Choosing piston

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Idaho

Idaho

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From another thread, I asked how the machine shop decides on a piston. We've determined I need somewhere in the 23 to 25 cc dish to arrive at 9.5 to 10:1 compression on a 408 stroker. He wants pistons there before boring to 0.30 over.

Got this reply and I understand it except how is deck height after milling determined without pistons? I've seen technical ways but do they actually do it?

MOPAROFFICIAL said:
By compression height based on deck height..then chamber volume piston head volume..head gasket volume...then ica...then consider head flow.
Click to expand...
 
Idaho said:
From another thread, I asked how the machine shop decides on a piston. We've determined I need somewhere in the 23 to 25 cc dish to arrive at 9.5 to 10:1 compression on a 408 stroker. He wants pistons there before boring to 0.30 over.

Got this reply and I understand it except how is deck height after milling determined without pistons? I've seen technical ways but do they actually do it?
Click to expand...

"Exact" is like within .006 lol. Jk
Compression height/pin height.
And that's based on a rod length being blue printed, a stroke blue printed, and an OEM deck height being blueprinted.... aka perfect.

There is always a hair variable.
So the rod is .002 shorter.... and the deck is taller by .008..max and .003 at the small end.. but the machinist just squared it and took .007 at one end and .002 at the other.. and the mains were line honed... etc... it's a stacking of clearances
 
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Deck height of the block is determined by measuring the distance from the top of the block to the centerline of the crank journals.

To lower the deck height for a zero deck piston height, the machinist needs the piston to determine the piston height. The height is or should be always listed by the piston maker. As with above, the height is measured by going from the centerline of the Pistons pin to the top of the Pistons fiat spot on top, not the dome if equipped. This is also measure along with the rods to be used.

The machinist could measure everything or he could bore the block and fit the Pistons to the rods and then to the crank and rotate it around and measure the distance left above the piston.

I like the second method with all 8 Pistons in the block. This will quickly show if the deck is what it should be, even, flat, level, un-twisted, tilted, warped, bowed or bubbled.
Other machinists will square up the block (you must request this.) and go from there.

I have spoken with a few ace machinists and it seems to be what ever they like.
 
There is a Scat 408 kit with a 20.5 cc dish piston . That should get you there with the correct head gasket.
 
Idaho said:
Got this reply and I understand it except how is deck height after milling determined without pistons? I've seen technical ways but do they actually do it?
Click to expand...
Use a long enough gauge and a reference in the mains.... BLOCK DECK HEIGHT GAUGE | eBay

But as said..... other items will very probably need adjusting/swapping-around along the way to get a tight tolerance of piston-to-deck height.
 
rumblefish360 said:
Deck height of the block is determined by measuring the distance from the top of the block to the centerline of the crank journals.

To lower the deck height for a zero deck piston height, the machinist needs the piston to determine the piston height. The height is or should be always listed by the piston maker. As with above, the height is measured by going from the centerline of the Pistons pin to the top of the Pistons fiat spot on top, not the dome if equipped. This is also measure along with the rods to be used.

The machinist could measure everything or he could bore the block and fit the Pistons to the rods and then to the crank and rotate it around and measure the distance left above the piston.

I like the second method with all 8 Pistons in the block. This will quickly show if the deck is what it should be, even, flat, level, un-twisted, tilted, warped, bowed or bubbled.
Other machinists will square up the block (you must request this.) and go from there.

I have spoken with a few ace machinists and it seems to be what ever they like.
Click to expand...

Thanks, its making more sense now. I kinda think he wants to do the install and measure pistons method.
So he orders pistons based on calculations including head cc, and uses pistons to properly deck the block.
He wants to order parts rather than me supply them. We just have to negotiate the parts price, he's kinda high compared to what I see online depending on the piston cost.
 
mbaird said:
There is a Scat 408 kit with a 20.5 cc dish piston . That should get you there with the correct head gasket.
Click to expand...
With a 0.040 gasket I was around 10 with 23 cc pistons. 20.5 seems low. Also when I spoke to Summit, their dish pistons do not have the valve reliefs I want.
 
I am running that Scat kit and the Icon pistons do have valve reliefs.
 
their dish pistons do not have the valve reliefs I want.
let's get on the same page
what do you want?
Trough instead of individual
???
 
mbaird said:
I am running that Scat kit and the Icon pistons do have valve reliefs.
Click to expand...

Do you happen to know the part number?

Wyrmrider said:
their dish pistons do not have the valve reliefs I want.
let's get on the same page
what do you want?
Trough instead of individual
???
Click to expand...

The problem I've had is very few of the ads for stroker kits give piston part numbers, or details like valve reliefs.
I called Summit. He said none of their dish pistons for SCAT kits have valve reliefs. Then he said all their SCAT kits are for LA engines, not recommended for Magnums. Wrong rep to talk to I guess.

Assuming my block will be shaved about .010 with 61 cc chambers, .040 gasket, we have calculated that I'll need something in the 23 - 27 cc piston to stay in the 9.5 to 10 CR.
Planning hypereutectic, unless forged are within $100 or so.
I'll have more conversation with the machine shop before an order is placed. I'm just trying to know what's available in advance.
 
If he is right and I am wrong then I am screwed ! Cuz I am running .648 lift ...
But it seems to run just fine so far ...No collisions between valves and pistons in the 100ft I have driven it. Lol
 
image.jpg
 
Idaho said:
The problem I've had is very few of the ads for stroker kits give piston part numbers, or details like valve reliefs.
I called Summit. He said none of their dish pistons for SCAT kits have valve reliefs. Then he said all their SCAT kits are for LA engines, not recommended for Magnums. Wrong rep to talk to I guess.

Assuming my block will be shaved about .010 with 61 cc chambers, .040 gasket, we have calculated that I'll need something in the 23 - 27 cc piston to stay in the 9.5 to 10 CR.
Planning hypereutectic, unless forged are within $100 or so.
I'll have more conversation with the machine shop before an order is placed. I'm just trying to know what's available in advance.
Click to expand...
Your dish cc numbers stand up with piston-deck clearance right at .000" for the SCR range you want.

Usually the pistons can be deduced from the SCAT data on particular stroker kits and perusing the catalogs. And they can tell you what they are.

Which block? Magnum or LA?
 
nm9stheham said:
Your dish cc numbers stand up with piston-deck clearance right at .000" for the SCR range you want.

Usually the pistons can be deduced from the SCAT data on particular stroker kits and perusing the catalogs. And they can tell you what they are.

Which block? Magnum or LA?
Click to expand...
Thanks for the verification. Magnum block.
 
Maybe you mentioned already ... but what heads are you running ?

Also remember we are at 2800ft + here in ID
 
Altitude... meaning that you should run a bit more compression ratio to make up for the lower cylinder filling pressures. Just be aware that if you drive down in altitude, it is going to work against you, or the next ownrr.
 
nm9stheham said:
Altitude... meaning that you should run a bit more compression ratio to make up for the lower cylinder filling pressures. Just be aware that if you drive down in altitude, it is going to work against you, or the next ownrr.
Click to expand...
That would be correct but we have to drive quite a ways to get to sea level from Boise.
 
rumblefish360 said:
I have spoken with a few ace machinists and it seems to be what ever they like.
Click to expand...

Rob, what local machinist do you recommend to bore a 360?
I want to go from .020 to a .030 bore.

Can I get that result just by honing?

Preferably one with a SB deck plate
Thanks
 
The machinist did say he found a piston that would put it around 9.7. The guy at Cleggs suggested the 23 cc piston saying he’d run “10:1 all day long”. I think quench is closer to ideal with 10:1 as well.
Makes me a little nervous. I know I can always go to a thicker gasket.
I hadn’t thought of the altitude. Maybe 10:1 is the way to go especially for more low rpm torque.
 
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