408 Piston Dish

[mopowers]

I'm building a 408 SB with Icon 745 pistons. The specs say they have a 20.5 cc dish. Is that 20.5 cc's measured from the top of the quench pad (red arrow), or the raised area around the perimeter of the piston (green arrow)?

I believe it's from the raised quench pad, but just wanted to verify. I'm in the process of measuring piston-deck clearance and want to make sure I'm using the correct reference surface.

 

[Bobzilla]

Measure the height with a caliper from the top of the piston pin hole to the top of the piston and then add .492 (half of pin diameter) and that will be the compression height to the lower flat of the top of that piston. If it closely matches the 1.465 advertised compression height, then that is technically where the deck height measurement would be taken from.

But, either way, that info won't ensure adequate piston to head clearance. Be sure to measure that directly on it's own during mock up, before final assembly. That looks like a raised quench pad, but I am not sure.

For compression ratio calculations, you could measure the half inch down fill volume and subtract that from a theoretical half inch down fill volume like the Mopar books used to have us do. You basically get the piston to actual top dead center then measure it's travel down half and inch (.500 on a dial indicator). In that position, apply grease around the top of the piston to seal it to the bore, then see how many cc of liquid it takes to fill that space to the top of the block.

 

[mopowers]

Measure the height with a caliper from the top of the piston pin hole to the top of the piston and then add .492 (half of pin diameter) and that will be the compression height to the lower flat of the top of that piston. If it closely matches the 1.465 advertised compression height, then that is technically where the deck height measurement would be taken from.

But, either way, that info won't ensure adequate piston to head clearance. Be sure to measure that directly on it's own during mock up, before final assembly. That looks like a raised quench pad, but I am not sure.

For compression ratio calculations, you could measure the half inch down fill volume and subtract that from a theoretical half inch down fill volume like the Mopar books used to have us do. You basically get the piston to actual top dead center then measure it's travel down half and inch (.500 on a dial indicator). In that position, apply grease around the top of the piston to seal it to the bore, then see how many cc of liquid it takes to fill that space to the top of the block.

Good point. I forgot about the advertised compression height. After measuring, the 1.465" is to the top of the quench pad. I guess that means the 20.5 cc's is based on the volume below the quench pad. Thanks.

 

[Kern Dog]

That is some confusing stuff, man.
My friend Rich is building a 408 and as is the case with other stroke builds, the pistons are dished to keep compression to a workable number. His pistons are stepped and dished too. His may also be the same model of ICON that you have. It almost seems like you'd need to call the vendor for an answer or mock up a piston at TDC in the block and CC the dished portion like you'd CC a combustion chamber. What liquid would you use though that would not try to leak through the ring gap?

 

[Valman]

For compression ratio calculations, you could measure the half inch down fill volume and subtract that from a theoretical half inch down fill volume like the Mopar books used to have us do.

That’s the best way to do it with dish or dome pistons.Very accurate.

 

[Dartnut]

That is some confusing stuff, man.
My friend Rich is building a 408 and as is the case with other stroke builds, the pistons are dished to keep compression to a workable number. His pistons are stepped and dished too. His may also be the same model of ICON that you have. It almost seems like you'd need to call the vendor for an answer or mock up a piston at TDC in the block and CC the dished portion like you'd CC a combustion chamber. What liquid would you use though that would not try to leak through the ring gap?

Good question.
Use grease to seal the top of the piston to the cylinder walls and use a 50/50 mix of water and alcohol with some food coloring mixed in.
Some people use anti freeze and it would be okay as well.
I've always used the 50/50 mix and a medical grade burette in the past and it worked well for me, because of the lower chance of a meniscus forming on the top as opposed to anti freeze, but we're splitting hairs here...

 

[mopowers]

That is some confusing stuff, man.
My friend Rich is building a 408 and as is the case with other stroke builds, the pistons are dished to keep compression to a workable number. His pistons are stepped and dished too. His may also be the same model of ICON that you have. It almost seems like you'd need to call the vendor for an answer or mock up a piston at TDC in the block and CC the dished portion like you'd CC a combustion chamber. What liquid would you use though that would not try to leak through the ring gap?

After looking more into it, the top of the quench pad is clearly what they base the advertised dish volume on. Besides the listed compression height, Icon lists a spec from the top ring land to the top of the piston at 0.272". That's to the top of the quench pad according to my calipers.

That's all I was trying to verify. With those pistons 0.006" in the hole, that puts me right at 10:1. A little more than what I was shooting for and without ideal quench, but hopefully with aluminum heads it'll be okay. Time will tell...

 

[Newbomb Turk]

It’s the entire piston. The ONLY away to know exactly what volume that piston has is to do a .500 down fill and do the math.

 

[33IMP]

So that piston has a dome AND a dish? I can see where that would be confusing..... and dangerous if you're using a closed chamber head!

 

[Newbomb Turk]

So that piston has a dome AND a dish? I can see where that would be confusing..... and dangerous if you're using a closed chamber head!

It doesn’t have a dome and a dish.

It’s a dished piston with a zero deck flat on it.

The reduced area around the outside is to increase the dish volume without making the dish any deeper than it already is.

 

[33IMP]

Thanks Turk. I understood that the "red arrow" area was a quench dome. My bad.

 

[Newbomb Turk]

Thanks Turk. I understood that the "red arrow" area was a quench dome. My bad.

I got that straight from Icon so that’s how I know it.

It’s not like I’m smart or something.

 

[mopowers]

Thanks Turk. I understood that the "red arrow" area was a quench dome. My bad.

Yeah, that's why I was confused as well. The raised area is the quench pad, but since it's the piston top and the dished volume is referenced to it, it's technically not a dome.

 

[MOPAROFFICIAL]

It's called a step-head piston. Step/dish
They intro'd the hypers as such, like the kb356.
In their case the step had a quench pad on thr step head .085 tall you can mill according to how much quench distance you need.. or leave it for open chamber stock sized chambers that are about .100 deep on the side. .039 gasket, .085= .046.-.100 =.054 distance roughly.. there's always a smidge taken off the heads during the rebuild anyways.
You could just measure everything missing from the step head or you could actually figure it out at its actual installation deck height for the actual running.

 

[Newbomb Turk]

It's called a step-head piston. Step/dish
They intro'd the hypers as such, like the kb356.
In their case the step had a quench pad on thr step head .085 tall you can mill according to how much quench distance you need.. or leave it for open chamber stock sized chambers that are about .100 deep on the side. .039 gasket, .085= .046.-.100 =.054 distance roughly.. there's always a smidge taken off the heads during the rebuild anyways.
You could just measure everything missing from the step head or you could actually figure it out at its actual installation deck height for the actual running.

It’s not the same thing. You don’t mill it off to make it flat.

Like I said, it’s built that way to increase the dish volume without making the dish deeper and by that, make the piston heavier.

 

[mbaird]

Sounds like you are referring to the Icon IC745 with quench pad ? I am going thru a similar issue . The quench pad ended up .040 + above deck . I wasn’t sure if I had the wrong piston or shop milled too much off of my deck . I called Icon and they said the quench pad should be at or under the deck . Then I have others say it will be above deck . Shop hasn’t told me what my deck height is yet ( bad sign ). And did not measure before decking which I would think is SOP for any machine work . So I can’t calculate my quench or compression to order a head gasket . Shop says they can cut enough off piston to make it zero deck but that may screw up the compression/ quench goal … .040 quench and no more than 10.5:1 compression .

 

[fishmens67]

Like I said, it’s built that way to increase the dish volume without making the dish deeper and by that, make the piston heavier

Either way it's a shitty design. imo

 

[mopowers]

Sounds like you are referring to the Icon IC745 with quench pad ? I am going thru a similar issue . The quench pad ended up .040 + above deck . I wasn’t sure if I had the wrong piston or shop milled too much off of my deck . I called Icon and they said the quench pad should be at or under the deck . Then I have others say it will be above deck . Shop hasn’t told me what my deck height is yet ( bad sign ). And did not measure before decking which I would think is SOP for any machine work . So I can’t calculate my quench or compression to order a head gasket . Shop says they can cut enough off piston to make it zero deck but that may screw up the compression/ quench goal … .040 quench and no more than 10.5:1 compression .

If that quench pad is 0.040" proud of the deck, something definitely happened. They probably took a ton off the deck surface. Is it a magnum block? I believe those have shorter deck heights too.

 

[Kern Dog]

I think it was that Rusty dude that wrote that the Magnum deck height was .019 lower than the LA series.

 

[mbaird]

If that quench pad is 0.040" proud of the deck, something definitely happened. They probably took a ton off the deck surface. Is it a magnum block? I believe those have shorter deck heights too.

It’s a magnum block but they are not .040+ shorter . I have an LA roller block in the shed I could use but don’t really want to if I can help .

 

[MOPAROFFICIAL]

It’s not the same thing. You don’t mill it off to make it flat.

Like I said, it’s built that way to increase the dish volume without making the dish deeper and by that, make the piston heavier.

The milling done isn't to make it flat..ta already flat.. and I never said that this piston in particular needed milling, though you could if for some reason you needed to. It's known to all it was designed to be zero decked to the top of that piston.
It's a step dish, but call it an ash tray for all i care.

 

[PRH]

Measure twice, cut once……..works for more than just carpentry.

To end up with an unwanted positive deck height …….that’s more like…….. “measure?”

 

[MOPAROFFICIAL]

Sounds like you are referring to the Icon IC745 with quench pad ? I am going thru a similar issue . The quench pad ended up .040 + above deck . I wasn’t sure if I had the wrong piston or shop milled too much off of my deck . I called Icon and they said the quench pad should be at or under the deck . Then I have others say it will be above deck . Shop hasn’t told me what my deck height is yet ( bad sign ). And did not measure before decking which I would think is SOP for any machine work . So I can’t calculate my quench or compression to order a head gasket . Shop says they can cut enough off piston to make it zero deck but that may screw up the compression/ quench goal … .040 quench and no more than 10.5:1 compression .

They probably mean the outer edge around the step should be zero deck..while the pad is positive.
Anything with a raised quench pad is designed for an open chamber head. All the others that are flat top zero deck or maybe positive .019- .024 ' like a 340 kb243 ...can be used for the closed chamber head and adjusted with a head gasket.
Your shop needs to equalize all the rod lengths, the stroke on the crankshaft if it isn't already then properly Square the block up based off the four corner measurements of 1,2,7,8 rods n pistons. Mock up is always involved as foar as im concerned

 

[mbaird]

They probably mean the outer edge around the step should be zero deck..while the pad is positive.
Anything with a raised quench pad is designed for an open chamber head. All the others that are flat top zero deck or maybe positive .019- .024 ' like a 340 kb243 ...can be used for the closed chamber head and adjusted with a head gasket.
Your shop needs to equalize all the rod lengths, the stroke on the crankshaft if it isn't already then properly Square the block up based off the four corner measurements of 1,2,7,8 rods n pistons. Mock up is always involved as foar as im concerned

I asked them specifically if the measurements were from top of quench or the rim around dish . He said highest point of piston .
But like I said it wouldn’t be the first time a techline was wrong . .

 

[Newbomb Turk]

I asked them specifically if the measurements were from top of quench or the rim around dish . He said highest point of piston .
But like I said it wouldn’t be the first time a techline was wrong . .

He’s right. It’s NOT an open chamber piston. It’s designed to be zero deck with a dish.

Like I said, the outer ring that’s lower was done so they didn’t have to make the dish deeper.