Flycutting cast pistons.

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Valvebounce

Valvebounce

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Has anyone done this?
I have a .030 over 318 that I was given. (Saved it from getting scrapped) Upon pulling it down, I found it was quite decent inside. (Pistons don't look very old at all)
I've since picked up an XE274 for it, and an LD4B. And I'll be using a pair of 675 heads with 1.6, and 1.88 valves. (Just what I had laying around)
Anyway, I've read that some 318's can have the piston as far as .080 below the deck, depending on the year. As these are replacement pistons of unknown origin, I'm not sure where they'll sit. (I never thought to check before pulling the engine down) So I'm planning to mock the engine up, and see where they're at.
Now, depending on how big the chambers turn out to be when I measure them, I'd like to have the decks machined to zero deck height. But with the XE274, I'm mindfull of valve to piston interferance. If need be I'll just pluck out the slugs and flycut some valve reliefs into them. But as this is usually done on HiPo forged pistons, I have some reservations about the chance of this compromising the strenght of the pistons. (Perhaps I'm just being paraniod?)
Have any of you guys ever done this with casties before? The whole build is a very low-buck endevour, (Like probably under a grand NZD total) so things like new pistons just don't come into the equation.
 
IMO, I'd bite the bullet and get new pistons, however, fly cutting the OE slugs depends on there thickness. Then, machining valve reliefs into them. IDK abot that. Doesn't sound good to me.

Also, IDK if the block can take a .080 cut to meet a zero deck height with the OE slugs.
 
I don't know that that is the deck height yet. But I hear what you're saying. What is the max safe cut that a 318 block could safely take in your opinion?
The chamber volume will also dictate the feasibility of running zero deck too. I do want nice stout static compression, but I also don't want a knock happy pig on pump gas. (I've driven a family members 340 which hates modern fuels with a vengeance)
However I seem to read in threads time and time again that the 318 was afflicted with some dismally low CRs from new.
 
I wouldn't cut the pistons. You won't be able to deck the block that much, I think the most I've heard of anyone cutting one was .030". Not to say you can't take more, but keep in mind if you cut that much you're going to need new pushrods, and may run into valvetrain geometry issues if you really start cutting on things.

How much compression are you looking for? 9:1? 10:1?

Have the chambers on the heads cc'd. The 675 heads ran smaller than some of the later ones, usually they're around 60-63cc's. Once you know the size of the chambers, you can start figuring out what your compression will actually be. Then you'll have to figure out how deep the pistons are in the bores. My '71 318 measured out with the stock pistons .090" down in the bores, I've heard of as much as an 1/8" (.125).

But, even with the stock flat tops, if your chambers are close to the actual spec (60cc's), you can run a thin head gasket (mopar is now making a .024" thick, 4.060 diameter head gasket, Mancini has them). Even with the pistons at .090" in the block, with a 60cc chamber and a .024" gasket the compression should be around 8.9:1. Not super, but not horrible. Shave a few thousandths off the heads and you can be at 9:1.

But as I mentioned, keep in mind as you cut the deck or the heads that if you make significant cuts, you're going to need new pushrods. Not sure about there, but a custom set of pushrods costs around $200 here. A new set of KB167's, which have a compression height of 1.81" vs the stock 1.741", runs around $277. And unless you're getting the machine work done for free, when you add the price to deck the block or shave the heads, you might actually be money ahead just buying the new pistons.
 
Thanks for your input 72bluNblu. I've got an engineer at my disposal who can make a custom set of pushrods for free. The motor I burgled the 675's with uprated valves from had custom pushrods already, to accomadate the re-ground can that engine had.
 
I've fly cut cast pistons before; so far have not had a problem. The build was a 318, cast rebuilder Sealed Power pistons .055 down in the hole, 360 heads angle milled to 45.5 cc, cam was a custom solid from a local circle track racer with .525 lift Int, .556 lift Exh, 248, 252 degrees duration at .050 lift. I run it like I stole it!!
 
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