Valve to piston clearance?

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nm9stheham said:
I truly appreciate your interest in making this accurate, MH. The closed intake valve minimum distance relative to the surface of a 675 head was .090". I asked for the same measurement for the 302 heads but did not get an answer. I THINK the valve position is pretty consistent head type to head type for the same sized valves. But, yes that is a variable that I can't answer without a similar measurement from the 302 head.

What I tried to do was get an answer good enough to see the valves were going to clear the piston heads in the OP's proposed setup by quite a bit, or was it going to be close. If it is <.100" at TDC, I would expect it to be much too close at some number of crank degrees after that. Cam timing can make it better or worse, but I think some eyebrows are needed here. Does not surprise me.... my '68 273 2 BBL pistons that set right at zero-deck had shallow eyebrows, and that was for the stock cam with significantly lower lift and duration.

Best I can do on paper with my parts on hand, and a few other anecdotal references. If I had the actual detailed cam profile, then this would be a snap to accurately compute the clearances at all crank angles in a spreadsheet.
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I'm sorry I missed this. What is the measurement you need on the 302s? I can check it out for you.
 
dartfreak75 said:
As of now yes. But I have a ways to go to make that decision. I will look at it more closely when i cross that bridge.
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No way would I ever just stab one in dot to dot ever again. I would at least install by the cam card.
 
dartfreak75 said:
I'm sorry I missed this. What is the measurement you need on the 302s? I can check it out for you.
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I was hoping to get the vertical distance from the lowest edge of a closed intake valve relative to the flat surface the 302 head. That distance on a 675 head was .090" and that is what I based the numbers on. I measured by placing a steel ruler flat across the head surface, measuring from that down to the valve with my dial indicator, then subtracting out the thickness of the steel ruler.

It may be a moot point if the pistons change.....
 
nm9stheham said:
I was hoping to get the vertical distance from the lowest edge of a closed intake valve relative to the flat surface the 302 head. That distance on a 675 head was .090" and that is what I based the numbers on. I measured by placing a steel ruler flat across the head surface, measuring from that down to the valve with my dial indicator, then subtracting out the thickness of the steel ruler.

It may be a moot point if the pistons change.....
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Yea I can get that and report back it's kinda a moot point considering but at least we will have the info just in case someone else needs it.
 
Built 318 trw flat tops,0-deck.030 off the heads .028 Mr. Gasket head gaskets Comp cams k20-224-4 kit. I had .088 on intakes and .102 on exhaust. If you get it to close or want to use a bigger cam. Just use a thicker gasket. But compression is horsepower. I figured I’m between 10.5 and 11-1.

3509A8A2-CD20-45AC-BF74-31430FAF13D3.jpeg


443B0406-5A05-4AD0-93A4-D4BD5BD7BB24.jpeg
 
Excellent data point. What heads? Would probably be a tad worse than the OP's proposed setup due to the earlier opening valve.

Hope you get the paint off of the interior surfaces of the filter adapter plate before it is all done.....IMHO no need to risk some paint eventually flaking off and going into the engine.
 
Sloan 1 said:
Built 318 trw flat tops,0-deck.030 off the heads .028 Mr. Gasket head gaskets Comp cams k20-224-4 kit. I had .088 on intakes and .102 on exhaust. If you get it to close or want to use a bigger cam. Just use a thicker gasket. But compression is horsepower. I figured I’m between 10.5 and 11-1.

View attachment 1715254623

View attachment 1715254625
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Looks good! Thank you for sharing. What fuel do you plan on running with that cr?
 
dartfreak75 said:
Ok I have been doing alot of reading and research on this topic and everything I see is how to check it after the engine is already assembled. Is there any formula or calculations to check and make sure you will have adequate clearance before you start buying parts? For example I have a piston a goal compression and a cam chosen that I want to use how can figure out if it will work without buying it and test fiting it?
Click to expand...
This may be what you are looking for.
dartfreak75 said:
Ok I have been doing alot of reading and research on this topic and everything I see is how to check it after the engine is already assembled. Is there any formula or calculations to check and make sure you will have adequate clearance before you start buying parts? For example I have a piston a goal compression and a cam chosen that I want to use how can figure out if it will work without buying it and test fiting it?
Click to expand...
This may be what you are looking for.... Let me know... Mr Gasket Software
 
Sloan 1 said:
Built 318 trw flat tops,0-deck.030 off the heads .028 Mr. Gasket head gaskets Comp cams k20-224-4 kit. I had .088 on intakes and .102 on exhaust. If you get it to close or want to use a bigger cam. Just use a thicker gasket. But compression is horsepower. I figured I’m between 10.5 and 11-1.

View attachment 1715254623

View attachment 1715254625
Click to expand...

What's your chamber volume??
 
nm9stheham said:
FWIW, I worked out the pistons to valve clearance for the OP's combination at TDC; that is easy to figure since the .050" intake opening angle occurs right at TDC. I won't go over all the numbers (unless someone wants to see them), but basically, with that cam and that combination and using a 675 head as an example head, the valve-to-piston clearance is going to be in the .075-.085" range at TDC.

Since the valve is moving faster than the piston for the first 5 degrees or so past TDC, then that small clearance gap is going close up tighter after passing TDC. They might even touch. So it looks to me like eyebrows are 100% necessary here. They probably can be only .100" deep, but still needed.
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I read somewhere that regardless of the cam specs, when checking piston to valve clearance the closest points will always be at 10 degrees btdc and 10 degrees atdc.
Since I read that I have built two different motors and using light checking springs and a long travel indicator have found that to be true. Just an FYI
 
Duane said:
I read somewhere that regardless of the cam specs, when checking piston to valve clearance the closest points will always be at 10 degrees btdc and 10 degrees atdc.
Since I read that I have built two different motors and using light checking springs and a long travel indicator have found that to be true. Just an FYI
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Here's a link verifying what I said about closest points. Clay Is For Kids! Proper Degree Check and Piston Valve Clearance
 
Thanks very much Duane. Looks like the described procedure checks at both 10 BTDC and TDC for exhaust, and both TDC and 10 ATDC for intake. And then says "This gives you your piston-to-valve clearance at the two points where the piston and valve are normally the closest." It does not look to be saying exactly +/- 10 are always the closest.... and that's not the be expected with all sorts different cams. But I'd think you're spot-on that it is going to be somewhere around +/10 degrees from TDC.

I use a similar procedure to what you linked.... I just rotate the engine a couple of degrees at a time starting at maybe 20 degrees BTDC on overlap, with checking springs in place and cam timed, and take note of how far each valve can open at each point. It is pretty easy to find at what degree points partially open valves have the least clearance to the piston, and get a measurement there. With performance type valve-reliefed pistons and modest cams, it is not a problem (that I have seen). Seems like the aftermarket piston manufacturers do a pretty good job of designing for this.
 
This graph might be helpful in visualizing this. It's a graph of piston-valve clearance in a standard stroke SBM with a cam profile I found on the internet for a 227@.050" cam. (Actual lift profile is for a SBF but it'll be very similar in the area of the minimum P-V clearance IMHO.) This is approximate for a flattop piston as set up by the OP, but with a bit different cam:
  • longer duration cam
  • a bit more advanced (ICL at 105)
  • still a fast rate cam (44 degrees duration difference between .006" and .050" lifter lift points)
This is just to help visualize things, so should not be taken as a general rule for the the actual clearance you would achieve in your particular engine, but the crank angles for minimum piston-to-valve are generally correct.

It confirms the info presented by Duane. Vertical scale is clearance in inches and horizontal scale is degrees relative to TDC. This is for the intake valve.
Example-Piston-valve-clearance.png
 
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nm9stheham said:
Thanks very much Duane. Looks like the described procedure checks at both 10 BTDC and TDC for exhaust, and both TDC and 10 ATDC for intake. And then says "This gives you your piston-to-valve clearance at the two points where the piston and valve are normally the closest." It does not look to be saying exactly +/- 10 are always the closest.... and that's not the be expected with all sorts different cams. But I'd think you're spot-on that it is going to be somewhere around +/10 degrees from TDC.

I use a similar procedure to what you linked.... I just rotate the engine a couple of degrees at a time starting at maybe 20 degrees BTDC on overlap, with checking springs in place and cam timed, and take note of how far each valve can open at each point. It is pretty easy to find at what degree points partially open valves have the least clearance to the piston, and get a measurement there. With performance type valve-reliefed pistons and modest cams, it is not a problem (that I have seen). Seems like the aftermarket piston manufacturers do a pretty good job of designing for this.
Click to expand...
 
nm9stheham said:
Thanks very much Duane. Looks like the described procedure checks at both 10 BTDC and TDC for exhaust, and both TDC and 10 ATDC for intake. And then says "This gives you your piston-to-valve clearance at the two points where the piston and valve are normally the closest." It does not look to be saying exactly +/- 10 are always the closest.... and that's not the be expected with all sorts different cams. But I'd think you're spot-on that it is going to be somewhere around +/10 degrees from TDC.

I use a similar procedure to what you linked.... I just rotate the engine a couple of degrees at a time starting at maybe 20 degrees BTDC on overlap, with checking springs in place and cam timed, and take note of how far each valve can open at each point. It is pretty easy to find at what degree points partially open valves have the least clearance to the piston, and get a measurement there. With performance type valve-reliefed pistons and modest cams, it is not a problem (that I have seen). Seems like the aftermarket piston manufacturers do a pretty good job of designing for this.
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The article mentions tdc, but in my limited experience my last two builds(assuming you have located tdc correctly on your degree wheel) the closest points were at exactly 10 degrees.
One engine was a .585 lift cam that did not have adequate clearance and required custom valve reliefs in the pistons $$$. Any my current build with 12.7:1 compression and .705 valve lifter that clears easily.
Go figure.
My Pistons were a custom Ross. I have to agree that the piston manufacturers have gotten much better. The cost for the Pistons was not cheap. But that cost was offset in the weight accuracy of the pistons( no balancing was required) and no deepening of valve relief costs.
My point in mentioning this is, and I cannot explain the math the way that you do, is that it cannot be correct to calculate piston to valve clearance with the piston at tdc because that is not the closest points, and that the stroke of the engine has nothing to do with it, nor the lobe profile. IMHO But I could be wrong, this is based on my experience that 10 degrees is it.
 
Understood and agreed 100%. When I gave a number to the OP, it was qualified with the remark that, whatever clearance was figured at TDC, it normally would get tighter after TDC. So if things were already too tight at TDC, it would only get worse.

BTW, so everyone will know, the above graph is computed from a spreadsheet, and is really just for illustration only. The piston height vs crank angle is a formula that is is easy to program in. The cam profile info is what is needed, plus some other measurements to compute this with ANY degree of confidence or accuracy.... Getting the cam's detailed profile info is not something you get off of a website! Makes it hard to predict P-V clearance in advance, as the OP asked.

So that is the value of buying pistons with goof valve reliefs if you are going to get some lift going.
 
nm9stheham said:
Understood and agreed 100%. When I gave a number to the OP, it was qualified with the remark that, whatever clearance was figured at TDC, it normally would get tighter after TDC. So if things were already too tight at TDC, it would only get worse.

BTW, so everyone will know, the above graph is computed from a spreadsheet, and is really just for illustration only. The piston height vs crank angle is a formula that is is easy to program in. The cam profile info is what is needed, plus some other measurements to compute this with ANY degree of confidence or accuracy.... Getting the cam's detailed profile info is not something you get off of a website! Makes it hard to predict P-V clearance in advance, as the OP asked.

So that is the value of buying pistons with goof valve reliefs if you are going to get some lift going.
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It just goes to show what a black art cam lobe profile really is.
Regarding the Pistons, well I guess the custom manufacturers have realized that a guy who orders a piston with say 13:1 compression is probably not gonna use a cam with .500 lift so we better give him the appropriate valve relief. This was my first time ordering a custom piston. Ross walked me through it and it could not have worked out better. It was mentioned earlier in this thread about having the Pistons before going to the machinist so he can fit them correctly.
In the past my local machinist and I have always done it that way.
But not this time. Ross told me what the bore size will need to be and the proper piston to wall clearance is designed by Ross into the piston. We still waited to get the Pistons before we started, but the Pistons were exactly what Ross said and proper clearance was obtained right at the nominal bore size that Ross said. Kudos to them.
 
the exhaust valves are closest at approximately 8*-10* BTDC, intakes at 8*-10* ATDC, depending on where the cam is installed
 
Marcohotrod said:
the exhaust valves are closest at approximately 8*-10* BTDC, intakes at 8*-10* ATDC, depending on where the cam is installed
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That could be, but as I understand it, changing where the cam is installed only changes the amount of clearance you will get. Some people will change the installed centerline just to gain the necessary clearance if there is not enough. I cannot say for certain as I have only ever looked at this on two builds.
 
70aarcuda said:
It is not max lift that cause problems...the piston is way down in the bore then...it is during the overlap period where the exhaust closing and intake is opening...
duration...LSA and retarding or advancing the cam all change the valve to piston clearance..
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^^OP: THIS IS IMPORTANT, right here^^
 
dartfreak75 said:
Ok I have been doing alot of reading and research on this topic and everything I see is how to check it after the engine is already assembled. Is there any formula or calculations to check and make sure you will have adequate clearance before you start buying parts? For example I have a piston a goal compression and a cam chosen that I want to use how can figure out if it will work without buying it and test fiting it?
Click to expand...

every moving part is going to introduce a little bit of chance for variance from the theoretical.

Just clay check the ******!
 
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