ckpitt55

iTrader: (4)

I got my block back from the shop recently, and I started going back over everything prior to assembling. I had them torque plate hone the cylinders for 3.905" pistons with the ARP studs I'll be using to mount the heads.

To measure the bores, I first torqued down the mains and the heads. Then using my dial bore gauge zeroed to my piston size, I measured the cylinder bores from the bottom to obtain my clearances directly on the indicator. I'm using a 0.0001" Mitutoyo dial indicator on the bore gauge and a 0.0001" 3-4" outside micrometer that I check for proper calibration every time I use it to compensate for temperature changes. How accurate that combo is compared to much more expensive metrology equipment I have no idea, but I'm willing to bet it's pretty close and good enough for my diagnostic purposes here.

Pistons are Diamond flat top street forged pistons, 2618 alloy, PN# 11502 for a 3.905" bore.

They call for a 0.0040" clearance to the cylinder wall, as measured at 1.25" below the oil ring land. I measured all the pistons first, they were all within a tenth of 3.9008". Realistically they might be a tenth or two larger than that because they felt a little cold in my hands. I avoiding excessively handling them to minimize the amount of heat I'd put into them for the sake of consistent measurements across all 8.

Here are my cylinder bore clearance measurements, with my bore gauge zeroed to 3.9008". The "90 deg" measurements are simply measurements taken 90 degrees from the thrust side of the cylinder bore.



Overall they seem to be ok to me, the only thing that stands out is the taper in cylinders 1 and 2. I'm close to the service limit of 0.0007" on those, seems to me that should not be there on a fresh cut. As far as out of round goes, what can rings tolerate? I tried calling diamond today but couldn't get through, their tech line is miserable. I'm assuming under a thou is ok? Advice, opinions, feedback would be appreciated. Is this good to run?

Thanks,

Chuck

Burken01

iTrader: (88)

Im still new at all this but I have the same piston in my ls6 block...

I was told .004 was too tight of a clearance, interesting.

ckpitt55

iTrader: (4)

That 0.004" is a minimum, you definitely don't want to go under that with forged pistons. But the larger you get the greater the chance of having issues with piston slap / accelerated skirt and bore wear. HP = heat, if you're not going to be developing the heat to warrant the extra clearance then you want to keep it on the tighter side of things.

Here's some pics of the paperwork they provided me:



Notice here how all the higher end applications recommend more clearance - power adders, strip cars, offroad, fi, etc.

Burken01

iTrader: (88)

thanks for the pm and thanks for those pics of the spec sheet, I needed them..

salemetro

iTrader: (5)

Bringing this back from the dead, as I'm currently having a bit of confusion on this. Notice on the piston "spec" card, it states that .004 clearance is recommended for a normaly-aspirated build without any power-adder....then, below for additional power adders, it states "Street-Strip power adder, add .001-.003"....meaning a desired .005-.007 clearance.

Here's where the confusion sets in: Look even further down on the 2618 alloy column, and it states "aluminum block (I'm assuming N/A, no power adder here).... .001-.003 LESS. So, are they meaning that an LS1 block should have an installed skirt clearance of .001-.003 ??? Seems tight to me.

Then, let's take it a step further: initial reccomended clearance is .004. Then, aluminum block; subract .002 (so skirt clearance is roughly .002)...and ADD .001-.003 if you plan on hitting it with a 250-shot of n2o....means that on an ls1 using 2618 pistons and a 250-shot, total desired clearance should be roughly .005 at the most....correct?

Using the spec card suggestion of .004, then subtracting .002 for an aluminum block (if no power adder).... .002 desired skirt clearance seems a bit tight to me. Can someone shed some light on this for me?? Or, is this final .002 desired clearance due to the greated expansion rate ot the aluminum block?

Sorry for the long-winded post....just trying to sort all of this out.

ckpitt55

iTrader: (4)

Forged pistons expand at a much greater rate than cast pistons. 0.002" is much too tight.

Given that these were designed for an LS1 (as stated on the card), I followed the minimum recommended clearance and followed advice by my machinist to open them up to a little north of 0.004". Power = heat = expansion.

salemetro

iTrader: (5)

I understand this....and 2618 more so than 4032. But bare with me here; cast iron blocks expand differently(less) than aluminum blocks. As such, it seems that the liner in an aluminum block could (potentially) expand more than a cylinder in an iron block. If this is the case, would it make sense that skirt clearance would be tighter in an aluminum block during assembly? Is it possible that Diamond is taking different block/cylinder expansion rates into consideration? Maybe I'm over-thinking all of this....just trying to be crystal-clear in my understanding.

Good point. I just wonder why they even bothered listing any adjustment for "aluminum block" on the other sheet if it (the main spec card) was indeed LS1-specific. Either way ya slice it, it's interesting to me that you would potentially subtract for an aluminum block....but it kinda makes sense.