dr.bombay

Hi All, long time lurker here. Been wrenching on cars a long time but into my first bottom end build. running into an iffy piston to bore situation that I appreciate some feedback on. I am doing a refresh ("rings and bearings") on a 2014 L96 to go into my 1972 monte. street setup with a mild cam and GM factory engine controls. I tore down the engine and bearings were uniformly worn and crosshatching on the bores was still visible. I wanted to raise the compression with flattops and put in new bearings etc while the engine is apart. I am using cast EngineTech 5041s in STD bore size as an inexpensive set that i can use without rebalancing the rotating assembly. I sent my block and the new pistons to my shop and asked them to hone the bores to freshen them up for new rings. I am in the process of final measuring and it seems my PTW clearance is out of spec.

First a few observations on the block (with main caps torqued). These are absolute measurements used to check roundness and taper (I will get to the relative measurements further down)
* measuring at 60 degree radial spacing and 4x 1" vertical depth for a total 12 measurements per bore.
* mitutoyo vernier mic (0.0001") and shars dial bore gauge (0.0005")
* out of round is consistently 0.0010" (thrust direction is one thou bigger than the longitudinal front/back direction) in all cylinders
* taper is less than 0.0005" from 1" below deck to 4" below deck
* all cylinders show the same measurements.
I dont know if bolting on the head will made the bores stretch out on the thrust direction or the longitudinal direction but i believe one thou out-of-round is in spec.

Now the conundrum:
* I measure all the pistons at the skirt location specified by the manufacturer as 3.9976" to 3.9981" (the 'grade' spec called out on the box is 3.9985 +/- 0.00035"). its possible that my gear calibration is off a few ten-thous (i use gauge blocks) but thats where we get to the relative measurements next:

* I set the dial bore gauge at zero at the average piston size in my set of 8 as 3.9979"

* Measuring bores in the thrust direction (the wider measurement vs the longitudinal) and consistently see 0.003" across all 8 cylinders at 4 different depths

* Measuring bores in the longitudinal direction (the narrower measurement vs the thrust axis) and consistently see 0.002" across all 8 cylinders at 4 different depths

* Piston manufacturers spec is 0.0015" and GM service limit is 0.0028" (latter for OE pistons which i am not using).

These are cast pistons. if I put this build together already at the wide end of spec am i asking for trouble (skirt knock, bore wear, weak compression)? I would like to avoid coating the pistons or reboring or going to forged pistons (calling for a looser PTW spec) if possible since this is a 450hp street / weekend build and not a race car but i dont want to set up for problems later on. I dont have a sense whether being off by 1.5 thou vs the piston manufacturers spec is a red flag in real life?

Ive read the fantastic info on this forum over the years and i feel like I'm in a gray zone with near stock specs and cast pistons. anyone with experience in a similar situation, please weigh in!

thanks

RB04Av

Sounds relatively OK to me.

If the OE crosshatch pattern was still visible, then the bores couldn't have worn more than its depth. Which is what, something on the order of 50 microinches. (.00005") Apart from any inaccuracy in the re-hone, that's the outer limit of its deviation from "stock" "original", or at least, was, while it still was "stock" "original". Not that "stock" "original" is any kind of paragon of production excellence, but if it went however many miles before, it's at least "good enough" for that, and come out on the other end without significant degradation.

If the cyls are all consistent, as it seems they are, then nothing is just "trashed' or ruined.

Installing the heads will DEFINITELY affect bore clearance. It's hard to predict exactly how or how much, but it's safe to say, the changes will be the greatest near the head bolts.

Relative measurements are more useful than the "absolute" numbers. Gauge blocks, measurement techniques, etc. can lead to uncertainties in the one, but if the other is consistent throughout the project, then I'd place more trust in that, than in the absolute numbers.

So, even if we were to tell you that's all crap, throw it in the trash and start over, whatever whatever whatever, what would you ACTUALLY do? That is, how much influence is anything any of us out here says, going to have on the course of your project? Would you go get another block, buy different pistons, spend $$$$machine work$$$$, ??? over some .0005" of something somewhere? Which is to say, everything I'm reading printed in bold red all in the white space all around the words in your post, tells me you're going to just put it together and run it anyway, regardless. All you want is some reassurance.

Given that, put it together and see what happens. Odds are it'll be fine. Looks largely OK to me in any case.

Che70velle

iTrader: (6)

Your data posted will be fine for a street build. I feel like your wasting time worrying about these numbers however, because the bore dimensions will absolutely move, upon you torquing the heads down. You could purchase a torque plate, install it, and re evaluate the cylinder dimensions but I’d bet you’ll find that your well within spec. I’d run it as-is and not worry about a thing. I’ve seen worse get assembled and run just fine. Heck…I just sent a 5.3 out the door here for a buddy’s turbo build. I went through it with new bearings/rings and I ball-honed the cylinders. He was looking for a gen4 rod engine that will last longer than his last gen3 rod engine did. I guarantee you that your numbers are better than this was. Skirts had wear and were tapered. He wanted a sloppy build and he got it. Not what I normally build, but it’s what he asked..and paid for. I’d imagine that most guys don’t even bother measuring cylinder taper or skirt wear. Good on you sir.

dr.bombay

Thank you both. This is why I love this forum - real world expertise to balance out my hand wringing and overthinking!

Deadeyeodell

You guys are awesome and are helping more people than you know. Thank you