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Cylinder diameter VS. piston diameter

54K views 32 replies 16 participants last post by  327NUT 
#1 ·
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?
 
#2 ·
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.
 
#3 ·
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.


XeroVolume said:
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?
 
#4 ·
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?
 
#5 ·
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.
 
#6 ·
XeroVolume said:
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.
 
#7 ·
Intense RT said:
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."
 
#8 ·
tyler71385 said:
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. :thumbup:

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.
 
#9 ·
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?!
 
#10 · (Edited)
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.
 
#11 ·
Blazin72 said:
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.
 
#12 ·
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...
 
#13 · (Edited)
sbchevfreak said:
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.
 
#14 ·
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...
 
#15 ·
bondo said:
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?
 
#16 ·
XeroVolume said:
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?
If you check the casting number at chevy engine block identification it could be a 327 because a 327 also has a 4.00 bore like a 350 the 327 just a shorter stroke
 
#17 ·
Everything I have found indicates the modern LS Series have .0007 to .0018 piston to cylinder clearance (meaning the piston is .0007 to .0018 "smaller" just to be clear). I have done interference fit assembly before and I am currently a Machine Tool Work Inspector. A 4.00001" outside diameter object will not fit in a 4.00000" bore without force or changing the diameter of one or the other through thermal contraction/expansion.

I understand thermal expansion of the cylinder increasing the clearance BUT how do you get the engine cranked over and brought up to operating temps for the cylinder to expand at initial start up? One cylinder with the piston .0014" larger than the bore will be pretty tough to get moving when it's cold to say the least but think how tough that would be multiplied by 8.

I'm not saying you're wrong, I'm not from Missouri and I'm not trying to be a butthead but I need a LOT more convincing on that one. If it's possible I'd be very interested in understanding how.
 
#19 ·
XeroVolume said:
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.
Probably be a good idea to let the machine shop determine what oversize piston you should order. If you stay with the 3.48 stroke I'd want to make sure you order at least a 1.560" up to a 1.563 Pin height. Not unusual for a machine shop to order a 1.540" piston putting you deep in the hole, lowering your compression, and putting your quench way above .040"..

It's possible that it is an old Jasper or other named rebuilt engine. I've seen several of those over the years with bores all over the place. They usually have a tag down on the side of the block or two rivet holes where it was.
I've also seen several engines which have been ridged and re-rung with cast rings three or even possibly 4 times with stock pistons. If that's the case you might see a heavy knurling pad on the piston skirts. Seen the Jasper engines with heavy piston knurl also. They often have random and different numbers on top of the pistons, like 8, 11 and maybe a couple of 14s, a 10 whatever.
We miced out my brother's 283 a while back trying to decipher the puzzle and they were all different in size but didn't seem to mic exactly what they read. Cylinder taper was way out there, like it has never been bored but worn that far from being re rung several times. I still don't know. The block I'm thinking about is still in the shop for several years now. I have to build it some time because it belonged to my brother. I'm thinking mine will clean up at .030" but I'll get my machinist to do the primary bore to determine and if good then hone to fit.. I'll watch to see how you come out on yours. Good luck.
 
#20 ·
I'm not saying you're wrong, I'm not from Missouri and I'm not trying to be a butthead but I need a LOT more convincing on that one. If it's possible I'd be very interested in understanding how.[/QUOTE]

I concur, how would you even punch in the slugs?

whats with Missouri hippie? just ask'n
 
#21 ·
If you understand that pistons are barrel shaped(much smaller at the top than at the bottom of the skirt), and know just how and where the piston is touching the bore when cold, you can see how that fact and oil retaining skirt coating and piston skirt flex will allow this slight interference fit. Much(or all) of the interference fit is in the thickness of the coating, which is sacrificial. After the engine has run for a few minutes the coating is wore away and burnished to a perfect fit for the rest of the engine life.
 
#24 ·
ericnova72 said:
If you understand that pistons are barrel shaped(much smaller at the top than at the bottom of the skirt), and know just how and where the piston is touching the bore when cold, you can see how that fact and oil retaining skirt coating and piston skirt flex will allow this slight interference fit. Much(or all) of the interference fit is in the thickness of the coating, which is sacrificial. After the engine has run for a few minutes the coating is wore away and burnished to a perfect fit for the rest of the engine life.
So the interference fit is actually only in a small contact area and is only present at initial assembly........ Right?
 
#25 ·
Hippie said:
Missouri is known down here as the "Show Me" state as the residents are hard to convince. It dates back to the early 1800's IIRC. If someone tells us something we question and we want proof we say "I'm from Missouri, show me." :D
Thanks thats a good one,

"rule of thumb"; 16 century expression, you could whip your wife but the stick could be no bigger round than your thumb... and here the piston can be no bigger round than the bore.
 
#26 ·
Hippie said:
So the interference fit is actually only in a small contact area and is only present at initial assembly........ Right?
Yes, that is correct. The spec they are giving is for new pistons into new bores, not for checking sizes and fits of used assemblies.

The very bottoms of the skirts are small enough to fit into the bore(pistons barrel shaped, remember), it is the coating the makes most of the interference.
 
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