JDRez42

the paper that came with the JE's says .0040-.0050 min clearance. Is this measurement diameter minus dieameter or is it clearance on each side of the piston?

Paul @ Thunder

Your clearance is the diameter minus diameter. You would measure your bore diameter and then measure your piston diameter and the difference will be your clearance. Hope this helps....


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Matt Gruber

i just slip in a feeler gauge

MotorHead

Problem is the bores are never perfect and where exactly do you measure the diameter on the piston ? I just use a feeler gauge

Paul @ Thunder

If the bore is not VERY near perfect then you need a new machine shop.



Paul

stingr69

You need the piston installed in the bore. You measure perpendicular to the pin at pin height with a feeler gauge. Pistons are actualy barrel shaped, not cylindrical as we would assume. This is for fresh bores.

-Mark.

lars

As correctly stated above, the clearance is diameter minus diameter.

No, you don't measure it with a feeler gauge unless your'e just trying to get a rough idea of whether or not you're out of tolerance. The bore is measured with a dial bore gauge, and several measurements are taken to assure that the bore is round and that the measurements are repeatable. If the bore is not round, the block needs to be machined.

The piston is measured with a mic at the top of the skirt just below the oil control ring groove. This is perpendicular to the wrist pin. This is the major diameter of the piston, and it's the measurement used to determine piston-to-bore clearance.

Subtract the one measurement from the other, and this is your accurate clearance.

JDRez42

thanks guys thats what i thought. JE says that you measure the piston .5" up from the bottom of the skirtThe block was honed by WORLD Products so i hope it is near perfect. I was planning on haveing the block checked for cracks or anything even though it is brand new(merlin 3) should i have it checked out.


thanks,
Josh

norvalwilhelm

Glad someone jumped in here. A feeler gage is no way to measure piston clearance. Alot do it that way but it is wrong. You need a mike or bore gage and the skill to use it properly.
I set my clearance for forged pistons on the tight side and for big blocks at .0045, no more.

MotorHead

My hypereutectic pistons have a 1.5 thous clearance. I had a bore gauge and all the mics necessary to do the job however I found it very difficult to get that kind of accurracy with tolerances allowed for machined bores and the pistons themselves

I guess I ended up taking alot of measurements with the bore gauge, and then on the piston and was not really happy with what I was getting so I checked with the feeler gauges to make sure there was enough clearance

mandm1200

I see no problem cheching the bore of the block at several areas to ensure a nice round bore. The problem with measuring the diameter or the bore and the the diameter of the piston is the amount of error in both the instrument and person doing the measurement. Granted with forged pistons, there is a little more leeway than say hypereutectic pistons where the clearance is 0.001". Anyone who has gone through high school science will know that the more measurements that are put together increases the the chance of error. The only sure way is to make one measurement. This will give not give the best result but the most accurate. Use a feeler gauge perpendicular to the wrist pin.

Matt Gruber

a pro with a feeler gauge can out-measure an amateur with a mic all the time.
the "pro" at my machine shop got fired after i checked his work with a feeler, and it flunked.
that was on the rod bearings. if the feeler won't fit, the clearance isn't there.
Mic's are generally used to show customers that they can't do this because they don't have the expensive tools "required"

vette rod

You don't measure the bore with a feeler gauge. You use a bore gauge, come on guys feeler gauge your joking

Matt Gruber

u don't measure piston to wall clearance with a bore gauge, do u? u must be joking
The micrometer salesman REALLY sold u guys. Prolly sold u a ring side clearance mic too!

GTR1999

I just did this with my 72 block. I set my 4" mic in a vise and set the dial bore gage to "0" then slowly rocked the gage in the hole to find the highest reading on the indicator. I mic'd the OD of the original pistons and subtracted. In my case the clearence was .004-.0045". I spoke to several shops and they all agreed that I could hone and rering but that would only last between 20-30,000 miles before I would have a problem. So I decided to bore and hone and go with Hyper flat tops.
Good luck,
Gary

mandm1200

Perhaps we're talking about 2 similair but different things. To me the piston-to-wall clearance and skirt clearance are one in the same.

Here is how Hastings defines skirt clearance 'The difference between piston skirt diameter measured in a plane perpendicular to the piston pin and cylinder diameter.'

There are several ways to interpret that. The way I understand it is to place the piston in the cylinder bore and measure the distance between the skirt and cylinder wall. For those who have special tools to get in there and make the measurements, great for you. I'm stuck using a feeler gauge. I find it hard to believe a bore gauge or mic will fit in an area that may be as small as .0005" and .010".

Edit:
I assuming we are talking about a block that has bored/honed and new pistons being fitted.

Twin_Turbo

What?? You measure the piston skirt diameter w/ a set of vernier calipers first and then the bore w/ a bore gauge, the difference is your clearance.

I wouldn't use a feeler gauge, the feeler is a flat strip of metal, if you use it to measure bore to skirt clearance the strip actually has to bend and even though it will bend it may not be willingly enough to bend to allow the feeler(s) with the exact clearance thickness to be inserted meaning you can just slide in one that's a tad smaller, this will lead you to believe you have a tight clearance when in all reality it may be slightly larger...and it's not inches we're talking about here. If you are serious in building your engine be prepared to spend some money to buy a bore gauge, you don't use a hammer to remove and install a damper also so why not use the proper tools to measure critical dimensions.

ZD75blue

So you guys take a micrometer and find the biggest diameter on the piston? Or do you go to the location that the manufacturer recomends?

Having never done it either way, I'll stay out of the cross fire. Just trying to figure out how you get the "Right" number out of all the possibilitys, cam ground and barrel shapped!

ZD75blue

What brand bore gauges are you guys using? I assume we're talking about dial bore gauges and not the snap type!

Twin_Turbo

ZD, if you use larger verniers (w/ digital readout preferred) and you angle the 2 tangs downward you will always measure the widest section.

With angled down I mean the tanks like this / looking from the side and not like this -

Look at starrett, can't go wrong there.