Which Way is UP ???

[joeycatt]

Gents - I've got Mahle pistons from my 1978 SC - I'm in the midst of the rebuild and am set to install pistons and cylinders. I've got Wayne's book (and am VERY thankful for that !!!) but my pistons do not look like the ones he shows in the book.
In the picture (posted here) you can see that the piston has an "E" cast into it - each one is like this - in this particular picture, the bump "up" section of the piston is to the right....
Can anyone tell me which way is"up" ? My guess (without any data to support the position) is that the "E" might stand for exhaust???? (E side down, and the piston pins line up correctly)

[ChrisBennet]

"E" is for intake. You can see it at about 1 o'clock on this piston.
In this picture I've marked the top of this 1981 Euro SC piston with a "T".*


-Chris

*Laugh if you want to but I label even the most obvious piston orientation. Half the time I'm working with the motor turned upside down and I'm just ditsy enough to screw up something this simple.

[cstreit]

Typically Porsche pistons will have the "bump" on the exhaust side of the piston. Why? Because the intake valve is bigger and needs more clearance to open. If the bump were on that side? Crunchy crashy.

[joeycatt]

Chris,
Thanks for the input and picture...very helpful.

I too, would have marked pistons, and everything else - but I bought this car in boxes - the PO did a full tear-down without marking a single piece. I've got boxes of nuts, bolts, washers, pistons, etc...you name it, it's there - but I've had to do major forensics to get this far in the reassembly.
OK - with your good direction, I'm off to the basement to install the pistons....CORRECTLY !!!!

thanks
john

[ChrisBennet]

Originally posted by cstreit
Typically Porsche pistons will have the "bump" on the exhaust side of the piston. Why? Because the intake valve is bigger and needs more clearance to open. If the bump were on that side? Crunchy crashy.

Hi Chris,
What you're saying makes sense but my memory says just the opposite. Maybe the hump on the bottom is an early motor thing? I forget whatpre 3.0 motor pistons look like but here are some data points from memory:
- 964 nearly flat with 2 humps top and bottom
- 3.2L "L" shaped dome with the "ridge" going across top and down left side.
- 3.0L has the dome is across the top half like a "bosom". (Associate something with sex and I seem to remember it better for some reason )
- 98mm JE's I've seen are either symetrical (not counting the valve pockets) or if the dome has been cut down it is flat or has the highest point on the left. This 98mm JE for a 3.2SS shows the latter case:



-Chris

[joeycatt]

Chris -
I just did a quick google of "INTAKE"....
The translator dictionary (online) says that the German equivalent is "Einlaß"
I'm not sure how to pronounce it....but it sure starts with an E !!!

thanks

[aturboman]

A quote from factory manual regarding this:

"K-jetronic equipped engines have a turbulence cavity in the piston top. The piston must always be installed so that the flat spot in the piston top faces upward (towards the intake valve)."

In other words the dome side faces the inlet valve, and the dished side faces the exhaust. Hope this helps.

[ChrisBennet]

Originally posted by aturboman
A quote from factory manual regarding this:

"K-jetronic equipped engines have a turbulence cavity in the piston top. The piston must always be installed so that the flat spot in the piston top faces upward (towards the intake valve)."

The way I read it, that sounds exactly the opposite of what it should be (for a 3.0L). That sounds like the flat spot (i.e. not the dome) goes near the intake. On a 3.0 the flat spot goes near the exhaust.

In other words the dome side faces the inlet valve, and the dished side faces the exhaust. Hope this helps.

While this is how the CIS pistons I've seen are, your conclusion seems the opposite of what you quoted above.

Hmmm, maybe I need to wake up more before I post.
Am I missing something?

OK, I dug out a manual with pictures. The "flat spot" referred to is a flat spot on the side of the dome. Not the flat part of the piston top.
I should scan the manual pages when I get a chance.
-Chris

[len911]

I'm in the middle of taking an '81 Euro 3.0l apart. Hope these pictures help. Exhaust side is towards bottom of picture (fully threaded stud). Lou

[aturboman]

I can't upload the factory photo (new computer) but the book clearly shows the dome pointing towards the inlet valve. I just recently build a Euro 3.0 with high comp factory pistons and the dome faced the inlet valve as well. These pistons have an offset at the piston pin also to help maitain balance so they need to be installed per the book. If you look carefully at the photos of the 3.0 Euro above you can see the dome has a side that is flat, this allows clearance for the inlet valve. This is what the factory manual refers to as the "flat face".

[ischmitz]

"E" stands for Einlass (German for intake)

[cstreit]

Originally posted by ChrisBennet
Hi Chris,
What you're saying makes sense but my memory says just the opposite. Maybe the hump on the bottom is an early motor thing? I forget whatpre 3.0 motor pistons look like but here are some data points from memory:

Chris,

You're right of course... I think the SC pisons have an oblong dome if I recall, and the last one I did had Max Moritz pistons, both with domes that go "closer" to the exhaust valve... However that's not true for all. (problem with making blanket statements.)

[ChrisBennet]

Originally posted by cstreit
Chris,

You're right of course... I think the SC pisons have an oblong dome if I recall, and the last one I did had Max Moritz pistons, both with domes that go "closer" to the exhaust valve... However that's not true for all. (problem with making blanket statements.)

This is all good though. If we didn't trade experiences we'd be like the fable of the blind men and the elephant - one feels the leg and thinks it must be like a tree, another feels the trunk and thinks an elephant is like a snake, etc.
If you hadn't shared your experience (Max Moritz) who knows, some day I might have concluded (wrongly) the orientation of some stange piston because I'd never heard of anything different.
-Chris

[anh911]

Fixed: