Valve clearence question
Thread Starter
Instructor
Joined: Dec 2007
Posts: 245
Likes: 2
From: Jersey shore NJ
Valve clearence question
This Might be a stupid question, but I am looking to put new heads and cam on my 76 L82. I want to go with the 64cc heads to boost compression. My question is how much lift can you put on the valves before you would have a clearence issue with the flat top pistons?
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
No matter how many people answer and no matter how many numbers are given to you, it will have to be up to you making the necessary measurements. The best way to make those measurements is with a dial gauge, a depth gauge, a micrometer or precision vernial caliper, and a metal straight edge.
1) Measure the thickness of the straight edge.
2) With the heads off place the straight edge along the top of the deck across a piston.
3) Rig the dial gauges so that it reads piston movement.
4) Bring the piston up to top-dead-center.
5) Measure the distance from top of the straight edge to the top of the piston where the bottom most part of the valve will hit. Then subtract the thickness of the straight edge. You will add the gasket thickness to this number.
6) place the flat edge on the head...place the edge in exactly the same location as on the block.
7) Measure the distance from the straight edge to the bottom most part of the valve.
The difference is at least how much valve lift you can have...to be more precise you need to degree the cam and measure lift vs cam angle and compare to piston position vs cam angle.
1) Measure the thickness of the straight edge.
2) With the heads off place the straight edge along the top of the deck across a piston.
3) Rig the dial gauges so that it reads piston movement.
4) Bring the piston up to top-dead-center.
5) Measure the distance from top of the straight edge to the top of the piston where the bottom most part of the valve will hit. Then subtract the thickness of the straight edge. You will add the gasket thickness to this number.
6) place the flat edge on the head...place the edge in exactly the same location as on the block.
7) Measure the distance from the straight edge to the bottom most part of the valve.
The difference is at least how much valve lift you can have...to be more precise you need to degree the cam and measure lift vs cam angle and compare to piston position vs cam angle.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
IMHO, the only correct way to measure this is doing a mock-up with clay or using a dial indicator on valves with set-up springs.
Generally on an SBC you can get up into around .550 before this is an issue with flat-tops depending on the duration and cam advance. .490 or so has *always* been safe for me. This assumes an .025 deck and around .040 quench.
But you *must* check!
Generally on an SBC you can get up into around .550 before this is an issue with flat-tops depending on the duration and cam advance. .490 or so has *always* been safe for me. This assumes an .025 deck and around .040 quench.
But you *must* check!
Race Director
Joined: Apr 2000
Posts: 17,672
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
The way I do it is put the head on with two valves with checking springs. Put a dial indicator on one valve retainer and slowly rotate the engine stopping at 2 or 3 degree intervals and press down on the valve.
Soon you will feel it hit the piston, keep turning slowly until you get the lowest dial indicator reading. Now add your compressed gasket to that and subtract .100" for piston to valve clearance ( .120" for high RPM motor ) and that is your max lift.
Keep in mind for a mild street 350 the piston to valve clearance should be checked but most street cams there won't be any clearance problems
Soon you will feel it hit the piston, keep turning slowly until you get the lowest dial indicator reading. Now add your compressed gasket to that and subtract .100" for piston to valve clearance ( .120" for high RPM motor ) and that is your max lift.
Keep in mind for a mild street 350 the piston to valve clearance should be checked but most street cams there won't be any clearance problems
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
I used to agree - but with the introduction of the roller-cam flat-tappet profiles and retrofit rollers, there are a fair number of what I'd consider reasonably mild street engines are running .500 or more lift...
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
The important clearance is on overlap when both valves are off there seat and the piston is TDC. The piston is chasing the exhaust valve closed. Correct stack height on the springs and important guide to retainer clearance at full open is just as important. 600+ lift cams are not unusual for street use with a properly prepared set of heads matched to the pistons.
A minor note is the best power is obtained with the cam straight up, not ahead or retarded for the street. Many ways to measure this as you can see on other replys depending on what their use to. Failure to make these important checks could kill a cam in nothing flat. And then either the 'cam' manufacturer or the 'oil' gets blamed ....
A minor note is the best power is obtained with the cam straight up, not ahead or retarded for the street. Many ways to measure this as you can see on other replys depending on what their use to. Failure to make these important checks could kill a cam in nothing flat. And then either the 'cam' manufacturer or the 'oil' gets blamed ....
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
Depending on the configuration it may have a slight impact on power...but cam advance is more about where power is made and not how much.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
While I generally recommend that cams are installed "straight up" (per mfgr) the purpose of advancing or retarding the cam is to move the power peaks up or down in the RPM range.
Depending on the configuration it may have a slight impact on power...but cam advance is more about where power is made and not how much.
Depending on the configuration it may have a slight impact on power...but cam advance is more about where power is made and not how much.
you can tailor the engine that way for the range you need..Top end or bottom.....
.
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Thread Starter
Instructor
Joined: Dec 2007
Posts: 245
Likes: 2
From: Jersey shore NJ
So Billla, On my other post you suggested that I use the IK 200 bodix heads and XE 12-246-3 comp cam. I believe this has 490" lift. I should be Ok with this combo clearing the L 82 Flat Tops?
Thanks Again for the replies.
Thanks Again for the replies.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
At max valve lift, the valve is nowhere close to the piston.
Because of that its impossible to tell you how much lift you can run.
A .650" lift solid roller could very well have more clearance than a .500" lift hydraulic flat tappet.
Valve clearance is affected much more by LSA and where the cam is installed.
Because of that its impossible to tell you how much lift you can run.
A .650" lift solid roller could very well have more clearance than a .500" lift hydraulic flat tappet.
Valve clearance is affected much more by LSA and where the cam is installed.
Race Director
Joined: Apr 2000
Posts: 17,672
Likes: 199
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Yes max lift was the wrong term I also forgot to mention the cam has to be installed and basically I was describing the steps for finding your piston to valve clearance. But this is also the way you can find out what max lift you can run. It is basically trial and error, if you don't have .100" clearance, it's too big. Experience plays a big factor here
Last edited by MotorHead; Sep 10, 2008 at 10:50 PM.