I had a question about pistons, hopefully you guys can answer. I am looking for new pistons for my 71 LA 318 small block. Since I need new pistons in my rebuild, I am looking at raising the compression. I am looking at these two pistons, first two Keith Blacks and third Speed Pro:

1. Silv-O-Lite Hypereutectic Pistons 3234HC-030
2. KB Performance Pistons KB167-030
3. Speed Pro Hypereutectic H814CP30

Here are the differences: #1 gives about .5 more compression, is about $160 cheaper over #2, has a coated skirt, flat top, listed for 85 and up engines, and uses metric rings. I talked to Summit, they said it doesn't matter the year of the block and can use them but are looking into which rings I would use (none are listed by their venders and SEMA is happening, everyone in Vegas).

I asked to make sure I wouldn't have an issue with the valves clearing since no valve reliefs and even mentioned that I want to use a roller cam with .515 lift (Lunati Voodoo #711, Hughes roller lifters with bar and lowered oiling hole). They told me shouldn't be an issue. My heads I plan on using are the Sidewinder/Promax alums.

Speed Pro pistons, #3 would allow me to use non metric rings but are also listed for the new vehicles.

Compression distances for each: #1. 1.741, #2. 1.810, #3. 1.755. I am not sure exactly what this number means.

Using the compression formula and a .042 head gasket, that puts me at 10-1 compression for #1. I am using 0 on the deck clearance as I do not know what that stock deck would be. This is will be street cruiser, no racing.

Do you guys see any problems using those pistons/ that compression ratio for what I want to do with the car? I realize I can go with a thicker gasket to take out a little compression, or a thinner one to add some.

Also, which type of rings should I get, moly or ???????

Thanks

If you want the engine to last, moly rings are a no-brainer. You do have to have the honing done with moly ring spec roughness.

Compression height is the distance from the centerline of the wristpin to the top of the piston, not counting any dome or dish. It is useful in calculating things like distance below or above the deck.

Your '71 318 has a deck height of 9.600". That means from the centerline of the crank to the top of the block is 9.6". Later LA and Mag engines used a slightly shorter deck, 9.577, to make up for the change in head gaskets used. Let's do some math: The height of the stroke/rod/piston usually equals or is less than the block deck height. Stroke is 3.31, half that is 1.655. Rod length is 6.123. Piston #1 has comp height of 1.741. 1.741 + 6.123 + 1.655 = 9.519" Piston is 0.081 below deck.
Piston #2 has comp height of 1.810.
1.810 + 6.123 + 1.655 = 9.588". Piston is 0.012" down in hole.
Piston #3 has comp height of 1.755".
1.755 + 6.123 + 1.655 = 9.533". Piston is 0.067" down in the hole.

There is really no reason to use the 1.741" comp height piston. We sweep that off the table. There are also pistons with 1.72" comp height, those don't matter either.

That brings us to the KB167 and Sealed Power H814CPs.

The pistons I'd like to use are the KB167s. They are lighter and have valve reliefs (have to, because they run so much closer to the deck.) If you were going to run aluminum heads, they'd be a no-brainer. But you might be considering using your stock heads, which have about a 71cc open chamber. Using a 0.040" head gasket you have 12 cc plus the head, which adds up to 83cc, plus 5cc for the valve pockets makes it 88cc. A 30 over 318 cylinder has a swept volume of 661.6 cc.
(661.6 + 88)/88 = 8.52:1 Not too bad for an open chamber iron head.

But there's another way to roll and it's the H814cp. With this piston you use the 302 heads from a late '80s Fifth Avenue, and a Mr. Gasket 1121G thin head gasket. The 302 heads are 62cc, the gasket is 6.2cc, the volume above piston but below deck is 13.4cc. So total volume above piston is 62 + 6.2 + 13.4 = 81.6cc.
(661.6 + 81.6) / 81.6 = 9.1:1 compression. That's very doable with the 302 heads' heart shaped chambers. That's what I'd pick. Why? The H814CPs are a stock replacement piston and can be used without rebalancing. They are available on Ebay for $132.00. The KBs run a minimum of $320.00 from Campbell or Hughes.
The KB pistons weigh about 90 grams less than the 814s, and though that's not bad, it means that besides the $190 more you pay for the KBs, you really should rebalance the engine at another $200. Then too, either set of heads will need to be gone through, and the 302 heads already have a hardened exhaust seat area. This has to be added to the '71 heads, meaning more cost to use them.

Because the 814s are down in the cylinder a ways, the valves won't run into them with the cam you're thinking about.


My story: My last engine was an '86 318. I kept the stock pistons, which have a comp height of 1.745, so on my late block they were down 0.055". I used the thin head gaskets and the stock 302 heads and it'd run all day on regular unleaded. My heads didn't need much work, I checked all the valve seats and guides, added new valve stem seals and new springs, and then I went wild with the grinder in the ports but you don't have to do that.
Using a Holley Street Avenger and a cast iron 360 intake I was surprised when the first tank of fuel mileage increased from Poly 318s 15 to 20!

So that's the story. Good luck on your build.

R.

Thanks for that dogdays. I do want to run the alum heads. Would you still want to go with the Speed Pros and a real thin head gasket? Or should I go with the 167s? The extra cost of the pistons plus rebalancing is almost half the cost for the new heads.

What Dog said, & have your shop rigid hone it for moly rings then I would hone it with a dingle berry hone (brush research manufacturing GB41832) & some good quality file fit plasma moly rings. Get a hand ring filer. there are some good utube vids on easy ring prep.

The head gasket I mentioned is the thinnest head gasket you can buy anywhere.

My thinking about compression ratios is the stock MOPAR open chamber 8.5:1, 302 heart-shaped chamber 9:1, aluminum head with closed chamber, 10.5:1.

I don't know what heads you are getting for so little, they must be ProComp/Speedmaster. These are starting to pick up support as quality of the castings seems to be improving. They are sold in pairs, most of the other heads are sold each. So Edelbrocks will run you around $1400. The heads should have the stem clearance checked and the valve job checked before you put them on.

When you do this, I think the KB 167s are the winner. They're a lot lighter, which will make the engine more responsive, and they optimize your compression ratio and squish. The 30 over 318 is 661.6cc, to get 10.5:1 compression, 661.6/9.5 = 69.6cc above the piston The valve notch is 5cc, the head gasket and down in the hole volume is 8.6cc, adds up to 13.6cc. 69.6 - 13.6 = 56cc which is the ideal chamber size. Speedmaster smallest head is 65cc. 65+13.6 turns into 9.4:1. It'll run on Regular, maybe midgrade.
The 814 slugs would be somewhere around 8.3:1. That's nowhere.

This engine will sing. High flowing heads, lightweight pistons, decent compression, all factors that help the engine feel lively. If you had an extra $275, a set of aftermarket connecting rods could take another 100 grams off each piston/rod combo. Hey, I'm going to work you into a $3000 engine if I'm not careful.

One nice thing about the KB pistons, they require a larger end gap on the top ring, so you don't have to try to find 0.005 oversize rings so you an file fit them

American gearheads who are used to the old V8s don't realize that with good flowing heads you don't need a lot of intake duration. I'd stick around 210 at 50 lift.

Don't hesitate to PM me with more particular questions.

R.

PS: A Magnum type head would solve some problems, 58cc chamber.
PPS: The skinny 318 rods weigh in at 726 grams, the fatter 496 or 645 rods a portly 758. I believe your engine still had the skinny shank rods.

My sons dart has the KB 167 pistons in it. Has edelbrock heads with .040 shaved from the surface. Comp ratio 10.5 to 1. Runs on 93 pump gas. 11.60 at 115mph. 254*--.520" racer brown solid flat tappet cam. Stock cast crank and stock 318 rods. Drive it anywhere. Been seriously abused for the last 8 years and still runs perfect

Try Autotec pistons...they can custom build what you need... rings, comp. ht., etc.

http://racetecpistons.com/pages/autotec.php

KB hyper domed 318 slugs work well. They were only .006 in the hole in my 85 roller block, not worth decking for that.

Slightly worked 340 floating rods and it balanced still lighter then the oem 318 bw.