inline pistons in flat engines

[David Kemp]

Briefly I am fitting Mazda pistons to my Jowett & fitted them with the Front of piston to the pulley.
The discussion then started....as the engine is flat the thrust is different on the passenger side to the drivers side.
So the drivers side (RHD) should have the pistons inverted, another argument is that f should be thefront on both banks, as the engine does not know it is flat. As the Mazda pistons are offset getting it right does make a difference.
Any opinion is welcome.

[Keith Clements]

I suggest you read this.
https://x-engineer.org/automotive-engin ... ne-piston/
and this
https://auto.jepistons.com/blog/piston- ... nd-bottoms

I use a slightly modified Kolbenschmidt Ford Fiesta piston.

This is an interesting post.
https://shoptalkforums.com/viewtopic.ph ... 8#p1142553

And from a link in it.

Location: Fond du Lac, WI
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.

My two penneth...In a horizontally opposed flat four the orientation of the pistons depends on whether the piston is assymettrical having piston skirts, gudgeon pin offset or valve reliefs. If there is an indication of orientation on the crown this can mean different things but of importance E is not likely to mean exhaust but inlet and an arrow can mean either front, top or facing clutch so you need to know what application your replacements were for and if they are aftermarket do not trust the arrows.
So, if your assymetry is piston skirt, the wide side must be on the thrust side of the bore. If you have wrist or gudgeon pin assymetry, then the crank rotation is the important factor. You should not fit relieved crowns to Jowetts so the E issue is not relevant.