Decking Heads ~ Effect On Piston Clearance

[Ohio HD]

We all know to check valve to piston top clearance when building a motor. One thing that isn't as readily known is when decking a set of heads, then just putting them back on the motor, the edge of the valve to the edge of the piston valve relief needs to be checked.

When the head is moved closer to the piston, there are multiple angles that you're dealing with. The intake and exhaust valves are at different angles in the head. And they aren't at the same angle to the top of the piston. What that means is the valves edge gets closer to the edge of the piston reliefs when decking the head. So the edge needs to be checked. This is most influenced when the piston is at TDC and the valves only open a little. The intake is opening, and the exhaust is closing.

In the example below, the heads have 26° intake angles and the exhaust has 30° angles. When 0.035" is removed from the head gasket surface on the head, the valve becomes 0.0159" closer to the pockets edge. It only matters if you have a close clearance to begin with. And the more you take from the head the closer it becomes. How large the pockets are in the head can also play a part in it all.




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[kd]

This is when a Trock valve travel checker tool is handy.  Rolling an engine over to clay valve pocket clearance can be heavy danger with higher lift cams and decked head surfaces. A valve can easily be bent and a guide damaged.  As said, when the head drops because the head gasket surface has been machined for any reason it moves the lower edge of the valve closer to the lip of the relief cut into the piston.

[Don D]

The best solution, relevant only if custom pistons are in the mix, is to specify the radial and depth clearance desired, then give the piston company the valve drop with the valve job and milling done and head gasket in place, angles, head model (assuming they have this in their library), and valve head diameters. Now you get a "custom" piston not a "shelf piston" adjusted. With engine at TDC, springs off, the actual point of contact and depth can be determined using Dykem. To fix an old valve, head cut off, and made to a point then marks the center and the reliefs can be recut on a mill with a boring head and piston vise assuming the top ring land remains sufficiently thick. Most "shelf pistons" have obnoxiously deep reliefs and some miss the mark on radial clearance. Of course all of this can be laid out in CAD too. Piston companies don't give out their .STEP files though.

[Ohio HD]

The best solution, relevant only if custom pistons are in the mix, is to specify the radial and depth clearance desired, then give the piston company the valve drop with the valve job and milling done and head gasket in place, angles, head model (assuming they have this in their library), and valve head diameters. Now you get a "custom" piston not a "shelf piston" adjusted. With engine at TDC, springs off, the actual point of contact and depth can be determined using Dykem. To fix an old valve, head cut off, and made to a point then marks the center and the reliefs can be recut on a mill with a boring head and piston vise assuming the top ring land remains sufficiently thick. Most "shelf pistons" have obnoxiously deep reliefs and some miss the mark on radial clearance. Of course all of this can be laid out in CAD too. Piston companies don't give out their .STEP files though.

Well, not to everyone.     

[mike jesse]

I've usually found it's the radial clearance that needs adjustment, not pocket depth.
Checking springs are cheap.

[Ohio HD]

I've usually found it's the radial clearance that needs adjustment, not pocket depth.
Checking springs are cheap.

That's one of the reason I use custom domes pistons, and leave the meat on the head. It gives you better clearance, and in the end you have the same compression. Even custom made or modified pitons are less costly than new heads and starting the port work over again.

[kd]

The best solution, relevant only if custom pistons are in the mix, is to specify the radial and depth clearance desired, then give the piston company the valve drop with the valve job and milling done and head gasket in place, angles, head model (assuming they have this in their library), and valve head diameters. Now you get a "custom" piston not a "shelf piston" adjusted. With engine at TDC, springs off, the actual point of contact and depth can be determined using Dykem. To fix an old valve, head cut off, and made to a point then marks the center and the reliefs can be recut on a mill with a boring head and piston vise assuming the top ring land remains sufficiently thick. Most "shelf pistons" have obnoxiously deep reliefs and some miss the mark on radial clearance. Of course all of this can be laid out in CAD too. Piston companies don't give out their .STEP files though.

An example, even though not custom pistons, when I sent my MVA heads to Dan Baisley he asked me to include my pistons.  He knew he was decking them +.040 (to get 89 cc) and changing the valve sizes up larger.  I said the deck height would be set .000.  He told me he didn't need anything else then.  He did the calc and cut the valve reliefs.  He cut the eyebrow radial clearance and checked the depth. I clayed everything as I assembled it and they were perfect.

[jsachs1]

I common problem that I find is valve pocket rear clearance to the inner area of the top ring land. Especially with large intake valves.
John

[Don D]

The best solution, relevant only if custom pistons are in the mix, is to specify the radial and depth clearance desired, then give the piston company the valve drop with the valve job and milling done and head gasket in place, angles, head model (assuming they have this in their library), and valve head diameters. Now you get a "custom" piston not a "shelf piston" adjusted. With engine at TDC, springs off, the actual point of contact and depth can be determined using Dykem. To fix an old valve, head cut off, and made to a point then marks the center and the reliefs can be recut on a mill with a boring head and piston vise assuming the top ring land remains sufficiently thick. Most "shelf pistons" have obnoxiously deep reliefs and some miss the mark on radial clearance. Of course all of this can be laid out in CAD too. Piston companies don't give out their .STEP files though.

Well, not to everyone.     

Well looks like you are ready to roll then.
I too insist on a design review so I can see what I am getting. I also try as hard as possible to avoid oil ring rails.

[ghostrider]

Are there issues with oil ring rails?   I don't know much about them, but I have some Ross pistons going into a build that use them.

[Don D]

Brian,  what are your thoughts on the design of the piston skirt reliefs. Keeping in mind the highest practical gauge point is about .850 on most pistons.

[Ohio HD]

The only thought I have on that subject is it requires a study of the application, stroke, bore, RPM, thrust surface pressures, and materials analyses. If I wanted to get answers to those questions I'd contact someone Like Randy Torgeson or Ron Dickey. I'm sure there are others as well in the industry. 

[jsachs1]

Are there issues with oil ring rails?   I don't know much about them, but I have some Ross pistons going into a build that use them.

No problems. Put piston and clips on rod. Oil ring rails next, Dimple down, so they can't rotate, then all the rings, like stock.
John

[Don D]

The only thought I have on that subject is it requires a study of the application, stroke, bore, RPM, thrust surface pressures, and materials analyses. If I wanted to get answers to those questions I'd contact someone Like Randy Torgeson or Ron Dickey. I'm sure there are others as well in the industry.

Did anyone notice that wiseco used to cut the rear piston face for clearance and now cut the front exclusively? KB 117 pistons make a good replacement for 124" pistons with a very small adjustment to the reliefs for clearance,  and base gaskets adjusted for the difference in CH. Now thrust face has support well below the gauge point. Sometimes I have to forge my own path. Cp sent me pistons of my design and 2 of the 4 sets have already been used in drag race motors. One had lateral gas ports and the rider won his class, street baggers, 117" 142hp 11.1 @ 118. He has run 120. The other set went into a 127 Bob built and it made 155hp.