Technical Piston to wall clearance

Depends on what material the piston is made from also for proper clearance. Cast, 4032 or 2618 forgings. It also depends on where you take the measurement from on the piston. Piston manufacturers are very specific on where measurements are taken from as pistons aren’t really perfectly round.

 

I think it was a site called “Zoro” has Fowler 4-5 in .0001 mic listed at 78$.
I’ve smaller sets, but largest I have now is 3-4 in. Just “normal” though, not the ones to use on something like a piston (real technical, I know).
Thanks.

 

I knew different castings might have different diameters, but had no idea they weren’t round or spec’ed to be measured at a specific spot on them. I just assumed the skirt.
Thanks.

 

You can get 0-4 and 0-5 .0001 mic sets very, very cheaply on Amazon and whatnot. Throw the standards in the garbage and check them against 1-2-3 blocks but other than that they're great mics.

Zoro is owned by Grainger

 

Before I head to bed, how much is a 1-2-3 block?

 

Was going to edit, heck with it!
But seems 1-2-3 blocks are from 1/2 inch to 3 inch. Mics are in inches. Ie 4-5 , etc.

So I couldn’t use a 1-2-3 block with a 5 in micrometer, could I?

 

You absolutely could but you're potentially introducing measurement error as a result of that being a fiddly operation.

Take a 1" or 1-2" mic to your 123 block first. It's probably+/-.0002 or something. You want to know what it is in case a piston measures right at a upper/lower tolerance limit.

 

That brings to mind my dad saying” If you can’t find time to do it right the first time, when will find time to do it over again” words I have lived by.

 

Piston are finished 'out of round' so that when they get to temp they are, hopefully, round.
Consider that the expansion is different across the length of the pin as opposed to 90° from the pin due to different amount of material.

 

I’m not getting it. What does equating a different micrometer used to measure the block, have me compare it to the micrometer I’m using for a piston?

 

This link should cover everything you need to know/do regarding what you're trying to achieve.

If you already have outside mics in the correct size for your pistons, get yourself a dial-bore gauge and you're good to go. The dial bore gauge will also come in handy when it comes time to checking main and rod bearing clearances as well, amongst other things.

 

1-2-3 blocks are 1" by 2" by 3"

They're sold in pairs so you can make any number from 1-6 an use that to assess the accuracy of your mics at the extreme open and closed. Blocks are way easier to make accurate than mics so the tolerance of a cheap chinese block is suitable for checking the calibration of a mic for non-toolroom use (even cheap chinese mics right out of the box don't tend to be out by a non-negligible amount).

You only need to check accuracy at the extremes. The pitch of the thread guarantees accuracy in between.

 

I prefer snap gauges but I will admit that a dial bore gauge is faster.

 

The mics don't even need to be accurate. Measure the piston with the mic and lock it down, insert the dial bore gauge inside the mic and zero it out, measure the bore with dial bore gauge, note the difference and divide by two; that's your piston to wall clearance.

You're not even really using the mic to measure the piston, per se, as you are using the mic as a baseline for the dial bore gauge.

 

If you have a proper ridged cylinder hone no reason you couldn't do it yourself. It does take skill and someone adding honing oil or a pump to do it.

 

A beginner shouldn't be scared.

Skill can be replaced by checking your work frequently on the first couple holes until you get a feel for how you should be moving it for even material removal and the rate of material removal.

Now that I think about it, boring a cylinder with a hone is a great exercise to teach honing because the material removal rate is so slow that you can hone the cylinder into all sorts of misshapen barrels and cones and hour glasses and still have ample opportunity to find and correct the error before you get too big.

 

As I mentioned, I’m good enough with mics and my dial bore gauge, so when I polished my cranks in my lathe, I had it dialed in pretty well.
Only concern I had, is if it was like a 218 I had machined, one piston felt tight after getting it in the bore, the shop then honed the cylinder for clearance.
I just took the piston and rod assembly in and they took care of it.
Since my friend has.a rod heater, he’ll do the new pistons and we knock a six back down and shoot the shit.
This engine is a very unmaintained 302 Ford, I’d expect some (a lot of ridge/taper) in it.
I’d like to just take the block in and say “take it .030 for cast rings”, I’m using cast pistons.
Once it gets to the heads, I’ll worry about it at a later time.

Oh, I’m not going to hone cylinders .030.

 

And this is what generally happens , all sorts of shapes , all sorts of frustration , all sorts of lack of experience measuring , all sorts of disappointment in the finished project & all kinds of wasted expense & effort , life's short , some things are better off farmed out .

 

Take the pistons with it and tell them to make it fit! I finish each cylinder to its piston so they all have exactly the same clearance

 

Having been a automotive machinist for 35 years my advice is find a good shop a pay them to do it. Take them the new pistons You said you have a Mic and dial bore gauge you can check their work when its finished. Honing cylinders to dimension, making them straight, and the proper surface finish is much more complicated than some here would lead you to believe.

 

You’re probably right, cost of a hone, the 6 pack that would turn into more…would rival the cost of just having the shop do it.
That’s a problem with working the night shift, being bored, and thinking of stuff