Trying to decide which pistons for 440 build

Bigsbetter · 2026-02-16T17:42:04-0600

I’m rebuilding a 1973 440. It is a very low mileage engine from an R/V. I have Aluminum heads with 75cc chambers. My question is about pistons. I want my compression to be between 10.0-10.5-1. I’m looking at the Icon 9947 which has a compression height of 1.991 but also a +5.6 cc dome. I’m the pictures of the piston, it doesn’t look like the pistons have any dome. So when I do the compression calculators, I’m not sure weather to add the 5.6cc or not? It makes a difference. This is a stock 4.32 bore and I’m assuming about a .040 head gasket. Or maybe someone else has a good recommendation for a piston that will get me close to 10.5 without going over? Thankyou.

BSB67 · 2026-02-16T17:54:17-0600

Icon IC836

70rrclone · 2026-02-16T18:12:09-0600

I did basically the same thing, I have a 77 440 out of a motorhome. I used wiseco pistons std bore. Trickflow 240 heads 74cc. With a .039 headgasket. I ended up with a pump gas friendly 9.84:1. The wiesco have a 2.05 ch.

Wiseco 5531AS

BSB67 · 2026-02-16T18:27:15-0600

Bigsbetter said:
I’m looking at the Icon 9947 which has a compression height of 1.991 but also a +5.6 cc dome. I’m the pictures of the piston, it doesn’t look like the pistons have any dome. So when I do the compression calculators, I’m not sure weather to add the 5.6cc or not? It makes a difference.

9947 - Not a dome, it’s the volume of the valve relief.

Maybe the Speed Pro 2266 would work for you

Iroc78 · 2026-02-16T18:59:05-0600

You would need to add the 5.6cc in the calculator. Based on the picture of the Icon 9947 the +5.6cc dome is positioned to provide valve clearance. If the +5.6 cc spec I am seeing on Summits website is indeed correct it would still be considered a dome while not looking like a traditional dome whereas a -5.6 cc would be considered a dish.

threewood · 2026-02-16T19:26:03-0600

If you want to save a bit of money, look at the KB237 hyperutectic pistons. 2.067" compression height. I've been beating mine up for about 10 years and they are great. Stealth aluminum heads, runs on 88 clear gas.

BSB67 · 2026-02-16T19:26:44-0600

Iroc78 said:
You would need to add the 5.6cc in the calculator. Based on the picture of the Icon 9947 the +5.6cc dome is positioned to provide valve clearance. If the +5.6 cc spec I am seeing on Summits website is indeed correct it would still be considered a dome while not looking like a traditional dome whereas a -5.6 cc would be considered a dish.

The piston is a flat top with valve reliefs. No dome. The combustion volume is increased by 5.6cc. The compression ratio will be lower.

BSB67 · 2026-02-16T19:38:19-0600

threewood said:
If you want to save a bit of money, look at the KB237 hyperutectic pistons. 2.067" compression height. I've been beating mine up for about 10 years and they are great. Stealth aluminum heads, runs on 88 clear gas.

The OP has 75cc heads. Is that what you are using?

threewood · 2026-02-16T19:41:14-0600

BSB67 said:
The OP has 75cc heads. Is that what you are using?

Stealth heads are 80cc

Bigsbetter · 2026-02-16T20:13:04-0600

As of right now, I’m leaning toward the Speed Pro 2355. Get me somewhere in the 10.2, -10.4 or so

RR Fan Dan · 2026-02-16T20:28:04-0600

I just went through this exercise a couple of hours ago, as I am ordering a 440 source stroker motor. They have some handy charts for the pistons they sell. The kit we got is around 9.7 according to their chart. But we are going to make up the difference to achieve 10.3 by using MLS head gaskets. So my point is. If you can’t find the exact piston you can make up the difference by changing the thickness of the head gasket.

Here is their chart just for a visual on how different bore sizes pistons affect CR.

Iroc78 · 2026-02-16T21:33:22-0600

" Piston Dome refers to the amount of added volume on top of the piston compared to a flat top piston." Quoted from the second sentence of link below among others.

https://www.performancetrends.com/Definitions/Piston-Dome.htm

By definition +5.6cc piston head volume is a domed piston.

While some may look at this piston the original poster was asking about and see a flat top with valve reliefs, I (as well as the piston manufacturer who labeled it +5.6cc) see a flat top piston with a 5.6cc dome added in an area where valve to piston clearance won't be an issue (circled on the upper RH side of pictured piston).

RemCharger · 2026-02-16T22:57:30-0600

BSB67 said:
9947 - Not a dome, it’s the volume of the valve relief.

BSB67 said:
The piston is a flat top with valve reliefs. No dome. The combustion volume is increased by 5.6cc. The compression ratio will be lower.

Agreed they're flat tops. Which means of course , there's nothing on top but flat. It's easy to understand when you're from saskatchewan.

BSB67 · 2026-02-17T04:53:47-0600

Iroc78 said:
" Piston Dome refers to the amount of added volume on top of the piston compared to a flat top piston." Quoted from the second sentence of link below among others.

https://www.performancetrends.com/Definitions/Piston-Dome.htm

By definition +5.6cc piston head volume is

I suggest that you go to the UEM catalog and read the specific description for the 9947 piston. If you read it closely it should become clear it is not domed.

Also, if you look around in the UEM catalogs you will also notice that true domed pistons have a negative cc values.

Bigsbetter · 2026-02-17T07:27:49-0600

This was excactly what I was confused about, weather it was a dome or a flat top with valve reliefs. I learned a lot from yesterday about compression height and quench and I understand it now. I believe these are flat top pistons and the valve reliefs measure 5.6cc so you have to add that to your combustion chamber volume witch of course decreases CR not increasing it.

Iroc78 · 2026-02-17T07:54:46-0600

The following quote is from the UEM catalog that BSB67 mentioned proves his point, and he is correct that Icon expresses a piston with a dish as a positive value making this a dished piston. I concede that this would not be considered a domed piston as I previously mentioned

"Piston Head Volume and Deck Clearance. Values for Piston Head Volume and Deck Clearance can be entered as both positive or negative values. Negative numbers indicate a reduction in cylinder head volume. Pistons with pop-ups or pistons sticking out above the deck will reduce chamber volume and will usually require a negative number. Pistons with a dish or pistons below deck increase chamber volume and receive a positive value. Located above each picture in the KB Catalog is the value for the piston head volume shown in positive or negative numbers."

While I was wrong in this instance, the below quote from Summitt.com has a note at the end stating some manufacturers use the "+" and "-" values opposite of how Icon does. My mistake was assuming that all piston manufacturers used the same language in describing their products.

"Pistons are available with different head volumes. This is the size of the top of the piston. It is listed in cubic centimeters (cc).

We list the head volume in the way it affects the size of the combustion chamber. This helps when calculating the compression ratio. For example,

A dished piston will increase the size of the chamber. So it has a positive volume (+).
A domed piston will decrease the size of the chamber. So it has a negative volume (-).
A flat top piston has zero volume (0).
- A flat top piston with valve reliefs may list a small positive volume.

NOTE: This method of listing head volume is NOT standard in the industry. Some manufacturers may list a negative value for a dish and positive value for a dome."

Iroc78 · 2026-02-17T08:21:14-0600

Yes Bigsbetter you are 100% correct. RemCharger and BSB67 are also correct, apparently Icon expresses their negative piston head volume (flat top with valve reliefs) as a "+" number that you need to plug into the compression calculator as a "+".

Some manufactures will express a dome, or positive piston head volume with a "+" number that needs to be entered into the compression calculator as a "-" which is what I was used to seeing.

Sorry to muddy the waters on anything for you.

BSB67 · 2026-02-17T08:24:34-0600

it’s been like this forever. It’s up to the user to understand how and what to use when calculating compression ratio.

Up next: cams, and what is “straight up”?

33 IMP · 2026-02-17T09:54:49-0600

Bigsbetter said:
As of right now, I’m leaning toward the Speed Pro 2355. Get me somewhere in the 10.2, -10.4 or so

How much cam are you planning?
That's more compression than I would run on pump 91, without at least a 250° at .050 cam.
For 225-230°, I'd shoot for 9.5.

BSB67 · 2026-02-17T10:34:30-0600

33 IMP said:
How much cam are you planning?
That's more compression than I would run on pump 91, without at least a 250° at .050 cam.
For 225-230°, I'd shoot for 9.5.

I get 10.3 CR on an uncut block with a 0.040” gasket.

IMO, that is on the edge with everything perfect. Intake closing point will matter, as well as ever other detail.

The 2266 position is at about 9.5 CR

Trying to decide which pistons for 440 build

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