Backward Pistons

[srocket24]

I just got done tearing apart my block, and I couldn't help but notice that 2 of my pistons were backwards. I just bought the typhoon 2 months ago, and the original owner told me that nothing has been done to the motor. Are the syty motors suppose to have backward pistons?

 

[MikeRenz]

Re: Backward Pistons

got pics? was the truck running when you bought it?

 

[srocket24]

Re: Backward Pistons

No pics, I don't have a digital camera. The truck ran fine for the first month, then is started smoking really bad. When I tore everything apart, the 3rd cylinder was caked in oil.

 

[Ry368]

Re: Backward Pistons

Two of the intake/exhaust valves in the cylinder head face the same way and the third intake/exhaust valve faces the other way.

 

[Don W.]

Re: Backward Pistons

We've had folks here put them together in the wrong order and the valves hit. Were there any marks on the pistons?

 

[JSM]

Re: Backward Pistons

I suspect they are probably correct and your just not sure what your looking at.

 

[Don W.]

Re: Backward Pistons

I kind of agree. It would be interesting to get an explanation from the OP as to why he believes some of his pistons are "backwards".

Pistons need to be installed in the correct "direction". Ie, "this way towards front of engine". There is almost always a mark, notch, dot, arrow, etc that indicates "front of engine" as installed.

There are several reasons that there is a "correct" direction. Dome shapes that only fit into the chamber one way, valve pockets, connecting rod offset, and on flat-top or dished pistons, most importantly: The pin hole is NOT in the center of the piston. It is off-set to one side to promote rock-over at TDC with minimum wear, stress, and noise.



I do a lot of engines for inboard boats. Vintage twin-engine installations often use one "opposite rotation" engine to achieve counter-rotating propellers. It is achieved with a different camshaft driven by a two-gear drive, starter motor, and crankshaft oil seals. Not much else.

I once built a pair of 283's for a 1963 Chris-Craft. I instructed the machinist to hang one set of pistons "backwards" on the rods for use in the counter-rotating engine. He felt it wasn't necessary or "correct". Although I had never seen instructions to do this, I felt it made sense based on "what I know about this". I insisted, and he did it. When I tore down the original 1963 engines, sure enough, the counter-rotating one had the installation notches facing the REAR. I felt exonerated. Turns out my 'instinct' was correct.

What do you mean by "backwards"?
DaveP

Interesting post Dave. For another comment see:

http://www.mahle.com/C12570B3006C0D49/CurrentBaseLink/W26TFG5L637MARSEN

I, being the oddball of the bunch, always thought it was a different reason. The connection rod - piston assembly exerts it's greatest force on the crank when the angle between the crank and rod is at 90*. By offsetting the pin that angle is reached sooner when there is greater pressure in the cyl so more torque is developed. Sounds good anyway. A Google search finds wear and noise, as you mention, often listed as the reason.

Piston notches for pin offset WOULD be rotationally important especially if wear and noise were the reason. If they are there for valve clearance then not. Valves are in the same place regardless of rotation. All the Chev flat top pistons that I've seen have two sets of valve reliefs. Presumably to allow for all the notches to be forward.

Aftermarket SyTy pistons do NOT have offset pins, nor notches, because we use different size valve clearance reliefs and those reliefs need to correspond to the valve location in each particular cylinder. If we did have offset pins then the pistons would have to numbered. And because of that we'd have two different pistons in our engines. I believe stock SyTy pistons are symmetrical without notches.

Sorry for the ramble. And/or the hijack.

 

[Don W.]

Re: Backward Pistons

It's always a pleasure to exchange thoughts and observations with you Don. Hope we're excused for our hijack.

Regards,
DaveP

Same here. :tup:Oops, I wuz wrong about two different pistons. (Should have thought about that one a bit more!)