measuring JE coated pistons

[rebelrouser]

Putting together a 4.6 bore big block Chevy with JE nitrous pistons. They have coated skirts with two small windows to measure the skirt diameter.

This video from JE, shows a special micrometer they recommend to use for measurement.


My standard outside micrometer has normal size ends and the hole in the coating is too small. And since this is a fairly large bore my calipers do not have long enough jaws. Does anybody use these special mics? And if you do who makes them? If not how do you get around it?

[mt-engines]

A blade mic or point mic. mitutoyo, spi etc make them

[FC-Pilot]

I would buy a standard 4-5” mic and grind down the ends, then send it out for certification. If it passes you are good to go. My previous employer certified measurement tools and we all brought our mics and torque wrenches in. I was surprises to see even the cheapest mics pass cert. if I were in your shoes I might grab one from harbor junk cause if I ruined one I would rather it be inexpensive.

Just a crazy idea

Paul

[FC-Pilot]

A blade mic or point mic. mitutoyo, spi etc make them

Even better!

Paul

[mt-engines]

I would buy a standard 4-5” mic and grind down the ends, then send it out for certification. If it passes you are good to go. My previous employer certified measurement tools and we all brought our mics and torque wrenches in. I was surprises to see even the cheapest mics pass cert. if I were in your shoes I might grab one from harbor junk cause if I ruined one I would rather it be inexpensive.

Just a crazy idea

Paul

I'd doubt Chinese freight has 4-5" micrometer. But you could have it ground.. Or just buy a decent mic.

Or call the manufacturer and ask what the minimum and maximum piston t wall would be.

[FC-Pilot]

I would buy a standard 4-5” mic and grind down the ends, then send it out for certification. If it passes you are good to go. My previous employer certified measurement tools and we all brought our mics and torque wrenches in. I was surprises to see even the cheapest mics pass cert. if I were in your shoes I might grab one from harbor junk cause if I ruined one I would rather it be inexpensive.

Just a crazy idea

Paul

I'd doubt Chinese freight has 4-5" micrometer. But you could have it ground.. Or just buy a decent mic.

Or call the manufacturer and ask what the minimum and maximum piston t wall would be.

I am with you, unless totally broke I would just buy a decent mic that has the right contact points to begin with.

Paul

[hoodeng]

If you had a bit of a chuckle watching someone check a piston with a vernier and thinking what they see on the dial is accurate enough to bore to,how's this!

I do bores for a lot of service centers, they are sent back with clear instructions that the bores need to be cleaned to a standard, also that the ring gaps need to be checked, pretty easy, its written inside or on the top of every box i send back. I also ask that an order, either written with the job or emailed to me stating the engine series, bore/stroke combo, comp required, cast or forged is included with every bore job. [if you have ten sets or more of just bores a week running through, you want better than a phone call.]

The other day i do a set of 88 to 95" 3.875" bores and supplied a 10.5:1 forged piston kit set at .0025" clearance, a few days later i get a call telling me they are going to send the pistons back as they are too higher comp for the cam they intend to use, and are getting a set of cast pistons with a lower comp. I ask 'so what clearance are you going to run with the pistons you intend to use'? the reply ,' you have already bored them to 3.875" so any 3.875" piston will fit won't it' I tell them the bore is not actually exactly 3.875" but .0025" clearance to the pistons supplied, the reply 'so you don't bore our jobs to exactly 3.875"? my reply 'no, i bore your cylinders to the specified clearance of the pistons being used, and if there is any variance pistons will be paired to bores.' ,,,, apparently i have been misguiding them all along.

These same guys install spring kits without at least measuring the installed height, apparently if a spring kit has 'Performance spring kit' printed on the package they will fit anything that will take them.

And these guys are on the big bucks.
Cheers.

[Geoff2]

My understanding is the piston clearance is in the piston, not the bore. So if a 4.000" bore, then a correctly finished piston that requires 0.003" clearance will measure 3.997"at room temp.

[BillK]

My understanding is the piston clearance is in the piston, not the bore. So if a 4.000" bore, then a correctly finished piston that requires 0.003" clearance will measure 3.997"at room temp.

Exactly correct. It is very rare now days to see a piston that is made incorrectly. If you finish the bore to the correct size the clearance will be built into the piston. Also, there are forged pistons that run very tight clearances and others that run loose. You simply cannot say that a forged piston runs more clearance. The ancient TRW / Sealed Power L2256F 350 pistons are speced at .0015 clearance and they have been around for at least 40 years ??

There are exceptions where you might want to run more clearance but that is a totally different story.

[Dutchman]

Usually I just read the instructions with the piston set.
No need to reinvent the wheel

[jed]

My understanding is the piston clearance is in the piston, not the bore. So if a 4.000" bore, then a correctly finished piston that requires 0.003" clearance will measure 3.997"at room temp.

Exactly correct. It is very rare now days to see a piston that is made incorrectly. If you finish the bore to the correct size the clearance will be built into the piston. Also, there are forged pistons that run very tight clearances and others that run loose. You simply cannot say that a forged piston runs more clearance. The ancient TRW / Sealed Power L2256F 350 pistons are speced at .0015 clearance and they have been around for at least 40 years ??

There are exceptions where you might want to run more clearance but that is a totally different story.

I agree and usually give the bore size a few tents more. It's better to be .0003 loose than .0003 tight.

[MadBill]

Putting together a 4.6 bore big block Chevy with JE nitrous pistons. They have coated skirts with two small windows to measure the skirt diameter.

My standard outside micrometer has normal size ends and the hole in the coating is too small. And since this is a fairly large bore my calipers do not have long enough jaws. Does anybody use these special mics? And if you do who makes them? If not how do you get around it?

With a 4"-5" standard mic and at least three hands (or Crazy-Glue), position a small ball bearing on each mic anvil. Measure, subtract...Done

[hoodeng]

I have received various reply's from piston manufacturers regarding hard and soft piston coatings. As there are a number of types in use the allowance in piston measurement can vary, from allowing nothing with a piston/bore size to size fit [no clearance] with abrade-able soft coatings, up to allowing .0015" for another manufactures similar abrade-able coating. some say use the given bore clearance over the coating included.
Give the manufacturer a call, he does not want to make his product hard to use, he knows full well people will walk if his parts are only gauge-able with tool room metrology.

Nearly every micrometer is manufactured with a .250" diameter anvil and spindle, and i would say nearly every machine shop uses the same, yes i know specialist measuring mikes are available and we could make a pretty simple jig that holds balls at the gauge point, but an email/phone call might be all that is needed.

Cheers.

[modok]

I have used micrometers with ball points or small tips to do this, but I don't feel it is very accurate, the surface is usually rather rough and the contact pressure is too high with the small tips.
So.....IMO
if you buy good pistons there will be a spec sheet that says the skirt size, it is accurate. go with it.

if you buy crappy pistons, use a file to remove enough coating below the measuring point to measure with a normal micrometer.

if they are used pistons, high dollar....not sure, I'd probably rig up a setup with a dial indicator to do it. the manufactures are certainly not using a blade mic for this, I am sure they have purpose built measuring devices that do not rely on hand feel.

[hoodeng]

The main reason i don't use a fixed bore dimension off the package is because the ambient temp can vary wildly throughout the year, 4°c to 45°c, i have measured pistons in the middle of summer that would have .0015" clearance if bored to dimension and conversely have .0025" if bored to dimension in winter. There is also a situation where the end of the box will give one clearance and the slip inside the box another,with a variance of .001" in spec clearance, the one on the box is correct for this manufacturer as the label is printed to match the batch.
Another manufacturer gives the specs on a sheet in the box, this is the one to work to,,, there have been instances over the years where manufacturers have closed down clearances on a later same type product.

I can bore to size no problem -.000" +.0002" we do it every week, but i still want the pistons that are to be installed when doing bores. There is a manufacturer that bores their cylinders to a finished size that they say does not need any alteration from as supplied, and that any of their piston offerings will fit without any correction,, some years back there were instances of seizures, quite rare but enough to prompt them to get cylinders checked against the pistons being fitted,,what happened is that the bores were largely on 4.000" or 4.060" depending on the kit, the recommended clearance was .0015" to .0025" what was found was that there were a number of combinations showing up with .0005" clearance. The kit installer would not know the fit was under spec as it would still slip together but it would show up as a failure in extreme conditions. I had told these guys to pay particular attention to new ring gaps as a variance of .001" in the bore would show up as .003" reduction in ring gap, the rings supplied with these pistons were quite accurate, even if they were not accurate the variance of gap suggests further investigation.

If a manufacturer recommends .0015" minimum fit it can go together with .0035" and you would not know, the engine will run the same and not make any noise indicating a loose fit, typically in this engine you would need in excess of .006" or more before slap was detected.

If we get the measuring tools out and confirm our work we will minimize faults to nearly nil,, the types of tools we use are going to be ones recognized in our area of industry as a standard,, there are some quite pretty tools out there that would make me look like a rocket scientist and would be kept in a glass case and only pulled out when we need a "the machine that goes ping!" I am guilty of owning some measuring tools that i look at in amazement, pictured is one, i know what it is supposed to do but can't make it do it,, now,, the Mitutoyo protractor is very easy to use, same tool, one good, one f,,,,ing hopeless.


Cheers.