JE Piston to Mahle cylinder clearance ??

[derm.1785]

It’s probably a daft question but I have a very good set of original 95mm Mahle cylinders and I’m about to buy a new set of JE pistons to go in, my question is, what is the clearance from stock pistons, I know they say it should be 0.508 but I don’t have the kit to measure them. I Know for a fact that my original CIS pistons had a clearance of 0.3 because it’s stamped on the top of the piston ( 94.97 ) JE just state “ set clearance at 0.508 “ they don’t tell you what it actually is out of the box, just to measure up from the skirt to find out, I don’t have a machine shop close so would like to know what they are stock out of the box ?? Thanks

[Trackrash]

Not sure where you are getting .508. The specified cylinder to piston skirt clearance is 0.0015" (0.0381mm). This can be checked with feeler gauge.

However, it is important that you make sure the cylinders are IN SPEC. The easiest way to do a preliminary evaluation is to look at the area of the cylinder where the top ring rides at the top of the cylinder bore. If there is still cross hatch through out that area you are probably OK. But then you need to have an experienced machinist measure them to confirm taper and roundness.

[derm.1785]

Hi Gordon, where your spec sheet says .0381 mine says .0508. It looks like they have changed the clearance spec since yours were made. This is where I need to confirm the sizes, I don’t have a gauge that measures that far, looks like I’ll have to purchase one. My barrels were re plated to original spec a few years ago and are still in great condition. Spec sheet on those states 95mm. My issue is the “set clearance “ statement !!! Surly this clearance is in the piston NOT the barrel ???? Confused

[stownsen914]

Looks like they're recommending .0508 mm piston to barrel (.002") in your case. It's hopefully more checking than setting, but it is important to make sure your cylinders are in spec for diameter and not having ovality. Many recommend a light hone on the cylinders to give some texture to help seat the rings. Whether you do these things or a shop.

[Trackrash]

Originally Posted by derm.1785

Hi Gordon, where your spec sheet says .0381 mine says .0508. It looks like they have changed the clearance spec since yours were made. This is where I need to confirm the sizes, I don’t have a gauge that measures that far, looks like I’ll have to purchase one. My barrels were re plated to original spec a few years ago and are still in great condition. Spec sheet on those states 95mm. My issue is the “set clearance “ statement !!! Surly this clearance is in the piston NOT the barrel ???? Confused

The 0.0508 looks like a turbo spec. Is that what you have?

[Zuffenwerker]

You need to measure don’t trust the math ideally you want .002” long feeler gauge is cheap and easy but really need to have someone use a bore gauge.

Daniel

[mikedsilva]

Originally Posted by Trackrash

The 0.0508 looks like a turbo spec. Is that what you have?

Each set of Carrillos I have used so far, have called for a 2 thou clearance and these are all for normally aspirated engines.

[derm.1785]

Hi guys, the issue is that I DONT trust the math. My engine is not turbo and the spec sheet is correct in 0.0508. Mike, you say your Carrillo pistons called for 2 thou clearance, is this in the piston or the cylinder??? I called my machine shop this morning and he’s NEVER heard of opening up cylinders to accommodate “ clearance “ he’s asked me to drop them down to him to measure but he won’t re bore the cylinders as they will need re plating and he certainly won’t cut pistons to make clearance work either. It’s late here in the UK and I can’t speak to JE to ask but it should be simple !!! It has to be the pistons that are under size same as the original Mahle pistons at 94.97. I don’t understand why JE don’t clearly state this on the spec sheet. Still confused.

[Trackrash]

Post the spec sheet you have from JE.

JE makes more than one type of piston and there are two aluminum alloys that can be used. But IMO the difference between 0.0015" and 0.002" is spitting hairs. You can see on the sheet I posted that some applications may require more clearance.

[mikedsilva]

Originally Posted by derm.1785

Mike, you say your Carrillo pistons called for 2 thou clearance, is this in the piston or the cylinder???

Not sure how to answer that question... I will try to answer below.

The clearance is 'between' the piston skirt (at a point determined by the manufacturer for measuring) and the cylinder wall.

If you start off with good cylinders, you measure accurately and then order pistons to be made to suit the cylinders.

Or if you have cylinders that need to be re-plated, or opened up and plated, then you order pistons first, and send the pistons along with the cylinders so they can be plated and honed to suit the pistons, with the required clearance.

I've only gone through this process 3 times where other members with much more experience would have done it dozens of times.

[derm.1785]

Her you see my spec sheet. Nowhere on the sheet does it actually show the diameter of the pistons, however it DOES state to “ set clearance” , how it this supposed to be interpreted?? Set where ??? There is a lot of detailed info on this sheet but the MOST important measurement is left to ambiguity. My Old Mahle pistons CLEARLY show what the diameter is, stamped on the top, why can’t JE be this clear. Am I being too picky about the piston/cylinder size ??

[derm.1785]

“ Some applications may require more clearance “ �� REALLY FFS

[Trackrash]

OK. I am working right now with a set of JEs (92mm) and replated Mahle cylinders.

The set came from EBS.

EBS stated that JE pistons are manufactured accurately enough that they will be in spec when used in Mahle cylinders that are also in spec.

The piston to cylinder clearance on the spec sheet is basically a double check.

Interesting that the spec sheet (won't bore you with posting it) that we have on these new pistons specifies the same clearance as on the sheet I posted above. 0.0015". FWIW.

[stownsen914]

Don't get too caught up in the "set clearance" language. Many cars' cylinders are iron and can be honed/bored to whatever you want after pistons are in hand. Obviously not so easy with nikasil cylinders, but Porsche cylinders are often in spec even used. In your case it will be a check of your cylinders to make sure they are in spec, and that they haven't gone oval. Have your shop measure them (making sure they check for ovality), and you can do the feeler check as well.

Once the shop has them, consider having them honed to give some texture for the rings to seat. If you do this, there is a specific Ra value you should shoot for. It's important to get this right on nikasil, both for the rings to seat, and so the nikasil isn't damaged.

[mikedsilva]

Originally Posted by derm.1785

It’s probably a daft question but I have a very good set of original 95mm Mahle cylinders and I’m about to buy a new set of JE pistons to go in, my question is, what is the clearance from stock pistons, I know they say it should be 0.508 but I don’t have the kit to measure them. I Know for a fact that my original CIS pistons had a clearance of 0.3 because it’s stamped on the top of the piston ( 94.97 ) JE just state “ set clearance at 0.508 “ they don’t tell you what it actually is out of the box, just to measure up from the skirt to find out, I don’t have a machine shop close so would like to know what they are stock out of the box ?? Thanks

I don't think you can say that you "know for a fact" that the cylinders are 95.00mm.

They are a nominal, not actual, 95mm.... and Porsche had 4 different size categories. Most of the cylinders I have measured are around 94.95 to 94.96mm bore, and if yours are, then instead of having a 0.03mm clearance, your old setup would be 0.07 to 0.08mm clearance.

Your best practice here, would be to have the cylinders measured properly. If you don't have a shop near you, then you need to find someone or buy the tools and learn to do yourself.

[LJ851]

Comparing any different piston clearances is not of benefit and potentially applies to oranges.

Piston material, piston manufacturing process, skirt profile, cylinder material and engine usage all affect the piston to cylinder clearance.

The piston is Never resized to match a bore, the cylinder is sized to the piston. Pistons are not round at room temperature.

If you can’t measure the piston to cylinder clearance yourself, you need to find someone who is capable to verify the clearance is within the piston manufacturers spec for your engine. If it’s not, the cylinders need to be resized to match the pistons. It’s really that simple.






..

[targa72e]

derm.1785

see if the information below helps.
While your existing piston is stamped 93.97 you will see that is also stamped with a 0. You should also see a corresponding size stamped on the cylinder on the side with the triangle. In picture below this is also a 0 cylinder



For 911 SC there were two different piston and cylinder manufactures and 4 different size classes. These sizes from the spec book are shown below.



So for your size class 0, the piston diameter when new should be between 94.965-94.975 assuming its a Mahle piston. Your cylinders would have a larger diameter resulting in a clearance of .025mm-.042mm.

I looked at fifteen 3.0 pistons I have and only one had diameter and clearance stamping like yours. Porsche did not build to this clearance they measured and matched, this is what you need to do as well.

You can also see that the comment that "the clearance will be in spec if the cylinders are in spec" cant be true as there is up to .035mm variability in the cylinder diameters for a 3.0L.

Porsche did matching of components to get tighter tolerances vs machining the parts to more exact size from the start. This method is less expensive but still results in better clearances. They also sorted items by weight as well.
When ever using used parts you need to measure them. Your bores may look perfect to your eye but be out of spec for ovality and taper.
As others have said the general procedure when purchasing off the shelf pistons is to send a piston to the plater with the cylinders and specify the clearance needed. You can also do the reverse and send a finished cylinder to the piston manufacture but then you are looking at custom pistons.
I would also double check with JE about the clearance listed on your spec sheet as that looks too large for aluminum cylinders. Because the aluminum cylinders expand more than a cast iron block you can generally run tighter clearances. I have a set of new JE's as well and the sheet with them match trackrash listing at .0015.
So measure and verify your cylinders are good to use. If good, see what the clearance with your new pistons and old cylinders is and verify the installed clearance with JE.

john

[derm.1785]

Thanks for all your info guys, I’ll send all of the parts off for confirmation of sizes and see what the outcome is. My assumption that you can put new pistons into an old engine is obviously wrong so I’ll have to wait

[stownsen914]

Worth noting that the variance in diameter across the cylinder groupings is like .001". I might be more concerned with possible ovality, but let's see what the shop finds for measurements.