpm seem to do well setting them up about as tight as you can get away with. I had situations with higher rpm engines where extra deck clearance seemed to improve performance and widen the tune-up window.
When you start get close you will see what looks like a light peppering or dullnes on the edge of the piston about 1/4" from the thrusting edge (so exhaust side the piston on the drivers side bank). Thats as tight as you want to be, this seems to show up when the deck is at .0095" per inch of bore.
Even though the piston is touching the head at that point its not really hitting. Its kind of controlling the amount of piston rock you have.

Every .001" of an inch (net) tighter you make it, that dull peppery look will migrate about 1/4" closer to the center of the piston. If you see that look more than 1/2" in from the edge your too tight mechanically.
 

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My engines are 4.070 bore size and I run them at .037. These are flat top pistons with .008 piston to wall clearance. RPM at shift is 8000 and trap rpm is 8700- 9000. Nothing is touching. Pistons are light steel Crower rods.
Check your pm please
 

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There is no correlation between cylinder bore size and quench. Most engine builders set quench measurement at around .040", due to the fact that this allows the fuel mixture to squeeze into the combustion chamber generating swirl and also allows for any growth (center to center) of the connecting rod.
So moving away from safe and approaching tight you would say there is no correlation or thought to be given to bore size (and everything else) vs piston to head clearance? One size fits all and we're good to go?
 

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Fellows, if you'll use your noggins and think about this, you'll realize that quench has to do with removing heat from a metal object, such as a blacksmith QUENCHING a horseshoe in water to remove heat. SQUISH, on the other hand has to do with squishing or squeezing something out from between two surfaces. If I step on a pat of butter, it would be squished out from between the floor and the bottom of my shoe. Nobody would describe it as being quenched out from between my shoe and the floor.
 

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Discussion Starter · #10 ·
while it is obvious that squish and quench are two separate phenomimes, They, no doubt are somewhat linked by total deck clearance... This should be easy to visualise, without any argument. Quench, on the other hand could "go either way" IDK, as squish increases or decreases. All things equalised of course.

In my original post, I was more thinking of mechanical interference as the primary concern and squish/quench, as little consideration given NA applications seem to like less as opposed to blown applications.

Thank you Mummert for the insight.
 

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Fellows, if you'll use your noggins and think about this, you'll realize that quench has to do with removing heat from a metal object, such as a blacksmith QUENCHING a horseshoe in water to remove heat. SQUISH, on the other hand has to do with squishing or squeezing something out from between two surfaces. If I step on a pat of butter, it would be squished out from between the floor and the bottom of my shoe. Nobody would describe it as being quenched out from between my shoe and the floor.
"They" at one time found out that increasing the "quench" pad that the engine would tolerate low quality fuel and not detonate as much and "they" thought that it helped "cool" the combustion chamber but what it really was doing is "squishing" the fuel mix more than before, so you can't have squish without the quench pad, lol.
 

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One old builder told me. If you don’t see the valves making marks in carbon on pistons your leaving hp on table. Make quench so pistons almost kissing head. 🤔ive had some close but work to hard for money just not that brave.
 

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I’ve been .033” on a 3.9” bore, and it was fine. I can certainly see that as bore size increases, so does piston rock, and additional clearance would be necessary.
 

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Discussion Starter · #14 ·
I thought as the bore size got bigger, there will be more rock also... and therefore more deck clearance will be required.. It seems noone knows what is needed by the increase in size... Or if they do they are not saying...
 

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I would not tell you what to run, I would tell what I've done and got by with, .029" on a 3-7/8" bore air cooled alum engine, on a 4" or so bore and steel rod check your piston rock (check each corner) and add .030" to it, on a 4.6 bore with alum. rod 8000rpm min. .060", any thing else somewhere in between.
 

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Know matter what methodology you or how tight you have your deck ht. its important to know what it looks like when you are too tight. This engine could use more clearance. This is what many engines look like at .032"-.034" deck ht. range. They come in running, they got away with it, but its not right.
 

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Doesn't the length of the skirt affect piston rock?
 

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The equilibrium of the piston is the most important. A piston with a 1.5" long skirt and a 1.0" compression height will be more stable and have better skirt life than a piston with a 1.5" long skirt and a 1.2 compression height. Top heavy piston will have more rock.
Its a relationship, modern 250 four stroke has a skirt that isn't much more than 1" long and with a .840" compression ht. they're reliable at 12000 + rpm.
 
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