Bore size precision...

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'66Cuda

'66Cuda

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What is your experience with bore sizes when you measure them after getting them back from the machinist? If you want .030" over on a 360, will it measure exactly 4.030" or will it be under sized by a couple thousandths?

Thanks!
 
'66Cuda said:
What is your experience with bore sizes when you measure them after getting them back from the machinist? If you want .030" over on a 360, will it measure exactly 4.030" or will it be under sized by a couple thousandths?

Thanks!
Click to expand...

'66 Cuda

In most {98%} of the cases,

The Machine Shop will bore the engine to size. And then finish-hone the
Cylinder to the 'desired' specifications of the Piston Manufacturer recommended
clearance.

The Machine Shop will 'have' the Pistons in hand, to measure for desired
clearance.

There are occasions, where a customer will just want the Block bored
{ie; .030" over}, but do not want the Cylinders finished to size.

As they prefer to fit the Pistons to the Cylinders themselves with their
own honing device.

But, if you want a Cylinder Bore to be 4.030". It should be 4.030" and 'not'
undersized.
 
I know in a perfect world the cylinders should be honed to each individual piston but I believe. ( correct me if I'm wrong) that for most stock to mild builds the bores are finished to the piston oversize ie:+.030 and the piston is actually a couple thousands undersize.
 
Thats right.^^(x2)
If my block came back, not exact, Id be taking it back to have them measure it with their tools, in my presence. Then Id have my tool compared to theirs. Then Id probably have to get mine recalibrated.
But if they were wrong, I would do all I could to have them make it right.
 
if the bore is supposed to be 3.040, that's what it should measure, a couple of thou under is too much deviation, do you mean a couple of tenths (.0001)maybe instead of thousandths(.001). Whats your piston size / bore size .... is the right clearances?
 
lil red said:
if the bore is supposed to be 3.040, that's what it should measure, a couple of thou under is too much deviation, do you mean a couple of tenths (.0001)maybe instead of thousandths(.001). Whats your piston size / bore size .... is the right clearances?
Click to expand...

I agree.

.001" is too much off.

If not "right on the money" should be within +/- .0003" or less for bore size, not clearance between the piston and cylinder...

When cutting the cylinders, the final measurements should be made to 4 decimal places minimum - micrometers not calipers are what should be used...
 
I appreciate all of the responses. I am new to micrometers and bore gauges and am taking a run at measuring all of my own clearances. I have a 360 .060 over with Speed Pro Hyper's, 4 valve relief pistons. The motor is a 3k-4k rebuild that I am rebuilding with the old/new pistons (long story). I measured the piston skirts right below the wrist pin . I took 3 measurements and then averaged them for the piston size. I took 6 measurments, 3 parallel and 3 perpendicular, and averaged them for the bore size. Here is what I came up with:

Hole Bore Piston
#1 4.0577 4.0557
#2 4.0577 4.0554
#3 4.0572 4.0551
#4 4.0572 4.0551
#5 4.0572 4.0547
#6 4.0572 4.0548
#7 4.0572 4.0546
#8 4.0572 4.0548

I was concerned just measuring the bore alone, that it wasn't 4.060". '69 Cuda cleared it up for me I think. According to my math, (Bore - Piston) /2 = piston to sidewall clearance, I should have around .001"-.0015" in all 8 holes. I read that Speed Pro recommends .001"-.0015". So it appears I am good. Is there anything here that looks wrong?
 
Yes
24 things look wrong
Not 1 hole is right
Not one piston is right
Not one clearance is right
To their credit, 6 holes are the same not right, and 2 others are nearly the same not right.
Perhaps your tools are not right
Perhaps your technique is not right
Perhaps the pistons are supposed to be measured 1/2 inch from the bottom of the skirt and perpendicular to the wristpin, and no other place, and no averaging.
KBs are measured at the balance pad, again perpendicular to the pin.


Good grief, if those piston measurements were accurate, they just might be the worst old/new pistons in America. Skirts are usually within 2 or 3 tenths of eachother. Thats .0002 to .0003
The KB107s that went into my engine were within less than that.
 
AJ/FormS said:
Yes
24 things look wrong
Not 1 hole is right
Not one piston is right
Not one clearance is right
To their credit, 6 holes are the same not right, and 2 others are nearly the same not right.
Perhaps your tools are not right
Perhaps your technique is not right
Perhaps the pistons are supposed to be measured 1/2 inch from the bottom of the skirt and perpendicular to the wristpin
Click to expand...

I needed a laugh!!!!
 
It takes some practice to use a bore gauge or micrometers. Keep it up and try taking the same measurement 3 or more times to be sure that it's repeatable. If you can't repeat the same reading within a few places on the last decimal, you may need more practice to refine your techinque.

You have to get it all square in 3 axises. The smallest reading that you can get in the particular diameter at that level is most likely the more accurate reading (if you follow what I'm trying to say)...

If you can't repeat the same reading with minimal variance, then keep trying until you can...
 
The shop I use bores to within .002" of finish size and hones the rest with plates. Wall finish and process for them evolved from a lot of testing and the use of their profilometer and dyno. If you're measuring right, and the work was good, there should only be some varience at the top of the bores near the head bolt bosses because they honed it with a torque plate. The math is piston size (properly measured) + piston-to-wall clearance = final bore size. You do not "average". You take the tightest because that's where the least space is. Pistons are measured where the manufacturer tells you to. That's normally 1/2" up from teh bottom of the skirt, perpendicular to the pin axis. Pistons are not round, and they are widest at the base of the skirt. Bores should be measured at least three places along the length, and there should be nothing different for the ones below the bolt bosses. No taper. No out-of-round. The top near the bolt bosses may be smaller. I normally see up to a thou (.001") of distortion there because of the torque plate. When the head is bolted one, it distorts to perfectly round which can be checked by bolting and torquing the heads on with the gaskets you are using, and measuring from the bottom of the bore (obviously before the crank goes in...).
 
Thanks for the input. Could it be the gauges? I doubt it. Could it be my technique? Most likely. I appreciate the knowledge, even if it comes in a "You're an idiot" sandwich. I will eat it and give it another go! If I was pretending to know what I was doing, then I wouldn't have posted. This forum and the internet is all I have to learn from. The knowledge is what I'm after, so laugh all you want...I still learned something!
 
not laughing at all. heck, your question probably informed a bunch of people. I know I learned something. where to measure the piston at to determine bore diameter. thanks.
 
I think the laugh from xLURK was in response to the "24 things wrong" post I made. He wasnt laughing at you, but rather at my bass-akwards sentence structure. Which is what I was hoping for; a bit of light-hearted humor. I think no body you was laughing at.
Setting up a dial-bore gauge, and matching it and the micrometer takes some practice as does using them once set up.
Hey, we all started out ignorant(thats unschooled). The fact that youre double-checking the machine shop, speaks volumes as to your dedication and will go a long way to making your assembly a success. Keep at it.
A lot of the joking that goes on in these threads, is between answering members, and hardly ever directed at the asking member.
Press on, flat-lander.
 
hey 66cuda, as far as Im concerned if your new to using precision measuring gear and getting repeated measurements of within a tenth of a thou... your doing good.....Ive worked with 3rd year apprentices who cant do such reliably.
 
Nope - no laughing from me. The first engine that I had dial bore gages for, and was done properly with torque plates, I measured and called my machinist and screamed about how bad the bores were. It was at that point that he laughed a lot...lol. We're all here to learn. Don't feel stupid. Lots of guys would never even ask until there was a problem with it running.
 
Dial Bore Gages

Not all are the same.

Dial-Bore Gages {ie; Sunnen or Mitutoyo} are high-quality measuring devices.

As for the 'Speed-Pro' Hypereutectic Pistons,

Yes they are designed to run on tighter clearances, and they require a
minimum of .0015".

Though some Machinists like to go up to .0025" with the 360 Mopar.

I go .0025", and have had nothing but good success with that
clearance. Nothing but solid running Engines with a tight compression
seal.
 
When I picked up my block the machinist did all the measurements for bore and the crank in front of me. He explained each one and when I did the assembly everything fell into place, well almost. (hung a piston backwards). Reading this makes me appreciate that guy! But the factory didn't spend much time throwing these motors together and most of them ran rather well. Thanks guys, I did learn a lot here and this thread was well worth the read.
 
What the OP was asking inpart (I think) has to doe with piston to bore clearance. There is nominal bore size (what the blueprint specs call out) actual bore size (what the machinist finishes the bore size at) and piston size (actual).

So lets say we are working on a pentastar chevy....ummm I mean a 360 (just a straight up chevy if it's a magnum with it's chey like crap rockers). It has a NOMINAL ( really means "in name only") bore size of 4.000, minus .000 plus .002, so the factory bore could be as small as 4.000 or as large as 4.002. The piston on the other hand SHOULD measure NOMINAL bore size MINUS operating clearance, which is spec'd by the piston manufacturer. For a cast piston, it should measure 3.998 to 3.999, which if you finished the bore at NOMINAL, you would have .001 to .002 clearance. If you were to use say..an old TRW piston that is forged, the piston might only measure 3.992 to 3.993 which should run at .007-.008 clearance.

What I'm saying is the running clearance is designed into the piston by the manufacturer. You will never find a piston designed for a 4.000 bore that measures 4.000.

I finish all my street engine stuff .0005-.0008 larger than nominal (always measuring the piston to verify) and race stuff gets plus .001 to as much as .0022, depending on application.

Keep in mind if you have a piston designed to run .002 and you add .001 extra, you just gave the piston 50 per cent MORE clearance than the piston was designed. If the piston was designed to run .008 and you gave it .001 more that is only a 12.5 per cent increase. That's why all those zeros after the decimal point have a big meaning.
 
AJ/FormS said:
I think the laugh from xLURK was in response to the "24 things wrong" post I made. He wasnt laughing at you, but rather at my bass-akwards sentence structure. Which is what I was hoping for; a bit of light-hearted humor. I think no body you was laughing at.
Setting up a dial-bore gauge, and matching it and the micrometer takes some practice as does using them once set up.
Hey, we all started out ignorant(thats unschooled). The fact that youre double-checking the machine shop, speaks volumes as to your dedication and will go a long way to making your assembly a success. Keep at it.
A lot of the joking that goes on in these threads, is between answering members, and hardly ever directed at the asking member.
Press on, flat-lander.
Click to expand...

Exactly!! No hard feelings.
 
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