2618 forged pistons w/0.001" clearance at skirt?

[Torquey]

I am putting together a 4.6L Ford V8 modular (alloy block). The bores have been bored to 3.572". The gauge point on the CP branded pistons is 0.5" from the bottom of the skirt and at this point they measure 3.5685". However, these pistons flare towards the bottom of the skirt where the clearance is only 0.001"!! They are so tight they nearly scuff when sliding them down the bores. CP Pistons Inc. says to disregard everything but the gauge point measurement. I feel squeamish installing a 2618 forged piston with only 0.001" clearance... measured at any point along the piston. Is this feeling justified? Thanks for reading.
Walter Carter

 

[MikeHalloran]

I think Ford fits their own pistons that tight at the factory... with automated machines.



Mike Halloran
Pembroke Pines, FL, USA

 

[Torquey]

I measured Ford's OEM forged Mahl piston at the tip end of the skirt at 3.5497". Going into the factory 3.552" bore would leave over 0.002" of clearance. CP Piston is supposed to be the top shelf product for this engine so I believe they know what they are doing. I'm just a little dumbfounded that a forged piston would require so little clearance. A first time engine builder, such as myself, would like to avoid at all cost a set of warm pistons locking in their bores.
Walter Carter

 

[MikeHalloran]

Well, if the bores haven't been honed to a nice crosshatch, you've got room to do it.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

An alloy block will expand more than an iron block so they can run a bit tighter.

It will depend on the aluminium alloy to some extent with high silicon alloys requiring a tighter fit.

The skirts have some flex and this varies with length, design and alloy. That is why piston manufacturers specify the very robust part of the skirt near the pin boss as the reference point as it is stable and does not flex so much.

Take their advice. They know what they are doing.

One word of caution, use the weight of oil specified. The tight clearance might be somewhat to do with light weight oil being used.



Regards

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[thundair]

I think the piston manufacture has a program that predicts the shape at running temperatures and machine the piston to that in the ambient room temperature. So you may see an oval or even a triangular shape to the piston.
I may have pin size or dome request to a piston manufacturer but as far as the bore fit I leave it to the pros

Cheers

I don't know anything but the people that do.

 

[Tmoose]

"The bores have been bored to 3.572". The gauge point on the CP branded pistons is 0.5" from the bottom of the skirt and at this point they measure 3.5685". "

That's 0.0035 inch clearance.

Squeeze the skirt tab/tails with one hand and measure.
They might be quite flexible.

Do they have slots, or hole for oil return in the oil ring groove?


The fancy ones in this picture call for 0.006 inch.

http://images.hotrod.com/techarticl...man_built_windsor+revised_x_skirt_pistons.jpg

 

[WilliamH]

A PROPERLY DESIGNED piston, forged, cast or whatever will require only .001" or so, actual skirt clearance, cold. By properly designed I am inferring that growth is designed into the piston in a predictable, designed manner.

Pistons do not expand evenly down, across or in any other geometrically definable manner. Alloy 2618 ("Mahle metal") is a superior alloy for forging pistons. However, not all piston forgers know how to take advantage of CONTROLLED piston thermal growth. So, to cover their A** they recommend huge clearances, i.e., .006".

CP knows how to design, and manufacture pistons. Follow their guidance and you can rest assured.

Regards,
Will

 

[BurnBros]

It has been my experience that liner/bore material is the defining component on clearance. On a cast iron liner/bore it has been .003-.004 depending on application. On a nicasil,Alusil,etc liner/cylinder I have run as close as .0007 with no problems.

I have heard of Ford modular engines having Nicasil liners custom made for them, is this the case?

 

[groundPork]

Actually Mahle typically uses 4032 (high silicon content) forgings. 4032 allows much tighter piston to cylinder wall tolerance than 2618. There is really only so much you can do to machine in compensation for 2618’s thermal instability, without eventually sacrificing overall stability within the bore. I’d like to see something other than a billet aluminum nikasil coated bore or alusil, that can run tighter than .002” at those bore sizes with 2618. Most Mod motors sound like spray paint cans much over .003" total clearance piston to wall, and I think the most important point is that the original poster may have mis-calculated his total clearance. As pointed out earlier, the dimensions measured make .0035" not .001". I wonder, did you tell CP you had .001", and they really said it'd be all right? or did you just give them the measured dimensions? They like tight tolerance, but even they aren't that suicidal...