KB243 Hyper Piston Confusion

[Siksty7 Dart 270]

Before I go inserting my foot in my mouth responding to a UEM Pistons representative, can someone tell me if I'm wrong here? I sent UEM an email asking if the compression height could be reduced by machining a KB243 piston shorter to achieve zero-deck clearance under a closed chamber cylinder head. Here's the response I received: "The deck of KB243.040 sits .012" below block deck @ TDC, on a stock LA/340 block. The .030" dome portion of the piston can safely be milled completely off , for use with a closed chamber head."

By my calculations 6.123(rod)+1.655(half stroke)+1.84(piston compression height)=9.618 assembly length. This is above the deck even with the UEM catalogs suggested OEM deck height of 9.599. Even the catalog indicates the piston is above the deck in that case. See screenshot attached.
My block before milling to flatten is 9.586". I was looking for a zero deck piston option for a street 340 without going to forged pistons.
Did the UEM rep look at a different piston, or am I thinking of this wrong?

 

[Siksty7 Dart 270]

 

[PRH]

Seems pretty clear to me that the piston would be sticking out .018” if the block were 9.600”.

 

[340Dart]

I have these pistons in my 340. They were .018 above the block.

 

[72bluNblu]

Before I go inserting my foot in my mouth responding to a UEM Pistons representative, can someone tell me if I'm wrong here? I sent UEM an email asking if the compression height could be reduced by machining a KB243 piston shorter to achieve zero-deck clearance under a closed chamber cylinder head. Here's the response I received: "The deck of KB243.040 sits .012" below block deck @ TDC, on a stock LA/340 block. The .030" dome portion of the piston can safely be milled completely off , for use with a closed chamber head."

By my calculations 6.123(rod)+1.655(half stroke)+1.84(piston compression height)=9.618 assembly length. This is above the deck even with the UEM catalogs suggested OEM deck height of 9.599. Even the catalog indicates the piston is above the deck in that case. See screenshot attached.
My block before milling to flatten is 9.586". I was looking for a zero deck piston option for a street 340 without going to forged pistons.
Did the UEM rep look at a different piston, or am I thinking of this wrong?

View attachment 1716483397

The UEM rep is correct for the KB243's.

I have the same pistons in my 340, and spoke to UEM 15 years ago on the same topic. The top of the piston is .018" over the deck. But that's to the top of the dome! The dome itself is .030" tall, measured from the flat part or shoulder of the piston (above the ring lands). That puts the shoulder of the piston .012" below the deck. If the piston were perfectly flat, that's where it would be. Since it's not flat, the top of the dome is .018" over the deck.

I had contacted them about the compression ratio, because I was getting conflicting numbers. This was their response (15 years ago!)

"the protrusion has to be treated like it is a dome so the piston is down in the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all equates to positive 7.3cc's.Use this to calculate your compression ratio"

 

[MOPAROFFICIAL]

I have these pistons in my 340. They were .018 above the block.

Yep. And if you deck your block the proper amount just to square it, the edge of the Piston will be zero and the actual raised flat surface positive deck will be about .024 above deck like my 340.

Blocks may vary of course.. ;)

 

[Siksty7 Dart 270]

The UEM rep is correct for the KB243's.

I have the same pistons in my 340, and spoke to UEM 15 years ago on the same topic. The top of the piston is .018" over the deck. But that's to the top of the dome! The dome itself is .030" tall, measured from the flat part or shoulder of the piston (above the ring lands). That puts the shoulder of the piston .012" below the deck. If the piston were perfectly flat, that's where it would be. Since it's not flat, the top of the dome is .018" over the deck.

I had contacted them about the compression ratio, because I was getting conflicting numbers. This was their response (15 years ago!)

"the protrusion has to be treated like it is a dome so the piston is down in the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all equates to positive 7.3cc's.Use this to calculate your compression ratio"

Now that makes sense! Thank you!

 

[RustyRatRod]

The UEM rep is correct for the KB243's.

I have the same pistons in my 340, and spoke to UEM 15 years ago on the same topic. The top of the piston is .018" over the deck. But that's to the top of the dome! The dome itself is .030" tall, measured from the flat part or shoulder of the piston (above the ring lands). That puts the shoulder of the piston .012" below the deck. If the piston were perfectly flat, that's where it would be. Since it's not flat, the top of the dome is .018" over the deck.

I had contacted them about the compression ratio, because I was getting conflicting numbers. This was their response (15 years ago!)

"the protrusion has to be treated like it is a dome so the piston is down in the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all equates to positive 7.3cc's.Use this to calculate your compression ratio"

This right here ^^^^ is your answer. You do not measure deck clearance from the dome of the piston. You measure from the FLAT of the piston head, which, in this case is the very small lower area all the way around the perimeter of the piston top. THAT is where you measure from. So the UEM rep is 100% RIGHT.

 

[RustyRatRod]

Those are nice pistons, too. If you're using an open chamber head like stock, I don't think I'd mill the dome off.

 

[72bluNblu]

Those are nice pistons, too. If you're using an open chamber head like stock, I don't think I'd mill the dome off.

For an open chamber head you definitely wouldn't want to do that. I run 65cc open chamber 308's and that puts my compression ratio at about 9.8:1.

But the OP is talking about using a closed chamber head, so he'll need to check for interference. The "zero deck" thing is a confusing way to look at those pistons, because if you zero decked the block the majority of the piston would be .030" over the deck. If you cut the piston .018" so the top of the piston is even with the deck, it's not really a true zero deck because you still have that outer shoulder section that's .012" below the deck.

 

[Siksty7 Dart 270]

Right on guys. If they listed them as "domed pistons" and not flat tops I might not have gotten crossed up. Yeah I'm thinking of using Magnum heads on a 340 block, going hydraulic roller cam with retrofit lifters, tapping and plugging the oil galley holes below the deck.

 

[PRH]

SRP 142068 are shown as 1.804” C/H

 

[72bluNblu]

Right on guys. If they listed them as "domed pistons" and not flat tops I might not have gotten crossed up. Yeah I'm thinking of using Magnum heads on a 340 block, going hydraulic roller cam with retrofit lifters, tapping and plugging the oil galley holes below the deck.

Yeah it screws up all the compression calculators too if you're not very specific about how you input everything, which was the issue I was having. Once you treat them like domed piston everything else makes sense.

That's a good plan as far as the heads and roller cam, although those might not be the best pistons for that particular application. If you can have them cut relatively inexpensively that would be fine, you'd have to look at if you can get a true flat top or not cheaper than having the 243's cut for clearance.

 

[PRH]

The UEM rep is correct for the KB243's.

I have the same pistons in my 340, and spoke to UEM 15 years ago on the same topic. The top of the piston is .018" over the deck. But that's to the top of the dome! The dome itself is .030" tall, measured from the flat part or shoulder of the piston (above the ring lands). That puts the shoulder of the piston .012" below the deck. If the piston were perfectly flat, that's where it would be. Since it's not flat, the top of the dome is .018" over the deck.

I had contacted them about the compression ratio, because I was getting conflicting numbers. This was their response (15 years ago!)

"the protrusion has to be treated like it is a dome so the piston is down in the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all equates to positive 7.3cc's.Use this to calculate your compression ratio"

Seems like some of the confusion stems from the “flat top” description…….and the C/H being listed as “1.840”.
If the portion of the piston sitting below the deck .012” is .030” lower than the top of the piston, it would measure 1.810” to the pin C/L.

On a stock 9.600” block, 1.84” C/H ends up sticking out .018”.

 

[72bluNblu]

Seems like some of the confusion stems from the “flat top” description…….and the C/H being listed as “1.840”.
If the portion of the piston sitting below the deck .012” is .030” lower than the top of the piston, it would measure 1.810” to the pin C/L.

On a stock 9.600” block, 1.84” C/H ends up sticking out .018”.

You would certainly know better than me, I'm no engine builder.

The 243's are supposed to be for the factory "10:1" 340's that originally had the pistons .018" over the deck from the factory. At least by the spec, anyway. So the later smog 340's with lower factory compression are supposed to use a different piston. I'm not really sure why the 243's are listed as flat tops, or why the rest of the published specs don't line up with the way that you actually need to do some of the calculations if you were going to actually treat it like a flat top. But that mismatch in description and actual specs/measurements has been around for a long time.

 

[PRH]

Just one of countless “typos” one can encounter when looking thru parts catalogs.

 

[JDMopar]

If you're using closed chamber heads, thumb on back thru the UEM catalog to the Icon pistons. You'll get a double bonus with getting forged pistons that have the modern ring pack instead of the old giant rings. They should have a better selection in the Icon section of the catalog.

 

[Dart Frog]

I've have ran those 243 pistons for years with closed chamber heads on 1970 340 block that has been surfaced though I don't know how much. I can't tell you that with a multi steel shim head gasket Of.051 my quench came to .032 which I feel is just right.