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Old 01-28-2010, 11:00 AM
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Cylinder diameter VS. piston diameter

I have a few questions about an engine I just picked up. The guy I got it from told me it was a 327, but the casting number points to a 350. So I measured the bore and the stroke and determined that it was in fact, a 350. Using a micrometer I found that the cylinders are within 4.020 to 4.030 inches and the stroke is almost exactly 3.48 inches. So my question is this: I know that a stock 350 has a bore of 4 inches right? So why am I measuring 4.020 - 4.030? Is this engine bored over? Or is the piston what's supposed to be 4 inches? If I were to replace the pistons, what "bore" would I purchase, and would that "bore" be referring to; the actual piston diameter, or the actual cylinder diameter?

To be more specific I suppose, what is the stock 350 ACTUAL measurement of the diameter of the cylinder as opposed to the piston, and what tolerance should I expect in between the two?

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Old 01-28-2010, 11:16 AM
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4" but it has been known for the factory to occasionally put in an oversize in piston in one hole...or more than one.

If the cylinder is in good shape, you may be able to go .040 over or even .060 over if it's not so good. Piston to cyl wall clearance is designed into the piston.
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Old 01-28-2010, 11:22 AM
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the engine is possibly a .030 over or it's been ridge reamed and honed to hell and back. It needs to be checked with a dial bore gauge to make sure it's same top to bottom.


Quote:
Originally Posted by XeroVolume
I have a few questions about an engine I just picked up. The guy I got it from told me it was a 327, but the casting number points to a 350. So I measured the bore and the stroke and determined that it was in fact, a 350. Using a micrometer I found that the cylinders are within 4.020 to 4.030 inches and the stroke is almost exactly 3.48 inches. So my question is this: I know that a stock 350 has a bore of 4 inches right? So why am I measuring 4.020 - 4.030? Is this engine bored over? Or is the piston what's supposed to be 4 inches? If I were to replace the pistons, what "bore" would I purchase, and would that "bore" be referring to; the actual piston diameter, or the actual cylinder diameter?

To be more specific I suppose, what is the stock 350 ACTUAL measurement of the diameter of the cylinder as opposed to the piston, and what tolerance should I expect in between the two?
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Old 01-28-2010, 03:39 PM
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So just to clarify, the ACTUAL stock cylinder of a 350 will measure 4.000 inches and therefore a piston that says it's bore is 4.000 will actually measure slightly smaller in order to fit inside the cylinder?

If I were to purchase new pistons for example from summitracing.com, and the piston specs say the bore is 4.000, will they fit inside a cylinder that is EXACTLY 4.000? They would have to be slightly smaller right? Sorry I'm just a little confused.

If I have my engine bored to .060 over, will I buy pistons that say .060 over or will they need to be smaller?
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Old 01-28-2010, 04:01 PM
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What you have is most likely a 30 over 350. The pistons are generally a few thousandths smaller than the bore. The rings maintain the seal between the cylinder and piston. Most oversized pistons are stamped with the size.
I would take the block to a machine shop and have them check it. .010 difference between cylinders seems like way to much of a difference.
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Old 01-28-2010, 05:04 PM
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Quote:
Originally Posted by XeroVolume
So just to clarify, the ACTUAL stock cylinder of a 350 will measure 4.000 inches and therefore a piston that says it's bore is 4.000 will actually measure slightly smaller in order to fit inside the cylinder?

If I were to purchase new pistons for example from summitracing.com, and the piston specs say the bore is 4.000, will they fit inside a cylinder that is EXACTLY 4.000? They would have to be slightly smaller right? Sorry I'm just a little confused.

If I have my engine bored to .060 over, will I buy pistons that say .060 over or will they need to be smaller?
To your first paragraph, yes. Second, yes also.

BTW, engines nowadays have come a long way. New LS engines actually have roughly a couple tenths interference fit when new and have skirt coatings. Hyper pistons expand much less and when at operating temp the cyl walls expand more than the piston. Tighter fit, less blowby, less emissions.
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Old 01-28-2010, 06:00 PM
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Quote:
Originally Posted by Intense RT
To your first paragraph, yes. Second, yes also.

BTW, engines nowadays have come a long way. New LS engines actually have roughly a couple tenths interference fit when new and have skirt coatings. Hyper pistons expand much less and when at operating temp the cyl walls expand more than the piston. Tighter fit, less blowby, less emissions.

Good to know. Guess the old machine shop mantra is out the window "Loose runs...tight don't."
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Old 01-28-2010, 06:27 PM
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Quote:
Originally Posted by tyler71385
Good to know. Guess the old machine shop mantra is out the window "Loose runs...tight don't."
Without sacrificing durability, the cup cars run as tight as they can. I have no numbers though.

I say if you're gonna miss it...miss it loose.

Miss it loose and You know, miss it tight and EVERYONE knows.

Those LS engines make power and hold it. They must have done their homework after all this time. As far as the piston to wall, as far as I know, it's the recent LS series' engines that are running that tight. Makes sense how it works though.
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Old 01-28-2010, 09:42 PM
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Thanks for the information, it really helps. I definitely plan on taking it to the shop before rebuilding and I will probably have it bored to .060 over. This will be my first full engine rebuild and I am picking up on the details as I go so I appreciate everyone's help! You have to start somewhere right?!
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Old 01-28-2010, 10:04 PM
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I highly doubt the Gen III/IV (LS) engines have an INTERFERENCE fit between the pistons and cylinders. The engine wouldn't run, especially when you account for thermal expansion.

The sleeves may be interference fit between with the block on aluminum engines but I doubt the pistons are interference fit with the sleeves/cylinder bores.

Last edited by Blazin72; 01-29-2010 at 01:26 AM.
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Old 01-29-2010, 10:20 AM
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Quote:
Originally Posted by Blazin72
I highly doubt the Gen III/IV (LS) engines have an INTERFERENCE fit between the pistons and cylinders. The engine wouldn't run, especially when you account for thermal expansion.

The sleeves may be interference fit between with the block on aluminum engines but I doubt the pistons are interference fit with the sleeves/cylinder bores.
This information came from my instructor Judson Massingill. He owns and instructs at the School of Automotive Machinists.

Don't know if you've ever press fit anything but a few tenths of interference fit is not much. Then you add the skirt coating that tends to retain a film of oil to itself and that would help with movement. With the operating temps, and again, again...hypereutectic pistons which do not expand more that a few tenths, the cyl will increase in diameter more than that by the time it reaches over 200* coolant temp. There is your clearance...the difference in expansion rate.
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Old 01-29-2010, 01:01 PM
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Believ it or not, they do run a slight interferance. From the engine mechanical specs for the 6.2L LS3:


Piston - Piston to Bore Clearance - Production
-0.036 to +0.016 mm
-0.0014 to +0.0006 in


They fit them from a rather tight interferance to a slight clearance. Different...
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Old 01-29-2010, 01:47 PM
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Quote:
Originally Posted by sbchevfreak
Believ it or not, they do run a slight interferance. From the engine mechanical specs for the 6.2L LS3:


Piston - Piston to Bore Clearance - Production
-0.036 to +0.016 mm
-0.0014 to +0.0006 in


They fit them from a rather tight interferance to a slight clearance. Different...
Thanks for the tech specs. Never got around to checking into it. I was also vague on purpose, lol, if I said a full thou or more (and I didn't know exactly) someone would have argued. I was just generally informed with no exacts given.

Just to add, people have to keep in mind the cyl diameter itself expands also. In a marine app running ~70* coolant you need more piston to bore clearance(~.005 with 2618 pistons) due to the cool temp coolant keeping the cyl diameter from increasing.

Last edited by Intense RT; 01-29-2010 at 01:54 PM.
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Old 01-30-2010, 08:32 AM
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Quote:
In a marine app running ~70* coolant you need more piston to bore clearance(~.005 with 2618 pistons) due to the cool temp coolant keeping the cyl diameter from increasing.
Ayuh,... Not to argue, but only to clarify,...

In marine applications, the T-Stat is either 140 for Saltwater raw water cooled systems,...
160 for freshwater raw water cooled systems,...
Or,...
180 for closed cooling antifreeze filled systems...

Even in late season 35 lake water, my motor runs at a steady 160...
Same as in the heat of summer in 75 water...
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Old 01-30-2010, 10:42 AM
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Quote:
Originally Posted by bondo
Ayuh,... Not to argue, but only to clarify,...

In marine applications, the T-Stat is either 140 for Saltwater raw water cooled systems,...
160 for freshwater raw water cooled systems,...
Or,...
180 for closed cooling antifreeze filled systems...

Even in late season 35 lake water, my motor runs at a steady 160...
Same as in the heat of summer in 75 water...
Power boat?
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