[alittleunwell31]

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?

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

[Hippie]

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.

[techinspector1]

Times 2 Hippie...

[Duntov]

Originally Posted by XeroVolume

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.

[Custom10]

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

[ericnova72]

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.

[Hippie]

Originally Posted by Custom10

whats with Missouri hippie? just ask'n

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."

[Custom10]

Thats interesting and not doubting your knowledge/explination, but how do you actaully install the piston if its bigger than the bore squish it? or does it go in from the bottom? I am still think'n its bigger than the bore here,, school me/us

[Hippie]

Originally Posted by ericnova72

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?

[Custom10]

Originally Posted by Hippie

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."

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.

[ericnova72]

Originally Posted by Hippie

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.

[Custom10]

OK so the coating must be 0.005 or so all the way round and it does actually squish down under some pressure, perfect fit, hmm LS1 magic,

If you understand that pistons are barrel shaped(much smaller at the top than at the bottom of the skirt),

So the middle is big and each end top/bottom is smaller?

[cobalt327]

I believe this is a moot point- no aftermarket piston manufacturer that I'm aware of is going to tell you to set the LS engine up w/an interference fit piston, as in the piston being larger than the bore.

IF there is a factory spec that allows for the piston to be larger than the bore- something that I personally highly doubt- it has got to be purely a factory deal, not anything that would ever be seen in practice outside of the OM.

IMO, it makes zero sense to build an engine that tight. No amount of explanation will ever get me to say, yeah- that sounds like a good thing to do. But then I'm kind of set in my ways. And I'm not even from the "Show Me" state.

[Custom10]

yeah me neither, i'll let it go, just caught my attention and you know what they say about inquiring minds

[Duntov]

Man I'm lost on a lazy Sunday, good thread.