Minimum valve to piston clearance?

[Jeffrey Osstyn]

I thought around .100 was the minimum, I have two exhaust valves that just touched the piston and I have measured a .155 clearance at 10 deg. BTDC. This is the point that the exhaust value comes the closest to the piston at valve lift of .155. Engine was never spun past 5500 rpm as far as I know. Using recommended springs for the camshaft.

 

[Don Jacks]

You'll be ok there,Jeffery.

 

[Jeffrey Osstyn]

So apparently there is a problem somewhere, but that is the closest the valve gets that I have been able to find. I checked from 50 deg. BTDC to 50 deg. ATDC. I must be doing something wrong in my check procedure?

 

[Fathead Racing]

A good start for an assessment would be where the piston is in or out of the hole and what gasket thickness you are using. Generally speaking you would measure piston to deck by rocking the piston in the bore then take an average. Pistons just brushing the head is right where high performance guys would love to be but the advice is you better know what your doing. I have ran with as little as .028 quench and had piston witness marks all over the heads with no consequences. I asked a very knowledgeable member on this site what he thought and he said, " well, that's a good place for the piston to stop"! That guy was our own Denny Ford. Gotta love that answer. But again we're talking about valve to piston contact here. I've always heard that it was .080" for intakes and .100" for exhaust with steel rods, more for aluminum. A cheap quick fix would be changing the rocker ratio on the offending valves, say from a 1.75 or 1.72 to a 1.7 ratio. Next would be a thicker head gasket. What is your quench number right now?

 

[Jeffrey Osstyn]

This is what I did, heads off found TDC, installed cam degree wheel. Installed head, lifter and pushrod. Using dial indicator checked lobe lift every 10 deg. starting at 50 deg. BTDC to 50 deg. ATDC. Took those readings and X 1.7 now I have valve lift. Positioned piston at prior cam wheel degrees and using light checking spring on exhaust valve pushed valve open until it made contact. Subtracted cam lift from max valve contact lift and as mentioned before smallest reading I get is .155? Why would it contact piston while running? Spring pressure too light?
I have checked deck height at the four corners, it is not straight, #2 down .0025 #8 down .016. Interestingly the #8 piston has the contact issue but not #2. I am using a .040 thick gasket, after compressed.

 

[Don Jacks]

Jeffery,What is the amount of cam lift at this point in the cycle?

 

[Fathead Racing]

Did you check piston to deck by rocking the piston in the bore? What rockers are you using? Rockers can be off a whole bunch. I assume you are using solid lifters? Flat tappet? Engines are never symmetrical, add up machining tolerances and we are always surprised. Valve stem length's are also all over the place too. Change that valve to another cylinder or do a good measurement, or change that rocker.

 

[Jeffrey Osstyn]

Jeffery,What is the amount of cam lift at this point in the cycle?

10 deg. BTDC Lobe lift .091 X 1.7 = .155. I can push valve open .310 until it contacts piston, minus the .155 cam lift leaves .155 clearance.

 

[Jeffrey Osstyn]

Did you check piston to deck by rocking the piston in the bore? What rockers are you using? Rockers can be off a whole bunch. I assume you are using solid lifters? Flat tappet? Engines are never symmetrical, add up machining tolerances and we are always surprised. Valve stem length's are also all over the place too. Change that valve to another cylinder or do a good measurement, or change that rocker.

Yes, rocked piston used average. Full roller rockers 1.7 ratio comp cam brand. Hyd flat tappet cam.

 

[Don Jacks]

Well,the way that I see this is that you have .155 clearance in an area where .100 is considered a safe minimum.Maximum valve lift occurs much later in the cycle.The only way that your clearance would be an issue[bent valves] would be if your timing chain broke during engine operation.

 

[Fathead Racing]

To much preload on a hydraulic lifter would cause the lifter to pump up, back off on the preload on number eight?

 

[Jeffrey Osstyn]

Well,the way that I see this is that you have .155 clearance in an area where .100 is considered a safe minimum.Maximum valve lift occurs much later in the cycle.The only way that your clearance would be an issue[bent valves] would be if your timing chain broke during engine operation.

Well, apparently the impossible has happened to me, I am going to re-check it all from the beginning. Found a min. exhaust valve to piston clearance stated at .120 in the Reher-Morrision race motor build book. But I have that covered. Not sure what is going on, but I do have two very small smiley's looking up at me from my pistons.

 

[64ss409]

Is the spring installed height correct? Too much would be like a weak spring. You don't want the pistons helping to close the valves.

 

[Jeffrey Osstyn]

To much preload on a hydraulic lifter would cause the lifter to pump up, back off on the preload on number eight?

Checking clearance is with the lifter fully pumped up now.

 

[Jeffrey Osstyn]

Is the spring installed height correct? Too much would be like a weak spring. You don't want the pistons helping to close the valves.

Yes, spring heights and pressures were all set during assembly, have not re-checked them but I will.

 

[dm62409]

Your piston to valve clearance is plenty , definitely recheck valve spring pressures, and check for tight valve guide clearance. I know bronze guides that hang down into the exhaust ports will overheat and seize down on valve stems, sticking them enough to cause valves to kiss the pistons.

 

[Skip FIx]

Intake valve is chasing piston down, on exhaust piston chasing the valve up.

I generally map how much total possible clearance with light springs and a dial indicator from 20 degrees either side of TDC by pushing valve down. Then measure the actual valve lift at all the same points.

I've run as close as 0.050 on the intake and 0.080 on exhaust

 

[tenxal]

You have a lot of variables in your checking setup. Try this:
-Use a solid lifter instead of the hydraulic to eliminate push rod cup movement.
-Install the valve spring and retainer you're going to run.
-Set the dial indicator up on the retainer not the lifter.
-Adjust the lash to zero.

Now you are checking it the way it's going to be run. If you don't have a used head gasket to use, add the thickness of the head gasket to your final valve-to-piston figures. About 20 degree BTDC to 20 degrees ATDC will be where you want to check in 5 degree increments.

Hope this helps.

 

[427John]

I'm with Dave on guide clearance my cylinder head guy tells me that is the most common mistake he sees on problem heads,they are setup where the valves move nice and smooth with minimal wiggle in the guide and as soon as you put some heat in them they don't want to move.He says there's a fine line between too much and too little.Also since it is limited to 1 valve you may have been unlucky enough to have a new spring fail right out of the gate,wouldn't be the first time.

 

[Greg Reimer]

In Stock Eliminator Chevrolet motors, .050" intake and .080" exhausts are considered OK. The exhaust stays open until the last possible instant to achieve maximum scavenging of exhaust gases,then it shuts very quickly. The intake starts to open before TDC so that when the pressure drop in the cylinder starts, it can achieve air flow as soon as possible. Very little air flow occurs below .050" lift on either valve,so that is why a legal camshaft has the rather brutal profile they have. The window area of the cam(the time that air flow occurs) has to be maximized for maximum air flow and horsepower to be achieved. In a more normal application, and where there isn't a constraint on valve lift, the cam lobe profile is much more gentle, so the engine is more streetable.As long as you aren't running it so hard as to get into valve float, there should be no problems with piston to valve clearance issues. Make sure you have adequate valve spring pressures to prevent this. The more brutal the cam,the more spring pressure you need.