Shtstr

Ls aluminium block expansion?
Had an engine builder tell me that when setting up for zero deck height on an ls aluminium block. It is best to set the piston height .010 thou out the bore.
This is for when the block and heads reach operating temperature it will grow in height by .010.
has anyone actually seen this and can confirm that is correct?
Given aluminium expands more than iron I think he might be right.
But have never head anyone else ever say this before
🤔🤔🤔🤔🤔

RB04Av

Depends on ALOT of different things... not least, how much clearance there is from the piston to the head, and the overall deck height... but yes, in a general way, he's right. I don't know the specific # for a LS block but .010" seems to largely pass the smell test. Same principle as setting valve lash on solid lifters for the same reason.

For the record, this is just one more reason why quoting compression ratio to more than 1 decimal place, is laughable and a sign that someone doesn't understand what they're doing. It CHANGES as the engine temperature changes.

MuhThugga

From the factory, the piston on an LS is out of the hole by .005" - .006".

grinder11

iTrader: (1)

Since the pistons are also aluminum, they also expand in every direction. I doubt very seriously that the .010" out of the bore is there for expansion purposes only......

RB04Av

True... and the factory is VERY conservative on that sort of thing. Not at all like some of us, going for broke. Furthermore they left it the same for the iron blocks, so I doubt that expansion figured into their reasoning very much.

As a point of reference, the old N* engines had their pistons proud of the deck by a similar amount. Those were also aluminum blocks.

True; but since expansion is a percentage / fraction / ppm or that sort of thing per °, i.e. in proportion to the overall size of an object, the expansion of a 1" tall (or whatever) piston in absolute .001"s of an inch, is MUCH less than that of a 9" tall block.

grinder11

iTrader: (1)

That is correct. But I still believe expansion played very little to no part in the pistons protruding above the deck.....

RB04Av

Depends.

It may have played little or no part in the DESIGN part of it. Butt that doesn't stop the REALITY. The main driver of piston height is the rod length. Rods (well, most, anyway) are some iron alloy; some kinda steel. An aluminum block is ... aluminum. Aluminum expands about twice what typical steels do; low 20s ppm per °C for aluminum, compared to 11ish for various steels. There's around 5 - 6 times as much ROD in the total "height" of the rotating assy, than there is PISTON. Meaning, the rods DOMINATE the change that the engine undergoes over its thermal cycle. Meaning, in an aluminum block, the block grows MUCH more than the rods, meaning the pistons end up getting SWALLOWED by the block as it gets MUCH larger than the length that the rods grow to.

Again, what the factory did, is not that important. Their goals aren't necessarily the same as ours. (duh ... otherwise we'd all have totally bone-stock factory motors, eh???) Things that they did for mass production reasons, while of course TOTALLY valid and proper on their own ground, DON'T apply to hand-built "blueprinted" engines, specialized for their particular purposes. So, whether in the design, "expansion played very little to no part in the pistons protruding above the deck" or NOT, in the real world of OPTIMIZATION, it DOES. It's one of many tools we can use to our advantage, or that if we fail to account for it, can kill our project or make it go sideways or just simply fail to live up to expectations, in ways we aren't prepared to understand.

I can't tell you how many motors over the years I've built that were the EXACT SAME bill of materials as the guy in the other lane, but my motors outran or outlasted theirs. Like the old Edelbrock RPM "kit" for SBC that was rated at 430 HP or whatever it was; butt about 90% of all street-strokes that bought it and "installed" it, barely broke 275. Why??? Attention to details. Deck height, optimum cooling, absolute consistency from one cyl to another, and so on.

So, yeah, there's method in the madness of deciding how much negative deck clearance WE WANT, as opposed to (or in some cases, in accordance with) "what the factory did", to get the results WE WANT. Much like bearing clearances, ring gap, valve lift, and so on.

Dian

pistons get hotter.

G Atsma

They do, but don't change much in the vertical dimension from the pin to the crown, which directly affects out-of-the-hole dimension, which is the focus of this discussion.

grinder11

iTrader: (1)

If the pistons had to protrude above the blocks deck for expansion reasons, every aluminum block aftermarket performance engine would have the same protrusion. Some do, some don't. My LS7s Diamond flat tops do not. I see your reasoning. But I personally think that when the engines were designed, the .005"-.010" above the deck happened due to a possible mistake. There would be easier ways to compensate for "block growth." Like a .005"-.010" thinner head gasket. I dont have the facts to prove anything on the following legendary rumor that has been floating around here in Michigan for over a half century. But legend has it that the reason the 396 BBC was morphed into the 402 BBC was due to an error in machine setup. Supposedly the 402 came into being due to many hundreds of blocks being accidentally bored .030" oversize. By the time the mistake was caught, there were a lot of blocks bored oversize. So instead of scrapping all these blocks, GM supposedly kept boring them oversize, and ordered pistons to match. Never underestimate one hand in the corporate ladder not knowing what the other hand is doing. But a thinner head gasket would accomplish the same thing as a protruding piston, and probably subject the piston to less heat than one sticking up into the combustion chamber. Who knows? I'm sure we'll never know for sure.