Piston to cylinder clearance???

[Jeff Alton]

Okay, I have searched around here and the internet and maybe I am just a crappy "searcher" but I can not find a *good* explanation of how to measure piston to cylinder clearance. Is the clearance spec just the difference in piston size to bore size? OR is it the clearance between the piston and the bore all around the piston? Do you measure the bore and the piston, or do you put the piston in the cylinder and measure the gap?

When reusing a P/C set I just sent them to the machine shop to measure for me in the past. But, this time I am using bored and plated 98's with some JE's and want to make sure I have the right clearance.

Cheers

[cgarr]

I just had my 95 bored to 98 and the spec sheet shows .0015 for the J&E pistons, so the cylinders are that much larger than the pistons, not .003, is that what your looking for?

EDIT: are you building the same thing I am? 3.4 10.5/1? you using stock intakes and twin plug?

[tom1394racing]

I always thought that piston to cylinder clearance was one half the difference between the piston and cylinder diameters. The piston is measured with a micromenter and the cylinder measured with a bore gage.

Is it more complicated than that?

[hobieboy]

Here's what I just did - Ask the machine shop to measure the bore of the cylinder (factory suggest to measure 3cm from top of cylinder but my machine shop measured them at multiple places). Meausre the pistons - around the skirt area (18mm from bottom). Calculate the difference between the 2. Factory says tolerance should be <0.12mm (this is for 3.2/3.3 ones but I think they are the same for all Mahle?).

If you are using different pistons/cylinders, then I think you need to ask for what is acceptable tolerance.

[Jeff Alton]

Thanks, I have the specs from JE and I have the right tools to do this, jsut want to make sure I am doing it right!

Cheers

[cgarr]

It is best to have the pistons when they do the cylinders to make sure, I just checked mine and the cylinders are .00158 larger than the pistons, I know the J&E's run this way, I build my 912 several years ago with these pistons and they all run very close or you will hear it slap when they are cold

[Jim Williams]

The little Porsche spec books, and everything else I have read regarding the dimensions and clearances on these engines (factory manuals), says the piston to cylinder clearance is defined as the difference in the piston diameter and the cylinder diameter. If it is any different from this, I have been doing it wrong all these years........... :^(

[Jeff Alton]

Thanks that is what I wanted confirmed!

Cheers

[JeremyD]

Jeff - make sure you measure these. I did not have the proper equipment when I did mine. Turns out it's common on the bored and replated cylinders to have cylinders out of round - and with taper... When I do this again - I will either have the proper equipment - or will send to a machine shop to measure the bores in at least 3 different places.

The JE pistons I have were very consistent. all within one half gram -

Too much room and you'll have piston slap, and blowby -

[Fritter]

Originally Posted by tom1394racing

I always thought that piston to cylinder clearance was one half the difference between the piston and cylinder diameters. The piston is measured with a micromenter and the cylinder measured with a bore gage.

Is it more complicated than that?

Digging this thread up....so is Tom right, is the piston to cylinder clearance simply the difference between the two diameters, or do you take the difference between the two diameters divided by 2?

[Flieger]

The difference in the radii is the difference in diameters divided by 2.

[(Dc-Dp)/2]=piston to cylinder wall clearance all the way around the circumference of the piston.

[Fritter]

Seems correct....so if I have an average of .115mm difference between piston and cylinder diameters on my 2.0, and .115 divided by 2 is .057mm, I am at the very top end of the .035 to .055mm factory piston to cylinder clearance....with the absolute maximum being .10mm.

So, I was going to rering and reuse the PC set in my mild street 2.0 motor. If I put 5K on it a year, it will most likely last a long time before another rebuild.

Correct me if I am wrong.

[Flieger]

Sounds good to me.

I am no expert, so the above statement does not mean much

[Fritter]

Originally Posted by Flieger

Sounds good to me.

I am no expert, so the above statement does not mean much

Doh!!

[Jim Williams]

Originally Posted by Fritter

Seems correct....so if I have an average of .115mm difference between piston and cylinder diameters on my 2.0, and .115 divided by 2 is .057mm, I am at the very top end of the .035 to .055mm factory piston to cylinder clearance....with the absolute maximum being .10mm.

So, I was going to rering and reuse the PC set in my mild street 2.0 motor. If I put 5K on it a year, it will most likely last a long time before another rebuild.

Correct me if I am wrong.

I may be mis-reading your interpretation of the wear limit, but if you have a difference of .115 mm between the measurements in your piston and cylinder diameters, I believe that is your clearance, not that number divided by 2. (See my post above.)

The Porsche factory workshop manual for the 2.0 liter engine says, or implies by lack of further definition, that the P/C clearance is the difference in the two measurements. I can't find anything about dividing that number by 2.

Maybe a more recognized engine rebuilder like Steve Weiner or Henry Schmidt will post here and remove the apparent confusion.

[mb911]

Its pretty simple You measure the piston with a Mic then set your dial bore guage up and measure the difference of the cylinder and the piston measurement. Done this for severel hundred engine with good success. the one thing that should be noted is that with different material types and forming processes that expansion will change accordingly and you could get to tight or to loose

[Fritter]

Yes, we need one of the engine experts to chime in here.

Another thing is that in Wayne's book, he (Porsche) states that pistons and cylinders for a 2.0 are out of spec when either are worn .1mm from their measurement range. So, .2mm in total. But, the clearance spec is .035 to .055mm, not much wiggle room.

Maybe I am just overanalyzing and denying the inevitable PC replacement....which I would rather not do on my first 911 motor rebuild. I'm just trying to keep costs down and see if I can actually rebuild one of these beasts myself.

My next step is to measure ring gaps, then I will really know if I am hosed or not.

Thanks for everyone's input.

[Flieger]

I think piston to cylinder bore clearance should be the difference in radii, rather than difference in diameter.

Hypothetical Example: Cylinder diameter - Piston diameter = .1 mm

Piston to Cylinder clearance can be the amount the piston can move radially (sp?) before hitting the cylinder.

If the piston starts in the exact center of the cylinder, it can move a maximum of .05 mm in any directon before it contacts the cylinder.

If it started at zero clearance on one side of the bore, then it could move by .1 mm toward the opposite side.

Which is correct?

For ring gap, wouldn't the difference in radii be more useful?

[tom1394racing]

Piston to cylinder clearance is the difference between the cylinder and the piston diameters.

If you check the nominal specs for piston diameter, cylinder diameter and piston to cylinder clearance in the factory spec books, it shows nominal clearance to be the difference between the piston and cylinder diameters.

[Eagledriver]

The measurement is the diameter difference like Tom said. It is not the clearance "all the way around". I measure it with a feeler guage between the piston and cylinder. The spec of .035mm to .055mm is the tolerance for new parts. The .1mm is the wear limit.

-Andy