Piston "Compression Distance"

[Trevor B]

Please explain piston height numbers and basics to me.
Summit lists the “compression distance” - what exact measurement is this?
For example:

Sealed Power Cast Pistons 526AP30 are 1.745”

Keith Black KB167 are 1.810”

So does that mean that the KB are 0.065” closer to the deck than the Sealed Power?

The goal is to get a little compression increase on a 1983 318 block if possible ('302 heads, were milled down 0.008" for flatness), while boring 0.030 over. The machinist wants to have the pistons in hand before doing the boring. Is there any danger of putting in too tall of pistons with either of these?

Are there any other taller pistons that will work that are more in the Sealed Power price range?

Thanks!

 

[mguner]

Yes you are correct. The bigger the CD number the taller the piston is from the wrist pin. I haven't shopped the small block pistons lately.

 

[pishta]

generally the compression height is from pin centerline to piston crown. So yeah, the KB's are 65 thou taller. The 302 head is a closed chamber head so if you get it to 0, you can still run the quench of the head gasket. Larry Shepard showed a 273/4, 318 and 360 piston mounted on one long pin showing the 3 different heights.

 

[mguner]

The taller CD is not "always" higher compression depending on the valve reliefs or negative dome volume but usually.

 

[MileHighDart]

I've been piston shopping for a couple weeks, and reading every thread I can find on 318 pistons. I think the kb's are your only choice if you want something like 1.810 compression height.

 

[nm9stheham]

Please explain piston height numbers and basics to me.
Summit lists the “compression distance” - what exact measurement is this?
For example:

Sealed Power Cast Pistons 526AP30 are 1.745”

Keith Black KB167 are 1.810”

So does that mean that the KB are 0.065” closer to the deck than the Sealed Power?

The goal is to get a little compression increase on a 1983 318 block if possible ('302 heads, were milled down 0.008" for flatness), while boring 0.030 over. The machinist wants to have the pistons in hand before doing the boring. Is there any danger of putting in too tall of pistons with either of these?

Are there any other taller pistons that will work that are more in the Sealed Power price range?

Thanks!

The 526's are pretty much all I have ever found that are that tall (around 1.745" compression height, or distance) in a low cost cast piston. I've looked. The KB's and similar are pretty much 'it'.

Here is a good summary page from Campbell Enterprises. Better prices on KB's BTW.

If you use the KB's, they and their pins will be considerably lighter than the stock cast parts.... like 753 grams for stock piston and pin, versus 658 grams for the KB pin + piston. So plan on a rebalance of the crank, but at least it will be taking weight off, so no Mallory metal to add.

IMHO, you are wise to work for more compression ratio. It'll sure help your low RPM torque and broaden the torque band. You're gonna boost your CR by more than a point with the KB's and get up into the mid 9 range. Plus get some small quench benefit.

 

[pomonamissel]

just built a 340 for street , used kb's put them .02 in the hole for todays gas here in calif . your the same stroke , we used 75-76 iron heads 1.94 in 1.6 ex small cam 110 lc , just a sweet street driver . it pulls from idle up to 5k . fun with that 4spd .

 

[MileHighDart]

The 526's are pretty much all I have ever found that are that tall (around 1.745" compression height, or distance) in a low cost cast piston. I've looked. The KB's and similar are pretty much 'it'.

Here is a good summary page from Campbell Enterprises. Better prices on KB's BTW.

If you use the KB's, they and their pins will be considerably lighter than the stock cast parts.... like 753 grams for stock piston and pin, versus 658 grams for the KB pin + piston. So plan on a rebalance of the crank, but at least it will be taking weight off, so no Mallory metal to add.

IMHO, you are wise to work for more compression ratio. It'll sure help your low RPM torque and broaden the torque band. You're gonna boost your CR by more than a point with the KB's and get up into the mid 9 range. Plus get some small quench benefit.

Ok, I'm a little confused on the balancing thing.
It was my understanding that on an internal balance engine, like a 318, the crank should balance on its own, same for the vibration dampner and the flexplate. So as long as all 8 pistons weigh the same, and all 8 rods weigh the same, what difference does installing lighter pistons make as far as balancing the crank?
Can someone school me on this?

 

[70aarcuda]

the crank journals are balanced based on the weight of the pistons and rod combo...there is a formula ...

http://www.hotrod.com/how-to/engine/ccrp-0803-engine-balancing/

 

[MileHighDart]

Thanks for the link, good article.

 

[Trevor B]

Great info - thanks!
I went with the Sealed Power so I wouldn't have to deal with re-balancing/light KB pistons.
Hopefully the Sealed Power weigh about the same as the stock pistons.