NPR Pistons PTW clearance

[DeNoZZo]

Hello, Does anyone know what the NPR pistons ptw clearance is or where I can find that information?


Thanks!

 

[RadDsm]

They are factory pistons, so use the factory specs.

With that being said, I used a little bit looser ptw with mine, just to give a little more margin for error.

I used:
Top Ring - 0.017"
2nd Ring - 0.019"
Oil rings - 0.010"

 

[BogusSVO]

the PTW is built into the piston, so if you have the std size piston, 85.5mm X.03937 = 3.366
your bore should be 3.366 and when you mic the piston 90* to the centerline of the wrist pin.

you eill find that the piston is small by about .003 so you should read appox 3.363

 

[DeNoZZo]

the PTW is built into the piston, so if you have the std size piston, 85.5mm X.03937 = 3.366
your bore should be 3.366 and when you mic the piston 90* to the centerline of the wrist pin.

you eill find that the piston is small by about .003 so you should read appox 3.363

Mine are .020 oversize?

 

[talonDSMerr]

.020 oversize is the same as 85.5mm bore. The numbers bogusSVO gave you are correct for .020 over, must have typed std by mistake.

 

[BogusSVO]

sorry I did type std by mistake

it will not matter, the PTW is "built into the piston, so it matters not if it is .5mm/.020 or .75mm/.030 or 1mm/.040 or 1.5mm/.060

 

[DeNoZZo]

Ok, I'm sorry I'm a little confused what numbers exactly should I give my machinist for the PTW clearance then?

 

[talonDSMerr]

What bugusSVO is saying is that the PTW clearance should be "built-in" to the pistons. This means that if your machinist bores the cylinders to 85.5mm or 3.366", the pistons should already be undersized by the ptw clearance they should be run at.

Stock ptw clearance should be .0012" to .0020". After mic'ing the pistons, the bore you should tell your machinist to run at would be: Piston Diameter + ptw = Bore

ptw in this equation can be anywhere from .0012" to .0020".

 

[BogusSVO]

he should bore to the bore spec on the box, 85.5mm/ 3.366

that is your std bore +.020

but he should mic the pistons to make sure they are the spec they should be appox 3.363

 

[DeNoZZo]

What bugusSVO is saying is that the PTW clearance should be "built-in" to the pistons. This means that if your machinist bores the cylinders to 85.5mm or 3.366", the pistons should already be undersized by the ptw clearance they should be run at.

Stock ptw clearance should be .0012" to .0020". After mic'ing the pistons, the bore you should tell your machinist to run at would be: Piston Diameter + ptw = Bore

ptw in this equation can be anywhere from .0012" to .0020".

Ok I see, thanks for explaining that.

What do you guys suggest for PTW clearance then since I have that much to play with. The car has a cut crank and 6 bolt rods. It wont see a ton of boost for a while but I eventually plan on upgrading the turbo past stock. If that helps?

 

[talonDSMerr]

If you plan on running more boost than stock, I would go on the loose side as far as ptw clearance.

 

[Spoolin98]

Sorry to bring back a dead thread but I just want to see if Im reading this correctly. I just got my hands on a set of .030 NPR pistons.

So to do the math I take 85.75 x .03937 = 3.375

So my bore should be 3.375", and my piston should be .003 smaller at 3.372"

Is this correct?

 

[BogusSVO]

.003 would be a tad loose, but not enough to cause piston slap.

As posted above factory spec is .0012-.002

That would be for Std bore or a .030 bore

 

[Spoolin98]

the PTW is built into the piston, so if you have the std size piston, 85.5mm X.03937 = 3.366
your bore should be 3.366 and when you mic the piston 90* to the centerline of the wrist pin.

you eill find that the piston is small by about .003 so you should read appox 3.363

So what was this equation all about?
Thats where I got the .003 from.

I did the equation above with 85.75mm in it and then measured my .030 pistons 90* from the wrist pin and they came out approx .003 smaller than the equation.

So im kinda confused...

 

[BogusSVO]

.003 PTW is common on alot of domestic engines and a number I use alot, in the org post I made, That is the go to number I use.

Keep in mind .003 is the thickness of a sheet of notebook paper.

If your mic is off in the slightest, or you twist the mic a hair too tight, it can and will give you a false reading.

If by your feel and your math you are .002-.003 you will be just fine

Go on with your bore/hone build.

 

[Spoolin98]

So basically I mic the piston 90* from the wrist pin half way up the skirt, take that measurement plus .0012-.0020 and bore the cylinder to that size.
Is this correct?