I have a set of Speed Pro 360 .030 over pistons that I am considering milling a small pocket to decrease compression. The rotating assembly has been balanced so I am concerned with that. There pockets are small so the weight will be small but not sure how it will affect things. Any comments?
I don't know much about balancing.

Dont know how much of a decrease in comp your wanting but maybe you can try thicker head gasket.
Cometic can make just about any thickness you need.

Before you consider a hick gasket, what's your quench distance now? If it's at .060", you lose it completely with a thicker gasket.
If you have .035-40", it will be less effective but still active.
Your bob weight will change by 50% of the weight change.

That's my problem. My chambers are too small with quench at a .030-.035". Instead of using different heads i was considering milling a small pocket, maybe 6cc or so.

The deck is .005 in the hole with a 58cc closed chamber iron head. too much compression with quench.

Not sure what you mean by "Your bob weight will change by 50% of the weight change."

I guess I need to do some calculations to see just what size pocket I need to mill to figure how many grams weight change will be.

Just throwing this out there. What about increasing your cylinder head cc, like deshrouding the intake valve. Not sure what heads you have if it's feasible or not.

To play with the numbers try the compression formula on the Diamond site. You can play with combustion chamber, head gasket thickness, piston dish, etc. You can figure out exactly what you need to do.

This is what I would do before messing with the pistons, that and maybe more thickness of the head gasket if you don't lose quench.

How much compression do you have?

Your pistons are .005 in the hole and you have .030-.035 quench??? Then that's already using a thin head gasket. Or are your Pistons standing .005 ABOVE the deck?

.005 under the deck or in the hole. The heads are some old W-2 ported open chamber heads with 70cc. no quench and low compression. I thought about milling .050 off them to make them closed chambered. but that puts the cc's about 56-58. but with a thin gasket and the piston in the hole .005 I coukd possibly deshroud the valves and get more chamber cc's. I haven set a head gasket on there yet to see how much deshrouding i can do.

I'd certainly look at unshrouding the valves. It's a lot of work (and should all be really close) but where and how much depends on the shape now, valve lift, stem angle, blah. Sometimes too open is a mistake, the flow leaving the seat wants to expand gradually and in the right direction, not turned loose all at once (like a vertical valve).
I wrote some stuff on the subject, some of it may be relevant: http://victorylibrary.com/mopar/chamber-tech.htm

Got a picture of a chamber?

I re-checked my calculations. I am right a 10.13 with a 70 cc head and no quench (.080). That is with a .027 thick gasket. If I mill .050 off the head the compression will go up 1 point. I believe that is too much for a iron head on pump gas. I read the info Polyspheric posted..... good info. I will buy a head gasket and see what I can come up with. Playing with the numbers I need roughly 10-11 cc less than I have if I mill the head to be closed chambered and have .033-.035" quench.

Should I just leave it alone?

Where did you think you were? Iron heads, 9.5 comp is the safe rule of thumb. If it were me , id go for quench, mill .030/.040 and then deshroud the head. I haven't done any of the math except the quench, trusting you on the comp figure. But quench is always better if its in range and you can figure out how to get it.

9.5:1 on a small block with iron heads is pretty conservative. But a lot of it comes down to how tight the quench is, and how big the cam is. I can tell you that I ran an open chambered iron head at 12.2:1 and .030"~.035" quench on pump gas without issues. This was done by equalizing the chamber depth on each hole, and milling the block to put the pistons .070" out of the hole. VERY unconventional way of doing it. I'd be leery to mill .050" off the heads unless you were sure that the deck was nice and thick to start with.

How deep is the chamber right now on the quench side of your heads?

Honestly if the cam is big enough, the 10.1:1 will probably fly ok without detonating. Worst case scenario you knock a few degree's of timing out of it on the street, and put a splash of race gas and full timing at the track.

The best thing you could do is to buy a set of quench dome pistons and re-balance, but it's also your most expensive option so I can see why you might not want to.

Call rebco machine, think he is Wichita. He cut deeper pockets in two sets of my pistons. He does great work.

A slight change in top end weight won't do any harm.

The quench side of the wedge shape is right around .050" deep. The deck has been cut to put the pistons .005" in the hole.

I want to post a pic of the chamber but do not understand how to do it.

[317243 head.jpg] http://www.infopop.com/artwork/footer_logotype.gif