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warlock 10-28-2002 03:39 PM

Piston /wall clearence
I am building a 383 stroker with a completely forged bottom end. I am using JE flattop pistons, Oliver rods and crank and am planing on using 250,300hp shots of NOS.The compression ratio should be around 11.0 to 1. My piston to wall clearence is about 20 thousants with a ring gap of about 18. Is this to much clearence??. The machinest that bored the block swears it needs the clearence for the pistons to expand. does this sound right to you guys?? I am new to this and dont want to put together my EXPENSIVE toy only to have it puffing smoke time i fire it off. Any advise would be Helpful. Thanks in advance. 10-28-2002 04:08 PM

are you sure its twenty thousants, that is wwwwway to much clearance. try .002 or two thousants. which is not enough for forged pistons. the actual piston to wall clearance depends on the design of the piston, the material its made of, cast or forged or hypereutectic cast. the bore size and what its going to be used for. consult the pistons manufacturer for correct wall clearance.. i'm always seeing keith black pistons with the top ring lands pulled off and the owner cussing the piston manufacturer as bad pistons, but they didn't read the basic instructions included in the set that says hyper pistons have to have different clearances and way more ring end gap, the top ring butts from the heat transfer and away goes the top of the piston. most forgings in a 4" bore are .0035 + due to the way the piston expands. way back in the old days we ran them at .008+ and thought nothing of it but materials are different now and ring tech is better.

warlock 10-28-2002 05:29 PM

I can fit a .020 feeler in between the lower piston skirt and the cylinder wall and its a loose .020! I think the machinest messed up the bore and is just trying to pass it off to me. Anyway, i will contact JE pistons by phone to confirm this. Thanks for the help.

[ October 28, 2002: Message edited by: warlock ]</p>

TurboS10 10-28-2002 06:18 PM

I am currently have a set of JE pistons that are at the machine shop with the block for boring. I found that the top of the piston in the ring land area is much smaller than the skirt(what little there is). I measured, but cant remember the exact number. I believe there was at least .020 difference from top to bottom. I would check the clearance at the skirt. If it is .020 there you are screwed, but I am betting it will be around .003 if your machinist did his job. You will hear that forged pistons need .005-.01, but new alloys used in pistons today are much different than those of yesteryear. Todays forged piston dont require the clearance that old ones did. My sbc 400 TRW forged pistons were fit at .003 wall clearance and there is no rattle when it is cold. Hope this helps.


Quepas00 10-28-2002 06:34 PM

.0065 would be tight .0085 will be loose.

Go to this site and check out dyno results on the 350 with 420 HP. It has all the specs they used on this motor.

engineczar 10-29-2002 03:27 AM

Most Pistons (possibly all)come through with the clearance built into them for the bore size ordered. If you order a +.030 piston for a 350 Chevy than the bore size should be exactly 4.030 and the piston will be smaller by whatever the piston maker determines is the correct clearance. JE pistons come with a sheet that shows where the piston is measured just so the machinist can confirm the size. Take a set of verniers, dial calipers, or dial bore gage and measure the bore.

engineguy 10-29-2002 06:15 AM

Dittos to all the messages above. The ring land area will generally be somewhat smaller diameter than the skirt area. Also, the piston manufacturers instructions as to WHERE to measure the skirt must be followed exactly. .020" is way too much, if this is what it turns out to be. Many aftermarket replacement pistons do have the clearance built in, but I am not sure about JE. Anyway, the machinist should have measured each piston individually and bored each cylinder accordingly. Used to be that you would find several thousanths difference between the diameters in a set of forged pistons, however newer pistons and good performance brands such as JE are usually right on the money.

slider in wa 10-29-2002 06:53 AM

It sounds like you have put some money and thought into this motor. The guys are correct on checking the clearance. Your .020 is to big a gap. In my opinion,most clearances are checked at or near the center of the cylinder.See what the manufactures of the pistons say about where to check clearance.You are right to question this. Track it down, and be sure.
Good luck, and let us know how it turns out. will ya?
Slider in Wa.

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