piston to bore clearance

[64buick]

What is the stock piston to bore clearance for the LC2 motor? 86 T-type

 

[Blazer406]

I think I saw the upper limit at .0047

I'm sure someone here can give the exact clearances....

 

[mikestertwo]

top land .046-056
skirt top .0008-.0020
skirt bottom .0013-.0035
This is for a stock motor.

 

[Blazer406]

top land .046-056
skirt top .0008-.0020
skirt bottom .0013-.0035
This is for a stock motor.

top land.... 46 - 56 thousandths? now that is one loose motor.....

 

[mikestertwo]

You read that correctly, .046-.056 clearance between the top land of the piston and the cylinder wall! This is straight out of the GM factory manual. The large clearance is to allow for expansion of the top of the piston where all the heat is concentrated.

 

[bison]

top land.... 46 - 56 thousandths? now that is one loose motor.....

Most turbo engines have clearances like this at the top. Also the rings are moved down quite a bit.

 

[64buick]

Thanks for the info fellas, when I took the heads off the motor and I seen all that space between the piston and the cylinder walls I got a little nervous, going to freshen up this motor had 180,000 + miles on it, oily and greasy as hell this motor leaked almost everywhere, but it ran real good,no smoke or oil burning whatsoever.

 

[turbofrank]

good luck stopping oil leaks:biggrin:

 

[VadersV6]

Its easy to be thrown off by those numbers. Theres no way to get those numbers by any kind of block machining. Those numbers will be the result of the clearance between the bottom of the skirt to the cylinder wall, and the piston dimensions, all over. They also are cut with a cam slope, so theres actually more numbers to it than mentioned above, but I dont know a machinist who bothers with those types of numbers. Just measure for consistency from piston to piston, and measure the bottom and top of the skirt. You flip a piston upside down, and measure about 3/4" down from the end of the skirt (depends on the piston), and then hone the cylinder to spec...the number being that number at the bottom of the skirt and the cylinder bore. A hypereutectic piston should have a piston to wall clearance of about .001-.0015. Nice and tight. The hypers dont require thermal expansion to reach proper clearance, which is great for leakdown and clean emissions. Some forged pistons will expand alot, requiring a looser bore, like .003-.005. But it all depends on the alloy, and the operating temp of the engine. If you machine for forged pistons with that much clearance, you'll have one noisy jacked up engine if you never exceed 160 degrees. The manufacturers numbers are based on keeping their piston free and moving with temps in excess of 230 degrees. So if you run a forged piston that calls for .005", but you're going to run a 160 thermostat, dont go bigger than .003.

 

[64buick]

Wow thanks for that explanation.

 

[Lee Thompson]

?

If you machine for forged pistons with that much clearance, you'll have one noisy jacked up engine if you never exceed 160 degrees.

So I am to believe that cruise @ 800 EGT and 25 PSI @ 1600 EGT will make no difference in piston expansion. IMO it will not matter if you are running a 160 or 200 degree stat. Combustion temp. is going to have a HUGE effect on clearance. It amazes me when people think they know more than the engineers that design the product. I have a few friend like that.

 

[Blazer406]

If you machine for forged pistons with that much clearance, you'll have one noisy jacked up engine if you never exceed 160 degrees.

So I am to believe that cruise @ 800 EGT and 25 PSI @ 1600 EGT will make no difference in piston expansion. IMO it will not matter if you are running a 160 or 200 degree stat. Combustion temp. is going to have a HUGE effect on clearance. It amazes me when people think they know more than the engineers that design the product. I have a few friend like that.

Great point Lee. I was thinking the same thing when the .003 forged piston clearance was suggested for a 160 thermostat.... but I have no real world experience that could back up my intuition.... so I kept quiet.....

 

[Lee Thompson]

me too

I sometimes open my mouth before I kick my brain in gear. Also sometimes comon sense will apply.

 

[Blazer406]

Great point Lee.
... but I have no real world experience that could back up my intuition.... so I kept quiet.....

A wise man one said "It is better to be thought a fool than to open your mouth and remove all doubt"

 

[VadersV6]

If you machine for forged pistons with that much clearance, you'll have one noisy jacked up engine if you never exceed 160 degrees.

So I am to believe that cruise @ 800 EGT and 25 PSI @ 1600 EGT will make no difference in piston expansion. IMO it will not matter if you are running a 160 or 200 degree stat. Combustion temp. is going to have a HUGE effect on clearance. It amazes me when people think they know more than the engineers that design the product. I have a few friend like that.

Maybe its just the hundreds of import/domestic, stock up to 700hp NASCAR or 1500hp big block engines I machined at JMS racing engines (you know, the guys who build all the engines for the biggest hot rod magazines out there) that never came back with a problem, but I do get the sense that i know what Im talking about. You're going to cruise at 800EGT (I think you meant O2 millivolts, right?) and 25psi @1600 EGT's? Thats pretty extreme to cruise at numbers like that. As far as what you're talking about, the clearances I said are correct. The big clearance up top is to allow clearance where the piston is the hottest at WOT...not the .003 at the bottom of the skirt Im talking about. Leave the machining advise to someone who has actually done it for a living, and not pieces together their knowledge from the internet and magazine articles. I have a few friends like you as well. Sounds like my best friends wife, who is a Denny's waitress and tries to teach my wife about opthamology, which is what she's done for the past 10 years and went to school for. All the Denny girls knowledge comes from TV, the internet and magazines. Think the entire thing through or at least ask questions before starting a dick swinging contest.

 

[Lee Thompson]

.

Ouch!!!!!!

 

[lazaris]

I ran the JE's at .004 piston to wall. Upon dis assembly the skirts showed signs of scuffing. Went to .005 and the scuffing is gone. I do believe EGT's play a part in the clearance you use as we do run high EGT's in turbo cars. A bit noiser yes. But JE also said they like more clearance an do run faster that way.

 

[Lee Thompson]

I know of

A few people with VERY fast cars that run .001 more clearance in # 1 and # 2 bores because of the cooling water hitting those bores first.

 

[bison]

I ran the JE's at .004 piston to wall. Upon dis assembly the skirts showed signs of scuffing. Went to .005 and the scuffing is gone. I do believe EGT's play a part in the clearance you use as we do run high EGT's in turbo cars. A bit noiser yes. But JE also said they like more clearance an do run faster that way.

Im suprised they scuffed on a 3.820 or 30 bore at .004. I have use that clearance for 8 years with no problems and i ran the thing on the edge for EGT's. Actually running an 8 year old set of JE's in an engine right now. 3rd engine theyve been in. I have also run the stock pistons around .004 on an engine after a clean up hone on a virgin block. They were noisy when cold and had more blowby but sealed up fine under boost.

 

[951 Blackplanet]

another piston to bore question.

im thinking of getting a set of trw 30 over used pistons.the block is standard now,what should the finished bore size roughly be to acheive the desired clearance?and what clearance is the best comprimise?.003,.004,or .005.not going to build a ten second car just a daily driver.and also what are the best rings for this type of application and part#?thanks.....