unshrouding valves - by how much?

[aribert]

I searched for unshrouding valves. I found an image by JackFish and one posting by someone who mentioned that he unshrouded by less than 0.030.

I am working w/ a '78 200 engine that has never been opened up. I am going to mill the head to bump compression so I can compensate for any chamber increase due to unshrouding. Since I have yet to pull the head - what is the unshrouding limit with respect to the head gasket ring? How close can one get to the sealing ring and still stay on the safe side? If the sealing ring is not the limiting factor, what is a practical number to shoot for (radially around the valve heads)?

 

[CZLN6]

Howdy Ari:

Q- "what is the unshrouding limit with respect to the head gasket ring? How close can one get to the sealing ring and still stay on the safe side? If the sealing ring is not the limiting factor, what is a practical number to shoot for (radially around the valve heads)?"

A- Do not exceed the bore diameter. The standard replacement head gasket will be slightly larger in circumfirence. For absolute limits, do any overboring first, then set the head onto the fresh bored block- centered as close as possible, and then reaching up through each cylinder from the bottom of the block, scribe each chamber to the bore. You can safely unshroud the chambers up to the bore limit. I've used magic markers, laundry markers as well as a sharp scribe tool. all worked fairly well.

If you do exceed the bore limit, the top of the cylinder bore will deflect and disrupt the intake charge. There is also a greater likelyhood of sealing problems.

FYI- Milling the head also effectively unshrouds the valves as it allows the valve head to drop into the chamber sooner. So does back cutting the intake valves. Unshrouding is most critical to the intake side, because of the larger diameter of the intake valves and the fact that intake is initiated by vacuum. Taking into account the thickness of aftermarket composite gaskets and the larger chambers of a '78 head casting, I'd suggest that may want to mill the head .075" to achieve a CR of 9:1. Grinding to unshroud the intake valves will likely only add a few ccs to the mix, but you will want to cc each chamber to equalize the volume as well as to know exact volumes.

Good luck and keep us posted your progress.

Adios, David

 

[wsa111]

David, excellent information on the bore diameter.

May i suggest that once you have the bore diameter marked for each cylinder, find a machine shop with a serta valve machine & they can unshrould the chamber & do valve seat machine work including the area under the valve seat in one operation. Doing this will ensure each cylinder is done to the same dimension.
As David said have them do a 30 degree cut on the underside of the intake valve after a lap in line is established.

This way you will have a professional job done & probably gain 10 HP, nice huh.