Piston Question

[dwjohnson]

 Let's say that there's not enough piston to valve clearance @ TDC for a certain cam, no more room to deepen the pockets because of the ring land, and trying not to move oil ring down into the pin bore.
Think it's possible to increase the comp. height, from the top ring land up, then deepen the pockets where you need them and then get the zero deck back with cyl. base gaskets or even spacer plates or am I forgetting or overlooking something?

[aswracing]

The more common fix is to sink the valves a little. Often your high TDC lift cams have more max lift anyway, and sinking the valves can help correct the geometry.

But of course sinking valves makes the chamber bigger, especially when you unshroud around them to restore the low lift flow you'd otherwise lose.

You can mill the head to get your CR back, but you're chasing your tail on that deal, as it takes back some of the clearance you gained with the sinking. The better solution IMO is to put some dome into the piston.

Which, at the end of the day, is about what you're doing by increasing compression height and raising everything up to match. Effectively that's dome.

[Phu Cat]

Seems like a lot of machine work.  Wouldn't it save time and maybe money to just buy a new set of heads?

PC

[nosjunkie]

You can also change cam timing to gain clearance. Why are you worried about the oil ring in the pin bore? All the 124" motors the oil ring is in the bore... Without issues..

[Don D]

Custom pistons. When a piston is truly custom valve drop is measured and the reliefs are properly placed and sized, not somebody's  generic with a few mods, all of these issues can be addressed. When I need to correct geometry I get the valves right for valve to valve and adjust the lengths to get the protrusion right. It takes custom valves. This allows fixing the spring pack height, tip length, at the same time so as not to use a whole stack of shims. It is costly but the guys that want the real deal and can afford to go there will pay. Same comment on piston to piston clearance. This can be done properly when the pistons are custom.

[dwjohnson]

 Thanks for the ideas.
I need a bit more clearance than I can get by sinking the valves, where as I already have the geometry close.
Option on changing heads is surly last choice because of the money and time I have in these heads already.
Moving ring pack down is still on the table but reduced pin bore strength and increased drag from thicker oil pack width is why I would hesitate.
These are custom pistons from CP, from a 3D head scan they did for me on the last cam combo. I know about CUSTOM. This is a 4" bore with 2.150" intakes and 41 cc dome pistons, turning 8000. It's all custom and all very close tolerances. Thought I'd see what kinda answers I'd get here on my thoughts before I suggest this to Mike and Don.
Thanks Again