Milling piston dome?

[Beekeeper]

I have a short block 1970 hemi that has a stroker kit in it and the domes on the pistons are
pretty high and I plan to CC them in a few days. I’m sure they will be high on compression for a pump gas build but they are in excellent condition so can the domes be milled down to lower compression? I know it would probably be a guessing game at how much to mill off but can it be done and work? Anyone have experience with doing this? I don’t know any specs on the pistons or who made them but they look practically new the skirts are like new as well so not a lot of run time on this short block.

 

[vintage chromoly]

You would have to be careful about not thinning the top of the piston too much.

also, quantifying how much volume you removed from the dome would be a challenge.

I was in the same boat with a marine engine. It had CP pistons that produced 13.5:1 compression. I ended up selling the dome pistons and putting the money toward new slugs

 

[Fran Blacker]

If you mill pistons save ALL the cutting. You'll need a cc tube partly filled then add cutting record the difference in readings. You know the rest.

 

[Beekeeper]

If you mill pistons save ALL the cutting. You'll need a cc tube partly filled then add cutting record the difference in readings. You know the rest.

That’s a great idea to measure the volume removed from the piston
once the known amount is removed from the first just let the chips fly on the remaining pistons.

 

[Fran Blacker]

Edit on last post: Save cuttings from one piston. Just need to do the others same amount.

 

[steve340]

You need to measure the thickness of the pistons top before you do anything.
Then work out what the C/R actually is and look at what you want to get it down to.
If you want to loose a lot as earlier posted you may be better to get new pistons.

 

[steve340]

Just something else I thought of. When I have modified piston dome I only work on one piston until I get things as I want them.
That way you only ruin 1 piston if you balls it up.

 

[33 IMP]

Agree with steve340, first you have to know how thick the dome material is. Older wedge piston with domes were usually solid, so you could mill whatever you wanted off. Newer large dome pistons, and especially hemis, are hollow dome (a hemi high compression solid dome would be ludicrously heavy). You may not have enough metal in the dome to be able to machine it to get it where you want.
Next step is to get as accurate measurements as you can, for dome volume, deck height, and chamber volume, to get a correct compression ratio for what you have. It might be where you want it now, or it might be so rediculously high that it will be obvious that you cant take enough material off. Measure, measure, measure.

 

[493 Mike]

If the dome is truly "hemispherical" this amount can mathematically determined.
Mike

 

[Fran Blacker]

There has to be some material because if you need valve to piston clearance. Run a .040 head gasket instead of a .029. @Hemirunner
It's like a hair cut some off the top and a little sides.

 

[Challenger340]

There has to be some material because if you need valve to piston clearance. Run a .040 head gasket instead of a .029. @Hemirunner
It's like a hair cut some off the top and a little sides.

This is excellent advice IMO.... why not calculate how much CR reduction is required and order Cometic Head Gaskets in the required thickness ?

Far easier than Milling Piston Domes, re-balancing afterwards etc., etc,.

 

[747mopar]

When I built my motor the pistons had to be milled, I remember the machine shop telling me that the piston manufacturer stated that they could be milled to achieve certain compression specs. Might just check with the manufacturer.

 

[33 IMP]

He stated that he has no specs on the pistons, he doesn't know who made them.

 

[Hemirunner]

The chambers are 170cc, so at 3.75 stroke, you’ll need quite a bit of dome for 10:1. Figure out the CH and the dome volume above it, the piston to deck relationship and you can calculate where you’re at and where you want to go before cutting or spending anything.

 

[Beekeeper]

this block has 7.100 rods not sure on the stroke yet and the pistons only have a
part/order number. I’ll snap a couple pics
this evening when i get home. the pin is in the oil ring and uses support rails. i can sonic test the dome but almost certain its a solid dome. I plan to CC the piston this weekend to get the volume and for heads i was gonna give the edelbrocks a try. I maybe ok on compression by using the correct thickness head gasket. thanks for the replies and advice so far

 

[33 IMP]

A big thick head gasket can be used in a hemi better than in a wedge, a hemi (2nd gen, anyway) has no quench to lose, but.....thick head gaskets may mess with intake alignment, and the vertical intake bolts.
@Beekeeper , is the shortblock together, or still apart?

 

[Beekeeper]

Its still together less one rod and piston.
I pulled one to try to identify it but no luck so far

 

[33 IMP]

No engraved stroke length on the crank cheek? Tape measure and a straight edge should get you close enough to determine stroke.

 

[Beekeeper]

No engraved stroke length on the crank cheek? Tape measure and a straight edge should get you close enough to determine stroke.

There most likely are it’s an eagle crank
i’ll check it this evening. this is my very first
hemi build ever so i really appreciate all the help you guys are giving me.

 

[33 IMP]

If that is an order#, and if they are ross, a call to them with the order# or part#, they will tell you all you want to know. I did it with some custom pistons of theirs that I bought at a swap meet.
I there a brand name under the head of the piston? At least that might tell you the brand.