samsamedd

hello. i have an 89 chevy 350 tbi and i had to replace a piston. it is a pressed style piston so it took a good amount of time getting the new piston onto the old connecting rod. when i went to put the piston in i realized i had pressed the connecting rod on backwards. it is clear that it matters which direction the connecting rod mates with the crank shaft but what i'm wondering is do i have to take it apart and flip the piston so the forward mark on the top is forward or can i just install it with the piston backwards while the connecting rod if forwards? i would really appreciate any help!

jimbolyo

I don't think it matters, I would check the ring end gap and piston to wall clearances to be safe though.

tech2

i think it does matter. I was wondering if the piston wrist pin is offset to one side. I think they do this as it helps quiet things down...during the transfer as the piston comes up from the compression stroke to tdc and down as the con rod moves from one side of the crank to the other.


does the piston have a arrow mark on one side of the top of the piston?

jimbolyo

I would agree with you if during a complete (new) engine rebuild. But I think replacing 1 piston out of 8 that have wear(or seated) the differences would be minimal.

tech2

if the piston pin is offset it would matter all the time. that is why the pistons have orientation marks.
here is a link http://www.kfz-tech.de/Engl/Desachsierung.htm

jimbolyo

tech2, Thank you for that information. I've built a few engines, but have never used offset rods, with pressed pins. Is there an advantage to this?

tech2

I know some old heavy duty trucks had engines with no offset. the timing wasn't advanced much....they could actually run backwards.


the advantage is the engine is runs quieter and shows less wear on the cylinder walls...most engine wear occurs at near tdc.