Engine: Correct Piston Orientation In The Engine

[JoeHall]

Many here are aware of our Stude's pistons' markings for correct installation; usually a notch on top of the piston, which is to be pointed to front of the engine. I never dreamed of installing them any other way, but often wondered why they were to be oriented that way. Today I watched several YouTube vids on piston pin offset, and the pros and cons. Then I looked closely at a used 289 piston and, sure enough, the pin is offset about 1/8" (would need to be relocated about 1/16" in order to be centered). Per YouTube, if no offset, the engine is easier to balance, but has more friction and heat; if offset forward, it makes slightly more power, less friction and heat (by reducing piston side load), but is harder to balance; if offset rearward, it makes slightly less power, less friction and heat, but also reduces piston slap related noise. Our Studes appear to be offset rearward for less friction, heat and noise, and slightly less power. This is per my interpretation of several YouTube vids, but it's all new to me. It doesn't appear to be rocket science, and likely some of y'all already knew this. So hopefully others here can comment on theory and/or experience.

I also learned it was once common practice for old racers to deliberately install the pistons backwards (notch to the rear), for more power. Of course this is macht nichts for most of us, since we'll continue just putting the notch forward. But it's good to keep exercising the brain by learning new things. LOL

[Commander Eddie]

Joe, I have seen piston orientation stressed in videos of various engine builds, but never with an explanation of why it was important. Your observation here is very interesting and the first I have seen. Thanks for posting this. Very enlightening.

[StudeRich]

This is a very interesting topic Joe, thanks for posting that.

This gets really confusing to me because the Manuals only refer to the "T" Slot having to be opposite the Thrust side when rotating Clockwise, so that is how they orient the Piston to the Rod.

So in my sometimes ambidextrous or dyslexic mind, the Piston Pins would have to be Offset Inboard or Outboard, not front to rear.
So, if that thinking is correct, one bank would have to have the arrow pointed to the rear, because all 8 are the same, so 4 would be backwards if we didn't face the arrows on one bank to the rear.

Since we no longer have T Slots, we have no other reference except the arrows to know which side goes inboard and outboard.

What we all understand is that Studebaker RODS have a Front/Rear Offset (left and right) for opposite banks.

Splain to me how/why that thinking is incorrect ? Per all accounts that is said to be wrong, but I can't get my mind around why.

[JoeHall]

Rich, The piston pin offset IS inboard/outboard, and can be seen by close scrutiny with the naked eye. I see no reason for either bank to be oriented opposite the other, since they share the same crankshaft, rotating in same direction.

Also, for decades some motorcycles, especially Yamaha, have onset the cylinder bank in relation to the crank. This, effectively, accomplishes same thing as offsetting the piston pin.

[StudeRich]

Thanks Joe.

Since with clockwise rotation, the Right side has the thrust Inboard and the Left side has the thrust outboard, so it makes sense that with "T Slotted" Pistons, the slot has to be opposite on each side so they all face away from the Thrust side to prevent damage.

But my thinking is, the Offset works the same way, and maybe THAT is what I "assumed" incorrectly.
So without the T slots for expansion, other means have been developed like Hypereutectic Pistons so we no longer need to worry about facing 4 opposite each other ?

[IndStudenut]

Most manufacturers for years have placed the pin offset. Many racers and others over the years have theorized gains in power etc if you flipped the offset. I watched an episode Engine Masters where Steve Dulcich, David Fryeburger and Steve Brule (dyno gurur) deliberately took a mid 80's dodge 318 from a junk yard designed with pistons with the wrist pin offset. They dyno'd it as bought then swapped the piston offset putting the pistons from the left bank in the right bank and vice versa. This was done to test the theory of increased power by switching. Then dyno'd the motor again to see power output. There was no discernable power gain or increase in heat what so ever. The final evaluation was that manufacturers offset the pistons to quiet piston slap and other potential noise. Actually it is such a mute point that all after market piston manufactures that I know of produce pistons with zero offset. So in theory it doesn't matter which way you put the piston on the rod as long as the offset oiling hole on the rod itself is correct for proper oiling.

[bensherb]

I saw that same episode,and concur they found no discernable difference in power one way or the other. They concluded the only difference it might make was in piston noise as the rod changes piston loading as it swaps from one side to the other and the piston "floats" for an instant.

[gordr]

Some of you may be aware of the Garage 54 Youtube channel. A bunch of good old boy Russians doing unspeakable things to Ladas. One episode they did just that. Took a tired Lada 4-cylinder engine, and reversed the pistons so the offset was wrong. It seized up, and threw a rod after being driven about 300 kilometers. And all the pistons had serious scuffing wear on the skirts, not present before the reversal.

[christophe]

Some time ago, à friend had a Peugeot 404 engine rebuilt by à "specialist". The engine turned well until it was hot and got stiff. You might have already guessed that pistons and conrods were assembled backwards.

[JoeHall]

Not sure reversing piston orientation woulda caused serious problems, but if the big ends of the rods were flipped so the oil holes were positioned incorrectly, that would cause failure sooner than later. IMHO

[JoeHall]

I too, found controversy on YouTube, with several "experts" weighing in on both sides. I also saw a guy use a chalkboard to articulate the related physics, and I don't see how it could NOT add power, although percentage may vary due to other variables, like rod length, stroke, amount of pin offset, etc... The one person in our community I'd love to hear weigh in is Ted Harbit. If there's one among us who'd know from experience, it'd be Ted (R.I.P.). As for most of the tests mentioned on YouTube, I'd need more info before deciding what to believe, i.e., the Mopar experiment mentioned above: did they use the same worn in/out rings for both tests? If so, I'd need more info. If not, I'd also need more info.

I'd also like to hear from Yamaha engineers, or see anything they've published. As for modern pistons now made for our Studes, I have rebuilt a 352 and a 289 within the last 2-4 years, both engines' new pistons were marked for correct orientation. Though I never even thought of visually looking for pin offset. It's safe to say it was once recognized technology by every car company I've heard of. Yamaha still uses the same technology, but also cites economy of space as one of their reasons for offsetting the bank in relation to the crankshaft. It's an interesting topic here, and good stimulation for our brains, at least for my brain. LOL