Piston to bore clearance in .010 rebuild - Hot Rod Forum : Hotrodders Bulletin Board
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
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 03-07-2009, 12:05 AM
Registered User
 
Join Date: May 2008
Location: alberta
Posts: 94
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Piston to bore clearance in .010 rebuild

I bought a set of custom JE pistons for my 417sbc build used real cheap.

They are for a 4.135 (.010 over) bore with 1.052 compression height.

Anyway, I ripped the 400 down today and put a piston ring at the top of the bore. Using the piston ring as a "machinests square" to measure the width of the ring plus the inside diameter of it I figure my virgin bore 400SBC to be 4.140-4.142 between cylinders.

It ran fine with stock 4.125 pistons just not great, now im after a performance build and want to clean up the bore. If I bore/hone it to 4.142 will the .007 piston to bore clearance cause any issues?

Using a basic forumula when the piston warms up to 300f/148c (wild guess) the piston will expand .008 inches. What kinda answers am I looking for here other than the obvious buy a set of 20 thou pistons. Thanks!

    Advertisement

Last edited by Nsomnia; 03-07-2009 at 12:11 AM.
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-07-2009, 12:39 AM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 9,198
Wiki Edits: 0

Thanks: 26
Thanked 417 Times in 382 Posts
You're #'s don't add up. 4.135" bore piston will be something like actual diameter of 4.128" so that they will fit in a 4.135" bore at .008" clearance. You are probably looking at .012" clearance now and probably .015"+ after just honing if you are lucky and there is no taper or out-of-round condition to the bores. If there is no taper or roundness issues then congrats, you've managed to find a needle in the haystack-- a 400 that doesn't need boring, but you are still too big for the pistons you have. If the 4.142" # is actual, you will be lucky to clean it up at +.030" overbore (4.155"). You have to have enough material there to cut solidly with the boring bar.

Sounds as if your understanding of how piston and bore sizing is arrived at is a little lacking. Pistons are smaller than the bore size they are meant to be installed in by the amount of running clearance specified by the manufacturer. Example: A piston for a 4.155" bore with a clearance spec of .005" will only be 4.150" in diameter.

Only place those +.010" pistons can normally be used is in an aftermarket block. Probably not what you wanted to hear.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-07-2009, 12:21 PM
406 bug's Avatar
9.82 @ 139mph
 
Last wiki edit: Finding vacuum leaks Last photo:
Join Date: Nov 2007
Location: phoenix, arizona
Posts: 582
Wiki Edits: 1

Thanks: 0
Thanked 1 Time in 1 Post
.007 - thats toooo much piston to wall. You need to contact JE and see what they recommend for that piston and your application. I would guess you need more like .0035. Also make sure you get info on how and where to measure clearance.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-07-2009, 01:09 PM
Registered User
 
Join Date: May 2008
Location: alberta
Posts: 94
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Well the skirts measure 4.135 and the crown measures 4.100 they are actually out of a ford nascar engine or soemthing? ebay steal. They have a gapless second ring.

I also read 4 Jaw chuck on a recent topic regarding piston to bore post "Generally 0.001" per inch of diameter is a good rule of thumb, for forged pistons it is more like 0.0015" to 0.002" per inch of dia to accomodate the denser material expansion. More than this and the rings will wear faster due to the piston rocking in the bores. I have successfully run 0.007" clearance in 4" bores with cast pistons with a light knurling on the piston skirt to prevent piston rock. Is this recommended? Not really, but it works.

If you want to go ahead and run it the way it is, she will still run OK but the rings will leak down more in that cylinder and won't last as long as the others. How much I can hear you asking? lets just say the pistons/rings will out live the valves. Personally I would just stick her back together and don't worry about it, too much clearance is good insurance against break-in seizure and most shops put in a little more just to be on the safe side. I always do on my own engines."



I suppose I'll see what the machine shop is, and throw the pistons on ebay with buy it now to make my money back if someone wants them, if not I may be taking my risks.

I will check for out of roundness today, I never thought about that, thanks!
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-07-2009, 01:22 PM
DaSouthWon's Avatar
I put up the tools against$300
 
Last photo:
Join Date: Apr 2006
Location: Memphis,Tennessee
Age: 35
Posts: 683
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
You may already know this but be aware that the skirts are not a perfect cylinder shape, they are barrel shaped. So the middle of the skirt is going to have a larger circumference than the bottom of the skirt. Forged pistons will also require a greater tolerance because they expand more than a cast piston.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-07-2009, 10:26 PM
racecar100's Avatar
Registered User
 
Join Date: Feb 2008
Location: Tennessee
Age: 53
Posts: 255
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by 406 bug
.007 - thats toooo much piston to wall. You need to contact JE and see what they recommend for that piston and your application. I would guess you need more like .0035. Also make sure you get info on how and where to measure clearance.
That is the standard clearance for JE Pistons at .007.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 03-08-2009, 02:06 PM
406 bug's Avatar
9.82 @ 139mph
 
Last wiki edit: Finding vacuum leaks Last photo:
Join Date: Nov 2007
Location: phoenix, arizona
Posts: 582
Wiki Edits: 1

Thanks: 0
Thanked 1 Time in 1 Post
Quote:
Originally Posted by racecar100
That is the standard clearance for JE Pistons at .007.

You are misinformed. There is not a standard. It depends on several factors like use/application and type of alloy used. For example, 2618 alum. can handle a rather tight clearance of .0035 - 0045.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-08-2009, 02:59 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,804
Wiki Edits: 326

Thanks: 747
Thanked 960 Times in 809 Posts
This is from JE's site.

Basic clearances....
BORE RANGE ...............2618 CLEARANCE......4032 CLEARANCE

SB applications 4.000"-4.200" .0035"-.0045" .0020"-.0025"

Additional clearances to be added to the above.....
Drag Race + .0005"~.0010"
Forced Induction/Nitrous + .0010"~.0015"
Circle Track/Road Race + .0005"~.0010"
Marine Applications + .0010"~.0020"
Filled Block + .0010"~.0020"

So it looks to me like the max clearance advised (with 2618 alloy) would be in a marine or filled block application with a figure of 0.0065". Max clearance 400 small block in an automotive application with unfilled block (2618 alloy) would be 0.0045".

The other thing I wondered about is the stack height. Assuming you're using 6" rods and a 3.832" crank, the stack is 8.968". That means a pretty good whack off the decks to get to a good squish unless you're able to find some really thin head gaskets. Whacking the deck a bunch will mean paying very careful attention to the manifold/head interface match-up to make sure you have a seal there.

Last edited by techinspector1; 03-08-2009 at 03:12 PM.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 03-08-2009, 04:15 PM
406 bug's Avatar
9.82 @ 139mph
 
Last wiki edit: Finding vacuum leaks Last photo:
Join Date: Nov 2007
Location: phoenix, arizona
Posts: 582
Wiki Edits: 1

Thanks: 0
Thanked 1 Time in 1 Post
Quote:
Originally Posted by techinspector1
This is from JE's site.

Basic clearances....
BORE RANGE ...............2618 CLEARANCE......4032 CLEARANCE

SB applications 4.000"-4.200" .0035"-.0045" .0020"-.0025"

Additional clearances to be added to the above.....
Drag Race + .0005"~.0010"
Forced Induction/Nitrous + .0010"~.0015"
Circle Track/Road Race + .0005"~.0010"
Marine Applications + .0010"~.0020"
Filled Block + .0010"~.0020"

So it looks to me like the max clearance advised (with 2618 alloy) would be in a marine or filled block application with a figure of 0.0065". Max clearance 400 small block in an automotive application with unfilled block (2618 alloy) would be 0.0045".

The other thing I wondered about is the stack height. Assuming you're using 6" rods and a 3.832" crank, the stack is 8.968". That means a pretty good whack off the decks to get to a good squish unless you're able to find some really thin head gaskets. Whacking the deck a bunch will mean paying very careful attention to the manifold/head interface match-up to make sure you have a seal there.

Good detail/ info Tech.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 03-08-2009, 08:13 PM
Double_v23's Avatar
Horsepower Enthusiast
 

Last journal entry: Finished BMOD001FH
Last photo:
Join Date: Jul 2007
Location: Mooresville, NC
Posts: 1,495
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Remove the gapless rings and place them in the nearest trash can. In order for the top and second rings to seal properly requires that some gases pass by the rings and create low and high pressure areas in the right spots. Gapless rings don't let enough of the gases to pass through for the rings to seal. Yeah they only leakdown 1% or whatever, but that means nothing when the motor is running.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 03-09-2009, 06:06 PM
Registered User
 
Join Date: May 2008
Location: alberta
Posts: 94
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by Double_v23
Remove the gapless rings and place them in the nearest trash can. In order for the top and second rings to seal properly requires that some gases pass by the rings and create low and high pressure areas in the right spots. Gapless rings don't let enough of the gases to pass through for the rings to seal. Yeah they only leakdown 1% or whatever, but that means nothing when the motor is running.
Really gapless rings are that bad ive done very little research on them, they are just what came on the pistons. I dont think i wanna reuse them but im not sure yet.

I measured every cylinder this morning. Best was 4.136 and worst was 4.142.

They average .00035 out of roundness so less than half a thou on all of them.

I think I may give it a try if the machine shop will bore them all to that size. Worst case it runs poorly and I have to rip it down again, only thing lost is my time and cost to bore twice.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

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.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

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
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
ford 4.6 2v piston to valve clearance sheldonw Engine 1 07-30-2008 01:39 PM
QJet Power Piston Won't Come Out of Bore JohnTN Engine 1 06-01-2006 05:38 PM
350 Piston Bore Silver_Camaro Engine 9 03-21-2004 09:00 PM
how to prepare a block blndweasel Engine 14 08-15-2003 12:28 PM
piston clearance stangfiend Engine 3 12-06-2002 07:30 PM


All times are GMT -6. The time now is 11:18 AM.


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.