Piston to valve clearance.

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comoxian

comoxian

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Hey all, been a while since I've been active here.

My 408 in my 1969 Dart had issues, and I replaced it with the build I started for my 65 Dart years ago. I decided to increase the cam from what I had in the current engine. Just called the mechanic to check on status and he informed me that there seems to be an issue with the valves contacting the piston. The lift on the cam in the previous engine was 0.519, a Blueprint crate engine. The current cam is 0.565 lift. That's roughly a heavy millimeter more lift. Would that be enough to cause an issue? I've had nothing but problems with this since I started the rebuild 18 months ago (mostly waiting for parts, wrong/damaged parts). I really just want my car back on the road.

Heads are Edelbrock Performer RPM. Is there a thicker head gasket? Or is it a spring issue? I'm mechanically inclined, and a tradesman, but not up on high performance specifics.

Hopefully, someone here with more experience than me, or my mechanic, can shed some light and get me through this.

TIA.
 
comoxian said:
My 408 in my 1969 Dart had issues, and I replaced it with the build I started for my 65 Dart years ago. I decided to increase the cam from what I had in the current engine. Just called the mechanic to check on status and he informed me that there seems to be an issue with the valves contacting the piston. The lift on the cam in the previous engine was 0.519, a Blueprint crate engine. The current cam is 0.565 lift. That's roughly a heavy millimeter more lift. Would that be enough to cause an issue?
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Yes
 
Lift has nothing to do with VP clearance.
Duration is what uses up clearance.
VP clearance is closest during the overlap period, when the valves aren’t open much at all.
(When the valve is at full lift, the piston is more than 1/2 way down the bore. So, on your 4” stroke engine…… that’s over 2” away)

That being said, what I see is that the cam not being installed correctly is often where these issues stem from.

If the new cam has not been degreed in(by someone who understands what that’s all about), that’s where I’d start.
 
PRH said:
It’s not, but please explain.

I’m referring to “peak/max” lift.
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Duration and lift both contribute to valve to piston clearance. To state that lift has nothing to do with VP clearance is false.
 
It’s all about how far the valve is open when the piston is near TDC, which is a function of duration and LSA.

It’s no problem at all to come up with examples where the lower lift cam has less VP clearance than one with substantially higher lift.

The point is, comparing peak lift numbers alone between two cams doesn’t tell you anything about how they would compare WRT VP clearance.

If you don’t agree with that, then I guess we’re agreeing to disagree.
 
PRH said:
It’s all about how far the valve is open when the piston is near TDC, which is a function of duration and LSA.

It’s no problem at all to come up with examples where the lower lift cam has less VP clearance than one with substantially higher lift.

The point is, comparing peak lift numbers alone between two cams doesn’t tell you anything about how they would compare WRT VP clearance.

If you don’t agree with that, then I guess we’re agreeing to disagree.
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This ^^^^^
 
A 0.046" increase in lift has the potential to cause VP clearance issues. Yes, duration is key. Clearance needs to be be measured. To state lift has nothing to do with VP clearance is clearly false.
 
PRH said:
Lift has nothing to do with VP clearance.
Duration is what uses up clearance.
VP clearance is closest during the overlap period, when the valves aren’t open much at all.
(When the valve is at full lift, the piston is more than 1/2 way down the bore. So, on your 4” stroke engine…… that’s over 2” away)

That being said, what I see is that the cam not being installed correctly is often where these issues stem from.

If the new cam has not been degreed in(by someone who understands what that’s all about), that’s where I’d start.
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Thanks for this information.

When I first took the car to this mechanic (recommended by a friend), I mentioned degreeing the cam. He stated that it wasn't really necessary, and that it only adds a little to the ultimate performance. I'm not an expert. But I did buy a degree wheel when I started the build on this engine for my 65 Dart. I never got that far. Could it be as simple as that? Just get someone that knows how to do this properly?

And when I spoke to him today, he did mention that it was the intake valves. It never occurred to me that they wouldn't be fully open at TDC. Something must be amiss.
 
comoxian said:
Thanks for this information.

When I first took the car to this mechanic (recommended by a friend), I mentioned degreeing the cam. He stated that it wasn't really necessary, and that it only adds a little to the ultimate performance. I'm not an expert. But I did buy a degree wheel when I started the build on this engine for my 65 Dart. I never got that far. Could it be as simple as that? Just get someone that knows how to do this properly?

And when I spoke to him today, he did mention that it was the intake valves. It never occurred to me that they wouldn't be fully open at TDC. Something must be amiss.
Click to expand...
A mechanic and an engine builder are not the same thing…. There eenlies your problem…
 
comoxian said:
And when I spoke to him today, he did mention that it was the intake valves. It never occurred to me that they wouldn't be fully open at TDC. Something must be amiss.
Click to expand...

Your closest valve to piston clearance on intake will be close to 8 degrees ATDC on your degree wheel, exhaust 8 degrees BTDC. As stated in this thread, it does not occur when the valve is fully open.
 
From a diy shadetree at best, I've rebuilt a few engines and only once I degreed it, that was the 340 I just built. Now that I know how if there are any future builds(hopefully 1 more) I will definitely degree it.
For an engine builder to say "degreeing is for performance" makes me wonder. It's for verification.
You can do it. YouTube, Princess Auto and fabo will get u thru it. It did for me.
Welcome back by the way! :thumbsup: :canada:
 
4spdragtop said:
From a diy shadetree at best, I've rebuilt a few engines and only once I degreed it, that was the 340 I just built. Now that I know how if there are any future builds(hopefully 1 more) I will definitely degree it.
For an engine builder to say "degreeing is for performance" makes me wonder. It's for verification.
You can do it. YouTube, Princess Auto and fabo will get u thru it. It did for me.
Welcome back by the way! :thumbsup: :canada:
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Princess Auto?
 
I think DC 340 should not have described PRH's post as "false." However, lift does matter, it's just that peak lift isn't what governs. Instead, it's the amount of lift at TDC, and one cam with higher total lift can have less lift at TDC than another cam with more total lift. Which is what PRH meant from the git go, although he did not use the term "peak lift" initially. And DC 340 did not initially use the term "lift at TDC." Once DC 340 and PRH clarified what they meant, it seems clear to me that they are saying the same thing.

Am I getting that right?
 
Scody21 said:
A mechanic and an engine builder are not the same thing…. There eenlies your problem…
Click to expand...
I had the bottom end redone by an engine builder (I had originally built it 12 years ago, not all that confident in my abilities in retrospect), but I figured that bolting on the top end wasn't a big deal. Live and learn. But the builder, in business for decades, sadly closed shop before issues arose with this build. Not a lot of choices around here for others.
 
comoxian said:
I had the bottom end redone by an engine builder (I had originally built it 12 years ago, not all that confident in my abilities in retrospect), but I figured that bolting on the top end wasn't a big deal. Live and learn. But the builder, in business for decades, sadly closed shop before issues arose with this build. Not a lot of choices around here for others.
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Do you happen to know what pistons were used. All modern HP pistons have generous valve reliefs. When pistons go up, valves are closing, so it would take one hell of an out of time situation to make piston to valve contact with modern HP pistons and most any lift camshaft. It's just not the lift that's most important NOWADAYS. Maybe 50 years ago when people ran large by huge cams with stock flat top pistons, but not today. It's just REALLY RARE to get piston to valve interference, unless something's assembled WRONG. However, having said ALL THAT, it's still a very advisable and accepted practice to measure anyway.
 
Agree with RRR. Were the pistons installed correctly, valve reliefs in the correct place?
 
RustyRatRod said:
Do you happen to know what pistons were used. All modern HP pistons have generous valve reliefs. When pistons go up, valves are closing, so it would take one hell of an out of time situation to make piston to valve contact with modern HP pistons and most any lift camshaft. It's just not the lift that's most important NOWADAYS. Maybe 50 years ago when people ran large by huge cams with stock flat top pistons, but not today. It's just REALLY RARE to get piston to valve interference, unless something's assembled WRONG. However, having said ALL THAT, it's still a very advisable and accepted practice to measure anyway.
Click to expand...


This^^^^^^^^^^

Virtually every single off the shelf piston today has valve pockets that are way too deep.

The new cam would need to have the valves open something around .075-.100 more at TDC than what you had.

That’s just not possible with the numbers you’ve posted.

Unless the old cam was ground on a 114 LSA and the new cam was ground on a 102 LSA and that may not do it.

Does duration matter? Yes? Can you look at duration alone?

Nope.

You can have a four different 280 duration cams and they can all have different lifts and none of them have the same p/v clearance.

PRH didn’t get it wrong. His post wasn’t false.
 
As has been mentioned…….
If the engine has any performance type piston in it with valve pockets, and they’re installed correctly(proper valve pocket orientation), it’s very unlikely that any semi-normal .565” lift cam would have a VP clearance problem…….provided the cam is in fact installed correctly.

My suspicion is it’s either a cam phasing problem(cam is WAY off), or there is some other procedural error going on in the VP measuring process.

How about the OP posting the rest of the cam specs for both cams?(advertised duration, [email protected], lsa)

Also, what are the observed VP clearances for both Intake & Exhaust.

Edit, after looking thru some cam lobe specs, I can see where something like a Comp XE295HL(251/257-110) might be iffy clearance wise with some pistons.
 
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Why you degree every cam…….

The very first cam I ever degreed in……..I couldn’t figure out what I was doing wrong.
I checked and re-checked everything.
I kept coming up with it being retarded 14*.

Out of frustration I tried a different timing set.
Came out right on the money.
Well, how can that be? I must have had something set up wrong first time around, so I reinstalled the first timing set and rechecked. 14* retarded.
So I removed that set and laid the two top gears on top of each other and lined up the dowel pin and bolt holes.
The dots on the upper gears were off one tooth from each other.
The gear showing the 14* retard was made wrong.

It’s not just for performance builds……it’s also a final check that all the timing components are at least “close” to being correct.
 
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