Hot Rod Forum : Hotrodders Bulletin Board - Reply to Topic
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine> Cylinder diameter VS. piston diameter
User Name
Password
lost password?   |   register now

Thread: Cylinder diameter VS. piston diameter Reply to Thread
Title:
  
Message:
Trackback:
Send Trackbacks to (Separate multiple URLs with spaces) :

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Additional Options
Miscellaneous Options

Topic Review (Newest First)
09-05-2010 09:31 PM
327NUT Just so you guys know, the Op asked the question in regards to his bore vs. piston size on 1-28-10. I would think he had it figured out by now....maybe not.
09-05-2010 05:18 PM
Custom10 OK well good info, and really somehow these tight/interference fit motors work so thanks

Now with respect the OP

Quote:
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?
Id say he could go with a 30 over slug and hone to fit? or should he get 40 over and really ruff it up?
09-05-2010 04:59 PM
ericnova72
Quote:
Originally Posted by 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,



So the middle is big and each end top/bottom is smaller?
Coating is thinner than that, but yes. you are correct.

Just some #'s for example off the top of my head, not in particular to the LS question, just general info:

Very bottom of skirt might only be .0005" smaller than the bore, but the top of the piston might be .010"-.015" smaller(some forgings are .035" smaller at the top) than the bore. A lot more thermal expansion at the top, this is why they are barrel shaped. Fattest part of the barrel shape is usually only about 1/2-1" above the bottom of the skirt.
09-05-2010 04:46 PM
Duntov
Some threads are more fun to watch than to play in!

Man I'm lost on a lazy Sunday, good thread.
09-05-2010 04:43 PM
Custom10 yeah me neither, i'll let it go, just caught my attention and you know what they say about inquiring minds
09-05-2010 04:35 PM
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.
09-05-2010 04:16 PM
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,

Quote:
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?
09-05-2010 04:08 PM
ericnova72
Quote:
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.
09-05-2010 04:01 PM
Custom10
Quote:
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.
09-05-2010 03:59 PM
Hippie
Quote:
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?
09-05-2010 03:56 PM
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
09-05-2010 03:56 PM
Hippie
Quote:
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."
09-05-2010 03:47 PM
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.
09-05-2010 03:38 PM
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
09-05-2010 03:26 PM
Duntov
Quote:
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.
This thread has more than 15 replies. Click here to review the whole thread.

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


All times are GMT -6. The time now is 12:30 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.