steel_3d

Doing a rebuild at 115k. Shop honed the cylinders and took out a good .002, unfortunately. After "cleaning" my old pistons, which means basically all the skirt coating is gone, I am left with .003-.004 piston to wall clearances, measured near the bottom of the skirt.

Am I headed for piston slap hell if I start this motor up, or is there hope that it will run fine for quite a while still?

ZZMike

I would not run them that loose. Hypereutectics do not expand as much as forged, so there is little likelihood that piston slap would go away as the engine warmed up.

If you are at .002 right now, power hone it a few more thousandths and put a set of forged slugs in it.

steel_3d

F me... I'll need to measure the actual bore diameter, but I'm guessing I'm around 4.067 right now. What diameter do forged pistons usually run, and what clearance do they ask for? Are there any (brand, alloy) that are likely to work in my bores as is? Should also work with the stock rods.

Where could I source them quickly? I'm under severe time pressure here...

Any other opinions on hypereutectic pistons at .004? The shops told me I'd be ok, but of course they don't know **** about stock motors...

Thanks!

glenB

book says .0031 max with skirt coating worn off

steel_3d

Interesting. The digital service manual I read says .0022 for the 6.2:

Piston to Bore Clearance - Service Limit with Skirt Coating Worn Off 0.0022 in

Now this is really strage:

Piston to Bore Clearance - Production -0.0019 to +0.000 in

Negative cearance??? How the hell does that work??

racebum

this is kind of a universal across all engines. generally you never setup a cast piston above low .0020 numbers. if it says service limit is .0022 that would be maximum and good for a race motor. less heat, more oil etc.

the last race motor i built was in a stripped down autocross honda. it used 4032 forged pistons setup at .0035 and rattled like hell until warm. very similar engine sleeve design to the LS is why i mention it. anyway, beings they were forged the rattle went away once they warmed up and the turbo put some heat into them. with a cast piston that won't happen and you'll beat the hell out of the rings and cylinder walls from the bouncing around

in short if you see .003 to .004 cast pistons are a bad idea, you'll be hating life. you are however right in line for a forged piston. check the spec sheets on something like a mahle powerpak and see what they recommend for clearance. unless you overbore and buy new cast pistons it's really your only shot and making what you have work.

your machinist should have said something about this as a cutting hone removes a lot more metal than a ball hone. i'm sure the argument would be that the machine driven cutting hones will give you a more true bore and that's all well and good but it's a moot point if it makes the pistons you have unusable.

racebum

one wonders how negative numbers fit in the bore. i grasp the concept of going in tight and letting the coating wear off. that's fairly common as piston manf's like mahle who do coated sides say to measure from the bottom where there is no coating. if they say .0030 at bottom skirt that may only be .0015 on the coating or some such number. still a negative number seems like it would be square peg in a round hole. a 4.00" object won't fit in a bore that has a 3 in front of it. would love to hear the explanation on this one as well

steel_3d

I've found these options for the stock bore and stroke. I'm hoping these forgings will expand more than the stock castings and not slap. But I'm not sure how they come from the factory. Do they already come undersized to fit in unchanged 4.065" bores? So they'll still slap in my 4.067"?? Or do they expect the bores to be honed somewhat?

I'd like to avoid completely disassembling the engine again and taking it to a shop to go to 4.070.

http://www.ebay.com/itm/360489939472
http://www.ebay.com/itm/330860296444
http://www.ebay.com/itm/390375370316

Opinions on any of these?

steel_3d

I just can't catch a break. This was the response of a seller on the Mahle:

"The wall clearance is figured into the piston already and it is 4 thousandths undersized. The finished bore and hone needs to be 4.065 and then put the piston in. Thanks"

Is this the case with all manufacturers?

jon6.0

Call Wiseco. It's only a few bucks more for a totally custom set. Get the good pins if you plan on boosting the motor or using nitrous. Erik at HKE is who I went through for my Wiseco pistons.

racebum

i like this idea and have to tell you. if mahle said what they did you have to follow the exact direction for them to work. they are spot on with their specs. i once had an engine builder that didn't follow their directions and thought he knew more than they did. that engine ran 6,000 miles before coming apart.

wiseco does do custom pistons and i think uses both alloys both 2014 and 4032. 4032 is superior for a street car as it expands less

the real thing you need to ask yourself is what your goal is with the car. if you want 100k+ miles and mild HP you want to use cast pistons period. the tight PWC is gentle on cylinder walls and extends ringlife

if you're building 600whp and or doing track days a 4032 piston is ideal. if you're running high boost forced induction and or really high compression you want 2014

build the engine around your goals, not around what's easy at the time. what's easy now may be a pain in your *** down the road.

since you know your bore size you could call wiseco and say look guys i have a bore that's ........ whatever it is. make me a piston to fit it

before you do, ask yourself about the goals though. the custom wiseco stuff will both cost a lot more than cast and by all means should have a shorter mileage life going up and down a freeway than cast.

if you're doing a build or race at all there is a big perk for forged. they don't break easily. cast pistons can frag ringlands much MUCH easier.

just be real with what you're really doing

steel_3d

I'm planning on having this thing get me to work till the c7 comes out

I called Wiseco, they said 3-4 weeks for custom pistons. I can re-hone this block much faster.

They also said they would not run a piston in a bore that's .002 larger for the street.

I'll call Mahle as well, but sadly I think they'll say the same thing.

racebum

even if someone said you could, i think you know deep down it's a bad idea.

choose the correct cast or 4032 piston for your hp goals and put the spec to where the manufacturer says. if it requires an overbore, fresh hone, doesn't matter. doing it right will not only work better, it will be MUCH cheaper in the long run.

that said if you're buying a c7 in a year forged pistons in a stock motor may scare someone as the rattle when cold could be mistaken for rod knock or clatter if they don't know any better.

if it's a built motor then the buyers expect such things as stronger pistons.

how strong of an engine are you creating here?

if it's a stock heads and cam overhaul stay cast. they are quiet, live the longest when it comes to mileage and are the cheapest

steel_3d

So to make things more fun, stock pistons and rings in 0.5mm (0.020") oversize are $250 per hole. Insanity.

Another crazy idea. Can I get my stock piston skirts re-coated and hopefully kill piston slap?

narfdanarf

Not that it matters now but I've never even heard of a machine shop honing a bore without having each individual piston to measure and hone to the correct p2w clearance. Every piston should be measured .5" from the bottom of the skirt and compared to bore size for the specific hole it's going in. I would find a new machine shop honestly.

Yes you can coat pistons to help fight slap and remove clearance (I think most coatings are in the neighborhood of .0015 thick) but it will eventually wear off and you'll be at the same point you are now.

racebum

most good machine shops that is. i wondered the same thing when he posted. what was the machinist thinking if he measured those pistons and honed that much out of the block? anyone with a lick of sense should do one of two things. 1 suggest a ball hone only if meaningful material can not be removed, OR, suggest oversize pistons if the cylinders are out of round or scared requiring it

almost sounds like the OP's best bet is to order oversize mahle powerpaks and machine the bore to fit them.

every other option is either expensive or time consuming

btw does keith black or trw make a hyper cast piston for the engine yet? that's an oem level piston you can setup tight for zero slap

HOXXOH

An alternate approach would be a new block. Sell the old one to someone wanting to build from scratch.

steel_3d

I'd be interested in cast options for sure.

I told the shop to just touch the bores so the rings would seat.

I've since read that that's not even necessary, and GM specifically doesn't hone when replacing rings.

He just kept honing till it was perfectly straight, which is not what I asked for. ****in shops, I swear... Neither of two shops realized the difference between cast and forged tolerances. All they know is race junk.

steel_3d

I doubt a new block would be as cheap as a set of pistons and a hone... And I'm not sure how great these old pistons will work with pretty much no coating left. Probably won't come out quicker than pistons plus hone. Pretty risky.

jon6.0

That machine shop better be helping you out on cost if they went against your wishes and caused this mess.