Sorry for bringing an old thread up but one of Bruce Anderson's comments in the latest Excellence mag (April) caught my attention:-
I wasn't aware of this. He was talking about JE's but did all Mahle's (Birals, Nikasils) in all sizes have an offset wrist pin? Even the new ones from Andial?

In American V8's, turning the piston 180 degrees will increase hp/tq due to offsetting the pin for better rod angle.

Blue

Read my post on page 1. Some did, some didn't. It is a pretty strange deal, since the pistons don't come in right and left side versions, the way Chevy did it until they decided it wasn't worth the extra inventory.

Birals are a kind of cylinder. You may mean Alusils.

If you are planning to use J&Es, are you really concerned about a little rattle?

Walt

I do not know about the 911 pistons but the 930 turbo flat tops are clearly offset as stated/shown in the factory manual. Here is the quote from the factory manual.

"Because the piston pin bore is offset, the piston
must be installed correctly. The installation mark
faces the clutch -see figure."

ZC - makes sense - if you have flat top pistons without different sized valve pockets, they will not have interference issues whichever way they are installed. This was exactly the case with the VWs. This means that the offset will be in the same orientation to crank rotation on both sides, so you will get its benefits (whichever you are after) on both sides.

The mystery involves non-symetrical piston tops, where you can't do this because of valve interference (think 2.0S pistons) or messing up the air flow (CIS pistons, if the valves would still clear). Why would there be any offset on them? But on at least some there is.

No, I didn't mean Alusils. When I was asking about Birals and Nikasils, I was referring to pre-1973 meaning that original p+c's were Birals (except 2.7RS) and new Mahles today (2.0, 2.2 and 2.4) are Nikasils. Do the new Mahles have 2 flavors of pistons?



Any idea exactly which did and which didn't? What about the racing versions - 86.7mm, 87.5mm, 89mm and 92mm?



No, I'm not going to use J&Es. Are you saying that Mahles, even those without an offset wristpin, don't rattle?

Blue72

I don't think I wrote down which (other than the 2.7s of mine) did or didn't. He probably measured 84s and 80s, though he raced a 2.8 so he might have had a race 92 on hand. I've got a Mahle racing 2.7 piston somewhere, and an Alusil 3.0, which I could check. I got the other information over the phone, I think, while we were musing over this.

It was of some importance to me when I was acquiring 85mm pistons for the 2.3 I built. I, thourgh a local shop owner, was negotiating with Malvern (rip) out on the East Coast to have Cosworth make up a batch. Which they did. Seemed easier to specify the pin in the middle - that meant having a couple of spares (I think I ordered 8 for myself) would work on either side. But having the "more power" offset was tempting, as you might imagine.

I've never heard a rattle that I associated with piston slap. But my mufflers have never been very quiet.

I read that pistons should be installed with the arrow towards the clutch. My pistons did not have an arrow, but were stamped with the letter "E". According to the Bentley book, this E should "always install toward the intake port." Curiously, Porsche chose the letter E, I am assuming the abbreviation for the German word for "intake" starts with an E. I wonder how many mechanics in America thought the USA car should have the pistons with the letter E point toward the exhaust?

If this is true we have offsets oriented (for noise? or for power?) in the flat top 965 piston engines, of which all six are the same. Stamping the orientation in the "top" means that the pistons are not simply swung over the top of the engine to keep the arrow pointing toward the clutch like other Porsche engines. Instead, the pistons are turned end for end when going from one bank to the other to keep the "E" up ^.

Would this affect the way the offset is oriented? It would seem that if the offset is toward the exhaust side on stock pistons, turning the pistons end for end (keeping the E up) would accomplish that. But if we want the optimum condition, i.e. with the offset on the exhaust for cylinders 1-3 and the offset on the intake for cylinders 4-6, then the E should point down on the 4-6 bank.

Or is it the other way around? OMG, half of my pistons are upside down!

Mark
http://forums.pelicanparts.com/uploa...1312731119.jpg

oops-just saw this
 

I too have the "E" on top of the pistons for both banks, which doesnt seem to make mechanical sense. I assumed that someone had been in here before me and installed them improperly, but as my car only has 60,000 miles, that seemed unlikely.

I'm confused.

I think intake is "Einlass" and exhaust is "Auspuff", so yes, the E should always be on the intake, top side.

"E" is up
 

See a picture from a Porsche manual and if fact, the E should be up on BOTH sides. Interestinghttp://forums.pelicanparts.com/uploa...1312845234.jpg

Thanks for all the help and this has also been verified by a gold miester mechanic.

The 3.6 pistons are all made with the same offsets. This means that 3 of them are correctly offset and 3 of them are not.

Been that way for years.

In some cases, we've machined the exhaust pockets to clear the intake valves and installed the left side pistons with the correct offset to reduct thrust wear and reduce drag. Naturally, those machined pistons do not have the exact same CR as the right side and thats simply a tradeoff.

Couldn't you just machine the other side's pistons as well to even out the compression ratios? Then you have even offsets and even compression, just a little lower, but you could compensate for that in other areas.

Yes, one could do that, however in high-performance engines, I hate to lower CR any further than I have to.

Like many things in life, its just another compromise.

So, Steve in our flat-topped turbo 3.3 and 3.6 cars we could turn over the pistons "E" to the bottom (exhaust) side on cylinders 1-3 and take advantage of a piston pin offset to our advantage?

Probably would not do that in worn-in piston/cylinder sets, but if the pistons/cylinders are new...

Mark

Hi.

I have a question for my recondition porsche 911 T 1968 my piston just one flat surface piston left picture :) :

http://forums.pelicanparts.com/uploa...1174429035.jpg

And my question is : flat surface orientation intake or exhaust valve cylinder ?

Thx for all :)

Moderator – a great question and a subject that I might be able to contribute to. Wrist pin offset is done in an effort to reduce piston noise. The small amount of offset (almost always less than 2 mm) does absolutely nothing for the power produced and has negligible effects on changing bore/piston wear. It is strictly done to minimize piston “slap”, the racket produced when the piston uniformly takes up its running clearance by moving from one side of the bore to the other. The mechanism causing this is comprised of 3 factors working simultaneously:

1) Gas pressure acting on the piston dome, with resultant force centered on the piston bore axis

2) Center of gravity (CG) of the piston mass, rarely coinciding with the piston bore axis

3) Angularity of the connecting rod, contributing the lateral force that drives the piston side to side

Laterally moving the rod off-center by offsetting the wrist pin displaces the line of action of the rod (at the dead center positions) with respect to the piston bore axis. When gas pressure is applied during the compression/power stroke and the rod goes through TDC, the offset in centerlines tends to rock or “cock” the piston in the bore, versus “slamming” the piston from one side of the bore to the other as the con rod angularity changes over TDC. Conversely, on the exhaust/inlet stroke when inertia forces are prevalent, having the rod offset from the piston CG also allows the piston the same rocking versus slamming effect. This greatly reduces noise especially prevalent in air-cooled engines having lots of cooling fins as noise radiators. Fortunately in the 911 engine using similar aluminum alloys for piston and cylinder materials, pistons can be fitted with minimal clearance so noise is not really an issue. But in the days of the VW iron cylinder engines, pin offsets were more of a necessity due to the larger working clearances required.

What has been stated is a simplistic description. The forces exerted on the piston are complex during a 720 degree rotational cycle and a computer simulation is necessary to fully understand the piston’s rocking motion in the bore at a given engine speed and power setting.

Wrist pin offset is commonly defined as being toward the direction of crank rotation, or opposite crank rotation. Offsets toward crank rotation reduce rod angularity on the power stroke, while offsets opposite crank rotation increase rod angularity. In actuality, an offset of 2 mm or less has only miniscule changes in piston position vs. crank angle, side force difference on the piston skirt during the power stroke, valve or ignition timing variations bank-to-bank, etc. This is easily proven mathematically by studying the con rod/crank throw triangle, or by constructing a graphical solution. It will show crank position varies by less than 1 crankangle degree for practical offset values.

So, bottom line, it makes no difference how the offset is oriented in the 911 engine. The important thing is to orient the pistons so the domes match the heads on that particular bank and consistency is achieved.

The offset can be grossly exaggerated by offsetting the entire cylinder axis relative to the crankshaft rotational axis. This is known as a Desaxe’ engine, obviously a French invention (at least so they claim). This was a somewhat popular way to build an in-line engine in the early days; the British particularly liked this design and quite a few auto engines were constructed in this form but has now dropped from favor. By using severe offsets, all in the direction of crank rotation, con rod angularity was substantially reduced on the power stroke with the claim that bore wear was reduced. The fact is that bore wear is greatest in the ring travel area and offsets contribute nothing here. There is a reduction in piston side loads and resulting friction on the power stroke, but this is somewhat offset by higher side loads on the compression stroke due to the higher degree of rod angularity at this point in the cycle. The Toyota engine is a Desaxe’ configuration (at least to my understanding) but I think the benefits to mechanical efficiency are, in reality, minimal. The big claims as to being more efficient sound more like marketing hype from the sales department.

To conclude, wrist pin offsets are done for noise reduction. There are no discernable effects on engine performance, efficiency, or wear reduction.

The flat indentation is for intake valve clearance.

Left and Right
 

The pistons that I used in my 3.3SS are Mahle pistons. Here is a picture of one showing the orientation marking. The arrow points towards the crank pulley end of the engine. I don't recall there being any offset on these pistons, but .032" is pretty hard to see with the eyeball! These are 10.1:1, 100mm pistons in LN Engineering slip fit cylinders.

http://forums.pelicanparts.com/uploa...1491796074.jpg

Thx steve :)