Piston to valve clearence help. - 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 03-05-2013, 01:06 AM
Registered User
 
Last photo:
Join Date: Feb 2011
Location: arizona
Posts: 76
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Piston to valve clearence help.

hey guys,
i checked my piston to valve clearence on my bbc.. it has kb pistons with a .250 dome height.
i have a set of 781 heads and i checked the clearence and it was 58 thous. i was reading you need a min of 80 thous. is there anything i can do besides fly cutting the pistons. and im running a 40 thous gasket already.
thanks for the help as usual.
Brandon

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-05-2013, 02:26 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,206
Wiki Edits: 0

Thanks: 5
Thanked 388 Times in 382 Posts
The absolute minimum needed on the intake valve is .060" net with lash accounted for. (set the running valve lash)
This assumes that you will be using a valve spring that will not allow the valve to float or bounce at high rpm.

You can move the cam a bit to get more intake VTP clearance.

The depth does not tell you about the valve radius clearacne in the piston notch.
If the radius clearance is also tight the valve may kiss the edge of the piston knotch.
Its not that big a deal to take it apart and make the valve relief a little deeper and or make the radius a
bit bigger. (you will only need to do it once as compared to the big mess if the valves do kiss the piston at rpm)
This is a common common issue on most all BBC pistons. ESpecially the factory crate motor pistons. like the 502-502HP motor.

you need more clearance on the exhaust side as the piston chases the exhaust valve.

Whenusing a KB piston with a oval port head or any non factory head you must also verify piston dome to chamber wall edge clearance @ TDC. Sometimes minor chamber wall edge clearance grinding is needed too.
You need to check this and the valve relief radius clearance around the open valve too.

you cannot do any of this using a hyd lifter.

Last edited by F-BIRD'88; 03-05-2013 at 02:31 AM.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-05-2013, 05:12 PM
vinniekq2's Avatar
Registered User
 
Join Date: Jun 2012
Location: BC,Canada
Age: 56
Posts: 8,519
Wiki Edits: 0

Thanks: 351
Thanked 835 Times in 801 Posts
retard the cam
you should properly adjuct clarance. if the cam is wrong ,change it.
if you dont fix this,what Bout the other mistakes?
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-05-2013, 06:20 PM
Registered User
 
Join Date: Nov 2011
Location: Virginia
Posts: 705
Wiki Edits: 0

Thanks: 0
Thanked 75 Times in 75 Posts
BBCs are very tempermental regarding valve train. We've had PROPERLY prepared high revvers touch the pistons with all 8 exhaust valves. I always give them .120" on the exhaust if going over 7,000. It's a good "habit" to keep, in any case. The tiny loss of compression is far offset by the less perilous position of the valve.

Fly-cutting may be inconvenient, but it's worth the effort. A good automotive machine shop has the capability or knows who does. If the pistons are pressed and not "floaters", this complicates matters. A REALLY good machinist with a REALLY good vertical mill can do it with the rods still attached. Shops like ours use a "piston vise", that "keys" on the pin bores, and therefore can't have a rod attached. My mill is fine for the piston vise, but I don't have a good mill vise (not my "department")...

Vinnie takes a little different "route" but is essentially saying the same thing. Fix it, don't gamble. You have too much money tied up in it to allow a "short cut" to ruin it.

Two questions: Did you have "light" springs on the head during the measurements? Did you degree the cam prior to taking the measurements? If the answer to either of these is "No.", start over and do it with these as part of the procedure.

Jim
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-05-2013, 06:28 PM
BOBCRMAN@aol.com's Avatar
Member
 
Last photo:
Join Date: May 2002
Location: Holly, michigan
Posts: 8,208
Wiki Edits: 0

Thanks: 27
Thanked 291 Times in 271 Posts
BBCs are very tempermental regarding valve train. We've had PROPERLY prepared high revvers touch the pistons with all 8 exhaust valves. I always give them .120" on the exhaust if going over 7,000. It's a good "habit" to keep, in any case. The tiny loss of compression is far offset by the less perilous position of the valve.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Listen good!! Many BBC's become history because of this problem. I always used the .100" on the exhaust but not on a BBC, they will at least clean the carbon off the piston pad on the exhaust side..
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-05-2013, 06:49 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,206
Wiki Edits: 0

Thanks: 5
Thanked 388 Times in 382 Posts
This is so true. If there ever was a engine that may get into trouble with valve piston contact at rpm its a BBC.
The valves are big and heavy, making them much more harder to control at rpm.
Even if you have to address the piston issue by mnakeing the reliefs bigger and deeper, by hand, it is well worth the time and effort. Another engine you really got to watch for VTP clearance is the Ford 429-460.

The piston chases the exhaust valve.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 03-05-2013, 10:47 PM
Member
 
Join Date: Apr 2004
Location: Cassville,MO
Posts: 846
Wiki Edits: 0

Thanks: 23
Thanked 49 Times in 45 Posts
Isky used to make something like a fly cutter that you powerd with a drill. You installed it in the head in stead of a valve and set the head on the engine then chuck the drill on the stem. It had stops on it that you set so you didn't cut too much. Check on Isky's site if you're not wanting to go to the machine shop.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-06-2013, 09:53 AM
OLNOLAN's Avatar
Registered User
 
Last photo:
Join Date: Apr 2009
Location: Louisiana
Posts: 1,074
Wiki Edits: 0

Thanks: 429
Thanked 111 Times in 92 Posts
Cut your losses

Quote:
Originally Posted by 1fast62 View Post
hey guys,
i checked my piston to valve clearence on my bbc.. it has kb pistons with a .250 dome height.
i have a set of 781 heads and i checked the clearence and it was 58 thous. i was reading you need a min of 80 thous. is there anything i can do besides fly cutting the pistons. and im running a 40 thous gasket already.
thanks for the help as usual.
Brandon
Brandon, I know this is not what you want to hear, but its the best common sense advise I can give you as a young hotrodder.

While I admire your tenacity at trying to do this yourself, maybe its time to reconsider everything about the build,cut your losses and pay a pro engine builder to assemble it for you. Usually assembly costs are not that high if all the parts are there. I mean that with all the expensive parts you've destroyed in multiple attempts to DIY, get a pro to put it together and if you ask him, he may let you come hang around while he's doing it and teach you a thing or two.

Also I'd like to point out that if you're still trying to use that big butt bertha mutha trucking thumper bumper cam(or whatever they call them) (249/255@.050-.663/683) ? Its wayyy too much for what you're trying to accomplish especially with the 781 heads.

If you just have to have a big dog cam, consider one a bit smaller than the Lunati, perhaps a Comp 292, but get it with a 112 LSA. This way you're not likely to have PTV clearance issues. With the wider lobe separation the piston is farther down in the bore when the valves are at full lift eliminating most PTV clearance issues. The little bit of low end torque you loose with the 112 LSA versus a 110 or 108 will help it hook out of the hole and you'll gain it back in the midrange and top end.

Nolan
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:
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
Piston clearence.. take a look 1fast62 Engine 12 12-15-2011 07:12 PM
piston to valve clearence/quench question babygene41042 Engine 9 07-23-2007 07:24 PM
LT-1 Piston-to-wall clearence lonewolf3197 Engine 2 12-27-2003 08:19 PM
350 Piston clearence riverway Engine 7 09-15-2003 01:04 PM
Piston /wall clearence warlock Engine 7 10-29-2002 07:53 AM


All times are GMT -6. The time now is 09:08 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.