Opinions on cast and alloy pistons

[Blacdout96]

Alright you gear heads, im looking or some opinions on something. looking at pistons for my car, I have a choice of getting cast pistons, and aluminum alloy pistons. both are hyperteutetic. the cast have that skirt coating and are made by sealed power, and the aluminum alloys are made by AE Clevite, and do not have the coating. The aluminum ones are chaper, and are the only ones that come in a set as to where the cast ones only come indivdual. also im looking to go .30. any opininos guys? would there be any gains and such?

[bubba428]

Get the Sealed Power pistons....they are great

[Featherburner]

From what I'm reading they are both cast aluminum pistons. So, It's really a flip of the coin. Does one weigh less than the other?

[Blacdout96]

doesnt say if theres a weight difference.

[johnjzjz]

if you buy them as singles the piston weights will be all over the place ( as much as 20 grams ) and the size ( overall outside diameter ) will also be different ( sets are checked so when the time comes to box up they are somewhat close to one another ) sometimes --- > the machining is done easier if they are close if they are all over the place in size you wont end up with a good motor seal, even if you get file fit rings and try and fit each to the exact cylinder, its going in, the machinist has to be able to fit each piston to each cylinder bore < only race shops < WELL SOME - do it that way --

most pistons are out sourced to china and are real junk today if you buy good stuff you have a better chance of getting good stuff -- just my take -- jz

[Blacdout96]

well im doing all the work myself, and want to make sure I use a reputable company. I know both names have been around in the performance industry, but I just want know which would be the better to go with. I do have access to a scale to check the weights of each now that I heard they could be off from one another. so jzjz, being the pro out of all the answers so far, if you had to go with a company, Clevite, or sealed power? is there any difference between cast and alloy?

[Tru2Chevy]

Originally Posted by Blacdout96

is there any difference between cast and alloy?

They are most likely both cast pistons. If they don't claim to be forged, then they are cast. Alloy just means they are not 100% pure aluminum, they are a mixture of aluminum and some other materials (I believe pretty much all pistons are made this way).

cast and alloy are describing two different things, so you can't compare the two based on that.

Give us links for the two and we'll be able to give you a better answer.

- Justin

[Blacdout96]

sure will:

http://www.partsamerica.com/ProductL...al+-+Oversized

[johnjzjz]

sealed power would be my choice of the two - make sure you weigh the pins as well you might be able to switch around the gram or so to help match weight at some point you want a set close to the same size and weight - a ring grinder as a thing to have if your looking to make it just right moly top cast second and chrome oil is the deal

try to get the bunch as close to the original set of pistons if they for some reason end up lighter thats even better the motor will be smother at high speed it just works out that way

[LTb1ow]

Why would you want a set of hyperteutetic pistons? Every guy that rebuilds LTs stays far away from them. Can anyone explain?

[Blacdout96]

should I purchase a micrometer as well to read to diamter of the pistons n such? im suprised that they wouldnt all be the same size, being cut form a machine or formed in a mold.

[crainholio]

Originally Posted by Shownomercy

Why would you want a set of hyperteutetic pistons? Every guy that rebuilds LTs stays far away from them. Can anyone explain?

Every choice is a compromise. With pistons, you're balancing requirements between strength, cost, and noise among others.

Forged pistons win on strength. They lose on cost (more expensive) and noise (bigger clearances because they expand more as heated, they're likely to slap during cold starts)

Cast pistons win on cost and noise...they're cheap and can be run with tighter bore clearance than forged ones. Less prone to slapping during cold starts.

My experience has been that you run forged pistons only when you need the extra strength, for example forced induction or nitrous oxide injection.

[crainholio]

Originally Posted by Blacdout96

should I purchase a micrometer as well to read to diamter of the pistons n such? im suprised that they wouldnt all be the same size, being cut form a machine or formed in a mold.

You need that measurement as well as each bore's inside diameter so you can properly set the clearance. You can't adjust the piston size, but you can hone the bore to get the clearance right. Run it too tight, and it will seize at temp.

Just measure and record all 8 pistons, then measure and record all 8 bores. Match them up to get the out-of-the-box clearances as close as possible, use a Sharpie to write the bore # on the crown of each piston so they don't get mixed up.

Bring your measurements and the block back to the machine shop, as well as your desired bore clearance spec, and they'll hone the bores to the size you need.

MAKE SURE the shop uses torque plates during final honing. This is important to ensuring good ring seal.

(the really fancy shops can heat the block to operating temp so the honing is done as closely as possible to actual operating conditions)

[LTb1ow]

Ok thanks, funny that they hold the LT back and other people use them all the time. Weird.

[WildBillyT]

Originally Posted by Shownomercy

Why would you want a set of hyperteutetic pistons? Every guy that rebuilds LTs stays far away from them. Can anyone explain?

In my opinion, it depends on the manufacturer. I was talking with a metallurgist who is also a motorhead and he explained it to me like this: Making a good hypereutectic alloy is like balancing two pins on the sharp ends. If you are good at it you can do it and make a great piston. If you are trying to cut costs (read: cheaper pistons) you can really screw it up and that leads to a weaker piston.

Sealed Power seems like a good brand. I've never run their stuff but from stuff I've read on other forums there aren't too many bad reviews. KB Silv-O-Lites on the other hand can be hit or miss. I've got one of those sitting on my desk- it came out of a 427 BB and didn't get the job done. The KBs also need a really tight ring gap from what I remember; kind of out of the ordinary.

[LTb1ow]

Well from what I have gathered, the hyperteutic stuff is real good for high compression motors, but when you introduce them to any sort of forced induction they fail.

[WildBillyT]

Originally Posted by Shownomercy

Well from what I have gathered, the hyperteutic stuff is real good for high compression motors, but when you introduce them to any sort of forced induction they fail.

High compression is a relative term. I would use them in a 9.5-10.5 motor but after that forged only.

It's not the FI that kills them, either. It's detonation.

[LTb1ow]

Learn more each day, cool.

[crainholio]

When you're done deciding between cast or forged, you'll want to decide your target compression ratio.

And the post above about detonation is dead on right. To mitigate this, some builders (like me and the shop I learned at) apply three strategies:

1.) Quench area: use only 2 valve relief flat-top pistons, keep the portion of the crown that sees only head deck perfectly flat. 4 valve relief pistons put two relief pockets under the deck and increase risk of detonation.

2.) Skinny head gaskets: no less than 0.025" gasket thickness compressed, ideally in the 0.03x" range but nothing 0.040" or higher. Fat head gaskets open up the quench area and nothing good comes from that.

3.) Zero deck piston height: after you've picked your pistons, bring the specs to the machinist with you and tell him you want them at zero height w/ the deck surface. Most pistons are a few thousandths in the hole, again killing your quench efficiency. DO NOT have them above deck without accounting for it in your head gasket selection.

My last 350 came in at 11.2:1 compression ratio and ran 89 octane gas with no problem...ran a 13.11 @ 103mph in that configuration. 58cc heads, super thin SCE copper shim gaskets, and Federal-Mogul flat top 2VR hypereutectic cast pistons.

[Pampered-Z]

Originally Posted by Shownomercy

Why would you want a set of hyperteutetic pistons? Every guy that rebuilds LTs stays far away from them. Can anyone explain?

The hyper pistons are fine for a N/A street engine. By street I mean a engine not built for all out racing, say, not turning over 7G. If you're going beyond that you want a stronger piston (and rods and wrist pin). Also if you plan to use a power adder then you need a higher quality piston that can handle the abuse. Power adders build allot of heat, and can stress the pistons around the ring lands. but the thing that seems to kill pistons is that detonation can break a piston and you're more likely to have that happen with Boost or Nitris then running N/A.

Originally Posted by crainholio

3.) Zero deck piston height: after you've picked your pistons, bring the specs to the machinist with you and tell him you want them at zero height w/ the deck surface. Most pistons are a few thousandths in the hole, again killing your quench efficiency. DO NOT have them above deck without accounting for it in your head gasket selection.

Just be carefull with the zero deck block, You must checked everything closely! When you deck the block to zero it can cause piston to valve clearance issues.

[johnjzjz]

the average piston assembly steel rod grows .040 when it gets up to temp so the heads shelf area must be at least that far away from it ( CLAY THE TOPS ) with a flat or dome piston - caution about Zero decking on a street motor as it only gives you one rebuild the one your doing < most decks once cut the .020 to .030 needed to square the top of the block so it can be bored parallel to the crank, are now subject to cracking and moving as its now thinner, and every time the motor warms up and the head and the block are moving in different directions across the head gasket wear is going on < we try to leave enough to re do it if needed again - also the main reason you want it decked is to get the cylinder tops and crank pin at the same distance away from each other < we use a block mike from the crank saddle to the top of the deck, and you would not believe how off most really are sometimes, as much as .020 front to back, we take the shortest corner and cut all to its # as a distance , use a good shop and they will do whats needed buying a mike not really knowing how to use it and not knowing the correct location to measure from is a thing -- but knowing the right question to ask is sometimes a better tool to have - jz

[Tru2Chevy]

The OP has a 3.8L v6....not an LT1. Since he hasn't said otherwise, I would assume that he's asking about pistons for his 3.8.

- Justin

[Blacdout96]

Justin, yes I totally forgot to mentino that, I have a 3.gr8 running under the hood. im just lookign to rebuiild the motor, and putting in a cam and rockers, and nutin else performance wise lol.

[Pampered-Z]

Same deal regaurdless of motor. A cast piston is a weaker piston, still usefull in certain apps, but not all.

[crainholio]

Originally Posted by Blacdout96

Justin, yes I totally forgot to mentino that, I have a 3.gr8 running under the hood. im just lookign to rebuiild the motor, and putting in a cam and rockers, and nutin else performance wise lol.

lol lol the link you posted for piston choices was for an LS1 engine not a 3.8 V6 lol lol lol u sure fooled us lol lol