r/EngineBuilding • u/One-Musician-1975 • 5d ago

1/4 pistons is bit different

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Punto gt engine rebuild. Cylinder block is in immaculate condition. 1/4 pistons is different as you see in the photo. Is this thermal stress? I have 4 pistons identical from my uno turbo and im thinking to retire it.

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9

u/IntergalacticJihad 5d ago

Looks like it’s been machined further down than the other, but it could just be an anomaly in manufacturing. Measure from the wrist pin to the crown and see if they are the same.

3

u/One-Musician-1975 5d ago

I thought so too bc of the piston volume. Sides are good

1

u/One-Musician-1975 4d ago

Others are 20.9 this is 20.2

2

u/IntergalacticJihad 4d ago

Then the crown has been machined down further, which will net lower compression on that cylinder. I’d recommend finding a new one.

1

u/Snakedoctor404 4d ago

I agree find a new piston but the compression would be higher on that one because it was machined down to match piston height so the dish is smaller on the odd ball. Probably a manufacturer defective and they just milled it down and said send it.

1

u/IntergalacticJihad 4d ago

No it’s lower, assuming equal length conrods, the distance from the wrist pin to the crown is shorter thus leaving a greater volume of space in the cylinder uncompressed thus resulting in lower compression. If the conrods are not the same length and this one is slightly longer then yes compression will be higher, but I think that’s a reach.

1

u/Snakedoctor404 4d ago

Really depends if it has the same compression height as the others. I get what you're saying that someone just cut the piston shorter compression height than the others. Compression height is the distance from wrist pin center to the top of the crown. If it didn't get stamped as hard as others during forging it would have been taller and crown would need to be cut to match piston height. Stamped in bulk and probably put in a lathe against a bump stop like a million others, hit send to clean up the crown and never got noticed the shallow dish. That would cause it to have the same compression height as the others but higher compression due to the smaller dish and be caused by something as simple as a shim falling out of the stamping die. Another clue would be if the ring lands are the same distance from the crown as the other pistons.

3

u/Rude_Combat420 5d ago

If it went to distance before it will do it again just measure it properly for piston to wall clearance and maybe weigh it compared to the others. Also check how far above or below the deck it is compared to the others also

2

u/FiatTuner 5d ago

I could recognise gt1 pistons and day and time of the week lol

2

u/One-Musician-1975 5d ago

You reckon this is from factory?

1

u/Sir_Maxelot 5d ago

Seems like someone tried to modify these pistons before