Noob question about arrow on piston

[Régis]

Hi guys,
I couldn't find the answer ,here or anywhere else, for a simple question : what does the arrow mark on the piston head of my 351C mean?

I'm rebuilding the engine but I forgot to take a picture when I disassembled it ...
I prefer to ask the question before having a disaster at the first start.

Thank you all.

Régis

 

[Hemikiller]

Arrow on the piston points to the front of the engine.

 

[Régis]

Oh OK. That's fine.

Thank you.

 

[MikeGriese]

This is important! The wrist pin holes in the piston aren't exactly centered - there is a slight offset to account for the forces exerted on the piston during operation. If the piston is put in backwards (arrow pointing to the rear), you will get a knock as the piston skirt slaps the cylinder wall. I got a 1987 Lotus Esprit Turbo really cheap that had the engine entirely disassembled because of an improperly installed piston during a rebuild. The owner didn't want to pay the $10k the shop was asking for to reassemble the engine (the same shop who disassembled said engine), so he boxed it all up, put it in a barn in Petaluma and sat on it for a decade before wanting it out of the way.

 

[Spike Morelli]

The wrist pins are, indeed offset slightly to one side. If I remember correctly, the offset was supposed to "quiet" the piston, but.........some experimentation has been made by various engine builders, and on the dyno, an increase in torque has been reported. ( I have an image in my files, but for reasons unknown to me, it won't upload, sorry ) This particular engine, a 289 Ford, has had it's pistons installed on purpose with the notch pointing to the rear, with no ill effects, and an increase in torque. Now, I personally would recommend installing the pistons properly, with the notch or arrow pointing to the front. It is sort of a moot point, as most every forged custom racing piston will be manufactured with the wristpins centered.

 

[basstrix]

I'm under the impression that pin offset is more of an available option on some aftermarket pistons, rather than the default for engines of our vintage?

 

[Spike Morelli]

Wristpin offset is not really accurately described as an "option", it is simply an engineered, mandatory, stock- as- designed method to quiet "piston slap", and likely still used in some engines. The pin offset isn't strictly a Ford nuance, it has been used in other makes of engines as well. As I pointed out though, it is not necessary, and isn't used in the manufacture of custom forged performance pistons.
Today though, there may be other reasons to install pistons a certain way, such as when piston cooling squirters are designed into an engine, it may have small notches in the bottom of the piston skirts to clear the squirters, and the pistons MUST be installed only a arrows forward, or when valve clearance "eyebrows" necessitate the piston go in according to valve arrangement.
You can probably Google Pistons Installed Backwards and gain more insight that some do it on purpose, with some engines.

 

[basstrix]

The linked thread clarified my own confusion with the offset direction and why there isn't a L/R piston needed. The picture from JE Pistons is the most important to look at.

Considering the number of cylinders I've bored, pistons I've pressed onto rods, etc....I'm amazed that I didn't realize this was common. The old Homer Simpson DOUGHT! is appropriate here LOL

https://www.modularfords.com/thread...tall-with-offset-wrist-pins.201401/?id=201401

 

[Régis]

Very interesting. Thank you for this amount of information. So if i buy aftermarket pistons( very standard as Sealed power pistons), there will be no pin offset?
This is a stock engine so i would like new pistons as the original ones.

 

[Stanglover]

https://7173mustangs.com/threads/piston-suggestions-351c.42580/This thread was ongoing a few days ago. It may have some relevance.