Mahle pistons

[fred cook]

I am about to start on an engine rebuild using a set of Mahle 10.1:1 pistons. Each piston has an arrow pointing either right or left (3 each). Do the arrows point towards the front of the engine (clutch end) or the rear of the engine (distributor end)?

Thanks!

[ajwans]

Does this help: Piston pin offset

andy

[fred cook]

Originally Posted by ajwans

Does this help: Piston pin offset

andy

But, I think that I will give Don at EBS a call and ask him. That is where the piston set and cylinders came from.

From the writeup, it appears that the arrows should point towards the flywheel so that the piston offset is correct.

Anyone else care to "chime in" on this topic?

Thanks,

[Gary De]

Fred,
Go to Mahle website site and they explain the markings on crown of the piston.
mahle-aftermarket.com

[Walt Fricke]

I don't see that the Mahle PDF catalog is much help. Arrow means front (someone did a rather poor job of translating here, if from the German).

I suspect the postings Andy referenced contain my standard complaint that wrist pin offsets without having right and left side pistons as well make no sense, and mechanically can't work unless the piston domes are symetrical. And SC pistons most assuredly are asymetric.

I would pay no attention to arrows, and install them so the intake side is up on every piston. You have to do that or you will have problems.

If, somehow, all six are not identical, but are really two sets of three, and you can, by being careful, get all intake sides up AND all arrows pointing forward, they that's what you should do. If that is, indeed, the case, be sure to let us all know.

Though, if you want to hot rod the motor at the expense of a bit more noise, you could keep the intakes up but have the arrows point rearward.

[fred cook]

Thanks for the info! The piston tops are symmetrical and may be installed w/intake relief at top w/arrows pointing to the front (flywheel end) on both sides.

How much power would be gained by reversing them?

Thanks,

[Gary De]

I have a set of brand new never out of the box Mahle 2.7 RS pistons w/rings and cylinders. The top of the pistons are not symmetrical. It will be interesting to see which side the offset is on, intake or exhaust. Are there (3) left bank and (3) right bank in a new box set? Are we making to big of a deal over the offset? It seems to me that the valve pockets having the correct orientation is more important then the offset.

[fred cook]

The Mahle piston set that I have are only 10.1:1 compression ratio and have a symmetrically curved top with shallow valve pockets. They are 100mm diameter pistons for a 3.3 ltr short stroke build.

[Walt Fricke]

Wrist pin offset is an interesting concept. It affects something a bit obscure, like rod angle at time of combustion, or at TDC, or something like that.

Offset affects things depending on whether it is ahead of or behind some point, so to speak, based on crank rotation. This means that, with any engine with more than one cylinder bank, if the offset is up on one side (let's call that ahead of the rotation), it must be down on the other side, in order to be in the same orientation relative to the crank's direction of movement.

So, if you have flat topped pistons (as the old VWs had), you can just have one set of identical pistons, with arrows on top all pointing the same way. That way, if you install all the pistons on the crank, with the crank bolted to an engine stand, you can rotate whichever pistons over to the other side, and the arrows will all still point to the rear (or front, if you installed things that way).

Now introduce pistons with asymetrical heads. Let's say they have large valve pockets for the intakes, and smaller ones for the exhausts. Let's say the pistons are identical, with the wrist pin offset on the intake side, and all arrows also pointing identically.

Now install with arrows all pointing to the flywheel. And flop half the pistons over. Now the arrows all point the same way, as before. BUT - the smaller cutouts will be where the intake valve wants to go. This isn't going to work.

So you install with intake side up. Now one side has arrows pointing one way, and one has them pointing the other way.

Smokey Yunick went into this with Chevy small blocks or some Chevy - for a while Chevy had offset pistons, and they were marked or labled right or left sided. Naturally race motor mechanics installed them the opposite of how Chevy intended. Chevy wanted quieter motors, with anti-slap. Racers wanted a bit more HP. Smokey said after a while Chevy got tired of having to stock two kinds of the same piston, and put the wrist pins back in the center.

Unless Smokey and the builders of VW Formula V Runoffs winning motors had it wrong (and those guys are expert at finding just one more horse in a nominal 40 HP motor, and use dynos), it just makes no sense to have one bank anti-slap, and one bank more power.

You can measure wrist pin location with an ordinary caliper accurately enough to tell if it is offset, and if so, to which side. Noticing that an addendum to the first 911 Factory Service Manual mentioned wrist pin offset as a change, and then measuring some pistons I had and finding this on CIS pistons no less, put this particular bee in my bonnet.

Anyone who has pistons with arrows ought to do this, just to see - are they offset? Are half the arrows pointing one way relative to the asymetrical piston tops, and half the other way? Or do they all point the same way.

So which offset is for which effect? Alas, the mind is a funny thing. I can recall all sorts of trivia, but some things I'd like to recall I can't. The reason for these effects, though I have read it often enough, eludes retention. My quick reference is How to Hotrod Your VW, and I can't find where I put that at the moment. Smokey's articles from long ago Circle Tracks are in folders, and I am not moved to hunt through those. One way pushes the piston less into a side of the cylinder, leading to less slap. The other way does a better job of pushing the piston down when combustion pressure is higher.

One of the discussions on this here should have the answers to these questions, though.

[fred cook]

I will measure the pistons for offsets tomorrow. I suspect that I will find that I have a set of 3 for the left side of the engine and 3 for the right side. This has been a most interesting and illuminating discussion!

[Walt Fricke]

The suspense mounts!

[fred cook]

Went down to the shop this morning and did some measuring of the Mahle pistons. Also took some pictures. Three of the pistons have the arrow pointing to the left with the intake relief at the top. These three pistons also have an "R" written on the inside with a marker pen? The other three have the arrow pointing to the right with the intake relief at the top. No marker pen lables on the inside. The first 3 pistons (the ones with the "R" on the inside have the piston pin offset just over 1mm towards the top or intake side of the piston. The other three pistons have the same offset but towards the exhaust side of the piston! Some pictures.

Piston with "R" marked on inside. Arrow to left.




Other piston without marking on inside. Arrow to right. Note pad on skirt.

[Walt Fricke]

This makes sense - three go on one side, and three on the other, and you can get the valve pockets right, and the arrows all pointing in one direction.

Historically, arrows point to flywheels for anti-slap. Unless this is a race motor, I'd install them that way if history is an accurate guide.

But then there is that R marking to confuse things. R = right in German as well as English.

[fred cook]

Originally Posted by Walt Fricke

This makes sense - three go on one side, and three on the other, and you can get the valve pockets right, and the arrows all pointing in one direction.

Historically, arrows point to flywheels for anti-slap. Unless this is a race motor, I'd install them that way if history is an accurate guide.

But then there is that R marking to confuse things. R = right in German as well as English.

I wondered if the "R" refered to "rear" of the engine? Also, I spoke with Don at EBS and he said that the arrows should point towards the flywheel. Perhaps we may assume that the question of the arrows is now settled!

[HawgRyder]

Umm...big relief on valve pocket for the intakes?
Bob

[fred cook]

Yes, the intake valve is larger than the exhaust valve!

Umm...big relief on valve pocket for the intakes?

Bob

[Walt Fricke]

I found my "Hotrod VW" book, which says, after saying that the arrow (or notation "vorn") points toward the flywheel:

"The [wrist pin] offset piston allows the connecting rod to change its inclination and tilt the piston easily toward the opposite side of the cylinder wall before reaching TDC. Piston slap which would otherwise occur is avoided. Vee builders turn the pistons over to place the offset in the opposite direction, claiming that the 1% to 2% HP gain offsets the added noise - which they can't hear above their open exhausts anyway."

6 HP gain on a 300 HP engine?