JE Piston to Cylinder Clearances
 

I'am about to send my cylinders off to be re-nikasiled, and I'am sending the JE pistons with them so that they can machine the correct piston to cylinder clearance. My car is a 1977 930, so checking the JE spec sheet indicates a clearance of between 0.004" to 0.0066" . Does this seem excessive? Bruce Anderson's book states that clearances greater than 0.004" indicate that the piston/cylinder need replacing.

JE's require a little more clearance being that their metallurgy is a little different than the Mahle pistons. I have my JE spec sheet at home that I can check if you need. EBS can also quote you the clearances.

Edit: read Steve@Rennsport's response half way down
http://forums.pelicanparts.com/911-engine-rebuilding-forum/139254-life-expectancy-j-es-street-engine.html

I think 0.004-0.006" is a little much for a street car. The noise one hears with clearances this loose is called piston slap, but I guess JE's metallurgy allows more normal tolerances at regular op. temperatures. Imagine how they'll sound with an engine with some miles on it. During this warmup period, the pistons rock in the cylinder and create add'l wear on the rings, so take it easy during warmups.

BTW, 1500 miles is not a valid indication of future piston life.

Sherwood

PS. The repair manual says the wear limit is 0.1mm (0.0039"); this with Mahles.

We run .002" +-.0005 with JE 4032 forgings on our aluminum cylinders, per JEs recommendation. About .0025" is as much as we like to spec, as customers tend not to like piston slap when their engines are cold.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

Thanks for replies guys. JE's spec sheet suggests an extra 0.001" to 0.003" clearance for a turbo charged engine, and mine is a 930. Anyone else running JE's in a turbo? I would be interest to hear what clearances turbo owners are running JE's at.

typically 'better' forged pistons have a higher silicon content in the alloy. These pistons have a higher expansion rate, requiring more cold clearances..

JE should be able to direct you to the right clearance specs..

Chris

"typically 'better' forged pistons have a higher silicon content in the alloy. These pistons have a higher expansion rate, requiring more cold clearances.."

So from this statement, you're inferring JE pistons are superior to Mahles because they expand more? I would take the contrary view; higher quality aluminum forgings don't expand as much. Carry this out far enough and overheated pistons will end up with zero wall clearance. If you compensate with excessive clearance (whatever that means), engine life is compromised due to ring wear and piston slap. Worn rings = compression loss/increased oil consumption. It's always better to have a more dimensionally stable slug of aluminum moving up and down at any crankshaft speed.

Sherwood

I think there needs to be a distinction pointed out here: JE offers two forgings, a high-silicon 2316 forging and a super high strength 4032 forging. The 2316 is the forging that doesn't expand as much, but should not be used for any application other than N/A engines. The 4032 is superior in strength, but requires a little more p/c clearance on cast iron cylinders, leading to piston slap when cold. On aluminum cylinders, you can run tighter clearances, like .0015"-.0025", again N/A, with the 4032, since the cylinders expansion rate is close enough to the expansion rate of the piston. Ask JE this, they will spec out on a 102mm bore a clearance of .0019", with a 4032 forging, on an aluminum nikasil plated cylinder. With turbos, since the piston warms up slightly faster, you just add JEs recommended .001" extra clearance, so ~.003", with the 4032 forging. Now here's the question of the century- if you can run .0015-.0025" p/c clearance with the 4032 forging on an aluminum cylinder, how tightly can you run a 2316 forging?

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

Thanks for the info Charles. The pistons I have for my turbo are JE 2618 non-silicon aluminium alloy (what ever that is). They don't make any distinction between aluminium or cast iron cylinder blocks in the spec sheet they sent me.

The 2618 is the high-silicon content piston. It is the 4032 forging that you want for use with your turbo. I would double check with JE to make sure that the pistons that you have are indeed safe for use with your turbo. The spec sheets that come with the JEs are better suited for SBCs and BBCs, not aircooled engines.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

Well now you got me worried Charles. The spec sheet enclosed with the pistons are for 2618 pistons, and the pistons were supplied by EBS as 7.5:1 pistons for a 3 litre turbo. There is also a job sheet that came with the pistons. The motor type states 3.0L but no mention of turbo. There is also clearance information on this job sheet that I previously missed, and this states a clearance of 0.0002" (.05mm), with some applications requiring more clearance (on their spec sheet JE refers to turbo applications requiring the extra clearance). The EBS invoice states these to be 3 litre 7.5:1 turbo pistons and the rings that came with them certainly look the same as the ones in my old pistons. The other thing I noticed was that the original Mahle pistons are slipper type pistons, the pistons do not extend down passed the pin holes on either side, while the JE's do. The 2618 spec sheet states the the pistons are non-silicon aluminum alloy, which I take to mean that they do not contain any silicon. These pistons are essentially on off the shelf JE product.

I have not been able to contact JE as they don't offer an email address on their web site.

If EBS sourced you the P/C set, they obviously have done some internal testing and know that the combo they sold you will work with your turbo. I would indeed double check with them regarding which alloy the pistons are made of and also that the p/c clearance that they spec'd is indeed appropriate for your forced induction application. Otherwise, you should be golden. As a side note, I thought that last time I spoke to EBS, that they used the 4032 forgings, not the 2316 forgings...maybe i'm out of the loop on this one.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

I have sent an email to EBS asking them to supply me with clearances the would use with these pistons. Thanks for the help Charles.

No problem- I hope they can clear up this confusion once and for all! Make sure you let us know what the outcome is. :-)

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

I have 2nd hand knowledge of a racer using .004 or so with JE's on a turbo-charged P-car and getting a LOT of scuffing and eventually one seized... He eventually ended up with about .007 and though the engine was a bit noisy when cold, he was quite happy with it.
Only one example though...

How much boost was he running? I heard the same thing with an autocraft customer of mine that was having a similar problem with a sand dragster. I'll be calling around today to EBS and a few of my other suppliers to try to get some more definative answers here.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

I think it was on a 924 or 944 turbo. It was a spec car, so running whatever factory boos there was. Unfortunately I don't know the value.

Ok guys, have an answer from EBS. They say to go for 0.0030" - 0.0036" plus an additional 0.001" - 0.003" for my turbo. This is as per the JE spec sheet for the 2618 pistons.

Wow!

Almost 7 thou at the top end of those claerances...

Ring life will be minimal (a few hours) IMHO...

I 'd be very dubious of this... I would ask them if they really mean that!

Kind regards
David