JE Piston Clearance.. what say ye?

[IXjamesXI]

Hello all..
Still slowly plugging away on a 3.2 short stroke rebuild for my 82 SC. Bought a set of Mahle Carrera jugs from Rothsport, and had EBS send them to be bored and re plated to 98mm, and got some JE slugs 10.5:1 CR to match.

The last few months I have been hunting ebay for metrology gear, and I have amassed a serviceable collection in short order, for much less money than I thought possible including several Starrett and Mitutoyo metric mics, gage blocks, and even a Mitutoyo bore gauge that goes all the way out to .001mm.

Today was a bit snowy so I parked myself in front of the TV to do some measuring and watch the start of the Daytona 24, which was mostly spins and botched pit stops sprinkled throughout with some actual driving.

I am coming up with about .060 mm (.0024 in) of piston to cylinder clearance and I wonder what you all think of that. JE's data sheet specifies .0381 mm (.0015 in).

Am I headed for piston slap issues? I hope running a little loose won't really aggravate wear faster. The pistons are just the normal JE forging I think.. 32P? I found some older threads on clearance but not sure what the latest thinking is as I know the forgings have changed over the years..

I did take care to measure the pistons .5 in from the bottom of the skirt, but I noticed that my pistons tended to be widest almost at the bottom of the skirt.. if I use the measurement from the bottom of the skirt I get more like .046 mm (.0018 in) of clearance... but I didn't expect that to be the widest part since JE themselves say to measure .5 in up from the bottom.

Perhaps my measuring skills just aren't that good yet. Any and all input is greatly appreciated.

[Flat6pac]

Your forged aluminum pistons are going to expand as the engine warms. The JEs can start with slight slap and then warm out quickly and go quiet.
Bruce

[Mark Henry]

Mahle's have way tighter tolerances than JE's.
JE's expand more when hot.

[Bob Kontak]

Originally Posted by IXjamesXI

I am coming up with about .060 mm (.0024 in) of piston to cylinder clearance and I wonder what you all think of that. JE's data sheet specifies .0381 mm (.0015 in).

I just wandered in here for the tech forum and I have ohhh, one 911 top end engine rebuild under my belt. Jon at EBS almost literally walked me through the whole thing in the mid 90's.

I do remember Jon telling me the JE pistons, out of the box, piston to bore clearance, were larger than the outside wear specs of the Mahle piston clearance (expansion). One point five thousandths seems pretty tiny for a slug that will get bigger when warmer.

Do a double check with EBS.

[KTL]

Charles at LN Eng. also recommends 0.0015" for JE pistons in Mahle cyls. I told him my previous install was around 0.0025" and he said that was too fat. When I got my replacement pistons from him & gave him my "new" cyl. bore diameters, the pistons were made to have 0.0015" Or the reverse could be done. Buy your "off the shelf" pistons and have the cyls bored & plated to establish desired clearance.

[Lapkritis]

JE has a manufacturer disclosed margin of error of +/- .0005". If you're finding .0018" measurement on .0015" spec then the hairs are officially split. Good to go in my opinion... many race engines (not Porsche specifically) run a bit looser as a general rule which produces a smidgen more power and also gives a safety margin in competition should you get things hot and tight.

Edit: the trade-off of way too loose is cold slap. I run a larger gap on my engine with iron cylinders (different spec) and do not have slap even cold but YMMV.

[BGCarrera32]

Originally Posted by Mark Henry

Mahle's have way tighter tolerances than JE's.
JE's expand more when hot.

Yup. I was told by Mahle that Mahle P's and C's are the same metallurgy and move at the same rate (in general). I have P to C total clearance on a few of mine at .0007" or within 2 grades. Measurements taken on a Renishaw CMM probe. I wouldn't do numbers that tight with JE.

[IXjamesXI]

So what really is the best method for measuring pistons anyway? I've read, and observed, that the piston is widest on the axis perpendicular to the piston pin, at a point some distance up from the bottom of the skirt. It seems like this widest point is very narrow, maybe not much larger than the face of the micrometer anvil possibly? I felt like I was slipping off this point every time I tried to measure it.

I gave it all a second try and I tried setting my micrometer at a value and trying go / no-go through the gap. I seemed easier to find the wide spot, but with a little wiggle it was easy to slip the piston through if you did not have it exactly square to the micrometer. This go / no-go method was really tedious but I had more confidence in my measurements. How do others do it? How do the pros do it? Have not seen a real how-to on here, but maybe I missed it.

[KTL]

The go no-go method isn't a good way to go because you can be somewhat forcing the piston thru the mic, futzing the skirt face and anvil face. The way I do it is set the piston on the table upside down and place the mic at the spec'd location from the bottom of the skirt. Hold the mic on the bridge and wiggle it as you rotate the mic thimble. You should be activating the mic's friction or ratchet release as you're "clamping" on the piston. The wiggling of the mic allows you to hone in on the narrowest dimension and eventually you can't wiggle it when the mic reaches the tightest measurement

Or you can place the mic in a mic stand (or your bench vise) and hold the piston instead. Procedure is still the same- wiggle the piston as you turn the mic thimble.

[Neffets]

I'm also looking into piston clearance for my 90mm JE's now.

I have the largest cyl/pist combo (group2), and JE's


I'm at 0.11mm / 0.004" if I mix large piston with large cyl etc.
Ovalness is within 1 hundreth of a millimeter so that part is good.

So what to do with the P/C clearance? get custom JE's?

[KTL]

Yep you need to have the pistons sized to your cylinders if you're going to use the cylinders as-is. 0.0040" is much too much clearance for the JEs. You want them at 0.0015"

[Jcslocum]

Piston to cylinder wall clearance
1. Piston diameter must be measured at gauge point, which, in most cases is .500" up from the bottom of skirt. (See fig. 1).
Dimensions listed are gauged at a temperature of 72 degrees fahrenheit. Note: aluminum expands and contracts with temperature variations.
2. Your pistons are manufactured from 2618 non-silicon aluminum alloy.
3. Clearance is built into piston based upon finished bore for normal operating conditions. (See Table. 1).
4. Clearances listed below are minimum. Some applications such as supercharged, turbo, nitrous and endurance applications may require
additional clearance (.001-. 003"). Coldwater pickup marine applications may require an additional .002-.004".


Table 1 bore range min clearance.
Sport compact 2.500 to 3.625" .0030 to .0036"
Sport compact 3.626 to 3.999" .0035 to .0045"
SB applications 4.000 to 4.200" .0035 to .0045"
BB applications 4.200 to 4.600" .0040 to .0050"


This is from the JE site. I have not called them direct for the porsche specific apps.

[KTL]

Originally Posted by Jcslocum

This is from the JE site. I have not called them direct for the porsche specific apps.

Those specs you listed from the site are generalized. You need to see a JE spec sheet for the Porsche pistons. They say right there on the sheet that clearance should be 0.0015"

[Neffets]

I've just sent JE an email for price and delivery time
I guess it's only the skirt that is increased? I'm going for 90.001 & 90.000

[IXjamesXI]

Good luck w getting getting the right pistons for your bores. I didn't see it mentioned here but if no one has mentioned it, before ordering custom pistons from JE, consider the alternative of having your cylinders replated and honed to match the off-the-shelf pistons you have. I am not sure, but I think that process can add a little material back, making your bore effectively smaller, but I could be wrong so perhaps someone will correct me here if I am, or you will find this out in your research. That way if you are going to throw down a grand or more, at least at the end of the process you get new pistons and new bores.. instead of just new pistons that have been sized to fit your worn out bores..

Thanks to everyone for chiming in.. the more I think about it, that go / no-go is the wrong way to do it for the reasons described. I think it might be time to invest in micrometer holder for operations such as this...

[Neffets]

I just got an answer fro JE , to get custom skirt diameter 2.5weeks delivery time price same as off shelf pistons
They will make all 6x the same size, so you can not get them with individuall diameters.

[JFairman]

If you order custom pistons from JE right now they might be a little busy with the Daytona 500 coming up