Piston notch position

Discussion in 'Sterndrives' started by Art1848, Jun 23, 2016.

  1. Art1848
    Joined: May 2016
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Wausaukee WI.

    Art1848 Junior Member

    Working with a Chevy 307 (1975 Slickcraft) Seloc Manual says " The notch in the piston MUST face forward for the right bank, and face aft for the left bank."

    wouldn't that change the Pin offset in relation to the crank and cyl. from left to right? Shouldn't the offset be the same way in relationship to the rotation of the crank and position in the cyl. for all the cyl. Witch would mean notch forward for all 8 pistons.
    Seloc Manual # 3400 page 3-76

    Need your thoughts.:?:
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    My understanding is they always face the water pump.
     
  3. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    The manual is correct. If you look at the piston pins you will see that they are offset about 1/8". the offset must be so the long side is to the outside of the block. The reason for this is so that when the piston travels up the bore against the compression it is tipped against the outside of the block where the combustion chamber is. When the cylinder fires the piston is already tipped so it is not slammed against the bore. For this to work half the pistons must be one way and the others reversed unless there are actually right and left pistons such as high compression domed pistons. Usually high compression forged racing pistons will have the wrist pin centered which results in a slight horsepower increase the cylinder bores are much more precision bored. The best way is to look at the pistons and you will be able to see the offset make sure the long side of the offset is to the outside of the block on both banks. If they are not installed this way you will have piston slap and if the cylinder bore is loose or worn the piston slap can break the piston skirt.
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    [​IMG]

    Again, my understanding of flat top and dished stock pistons is notch forward and rod orientation. I've also seen symmetrical valve relief pistons with a notch (below), though most domed, pocketed and higher performance pistons require a different approuch (no notch), with valve relief specific arrangements (siamesed ports on SMB).

    [​IMG]

    Symmetric valve relief pistons, arranged for installation, note the notch.

    [​IMG]
     
  5. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    For the offset of the wrist pin to work as designed and all notches to the front then you must have left bank and right bank pistons, If all the pistons are the same then for the offset to work as designed the pistons must be reversed on one bank. Another possibility is that the pistons only have two valve reliefs instead of four and they used all the same pistons. If so then for the valve reliefs and the wrist pin offset to be correct then you would have to have the one bank with the notch to the back of the motor. As far as I know there is no other reason for a piston to go either way other than for the offset of the wrist pin as long as it doesn't have a dome or only two valve reliefs that is.
     
  6. Art1848
    Joined: May 2016
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Wausaukee WI.

    Art1848 Junior Member

    After reading a ton about offsets, the main reason for the offset is to let the rod have a slight angle of attack when the power stroke starts so the piston is not pushing straight down on the crank. For that to happen all the pistons must face forward so the piston pin offset in relation to the crank is the same for all. It takes a lot of reading on " the reason behind piston pin offset" any other articles someone may know about, I would be more than happy to read and think about. Thanks All.
     
  7. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    Piston pin offset

    My understanding of why the piston pin is offset is to avoid piston slap when the cylinder fires. Therefore the long side of the offset must be to the outside of the block where the combustion chamber is so the piston is rocked to the outside by the compression and for this to be correct with all the notches to the front then you must have different left and right pistons because if all the pistons are the same with the notch to the front then one bank will have the short side of the wrist pin offset to the outside and then the compression as the piston moves up the bore will rock the piston to the inside of the block and then when the cylinder fires you will get piston slap as the piston is rocked to outside of the block. I may be wrong about this but I have rebuilt over a hundred small block chevys in my day and I have always gone by wrist pin offset to determine which way the pistons go and not the notch and of coarse the piston must be install on the connecting rod accordingly.
     
  8. Art1848
    Joined: May 2016
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Wausaukee WI.

    Art1848 Junior Member

    When you change the position of the offset you will also change the deck height of that piston, So you must have been running one deck height on one bank and a different deck height on the other. This is also what I have read about piston pin offset. Compression force is spread equally across the top of the piston on the power stroke. Also, you can get piston slap on a engine with no heads on it by spinning the crank. It would be interesting to sit down with the engineers of engines.
     
  9. Dave T
    Joined: Sep 2011
    Posts: 250
    Likes: 14, Points: 18, Legacy Rep: 158
    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    You will not change the deck height of the piston by reversing the piston. At TDC the distance from the center line of the crank pin to the center of the wrist pin remains exactly the same you only move the offset of the Wrist pin from right to left or left to right. When the cylinder fires there will definitely be much more pressure applied to the half of the piston that is under the combustion chamber as the other half of the piston is shrouded under the cylinder head and if the piston is installed with pin offset, short side to the outside of the block the compression on the upstroke of the piston will rock the piston to the inside of the block and when the cylinder fires it will be rocked to the outside of the block causing piston slap which results in noise and broken piston skirts especially on motors with a lot of miles on them. Ive seen it many times.
     

  10. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    This conversation can get very complicated. In a nutshell, some pistons have offset pin bores, while others don't. The idea is to decrease the load on the thrust side of the cylinder by altering the rod angle at a given point of rotation. Most single valve relief pistons have an offset. They only go in one way. twin relief or flat tops usually don't. If it's a flat top four valve relief piston that has a piston offset and must be installed in a certain direction although it can be installed either way because of the four valve reliefs. Most higher end forged pistons, have no piston pin offset and only 2 valve reliefs or a single trough. Typically when installed correctly, the valve reliefs will be closer to the lifter valley with the bearing tangs on the rods facing the outside of the engine. The reason they need to face the outside is because the tangs are machined in such a way that they offset the bearing away from the crank filet. If installed facing the inside of the engine, the bearings would ride on the filet, score and probably spin.

    It's a lot more complicated than this, with chamber design, rod design, pin and piston design all playing roles. It's also a subject that most piston manufactures don't like to talk about, because of the complexity and more importantly the "tricks", some use to get free power out of their engines, while still retaining the stock class spec's they run in. I know a guy that drills his bores slightly offset to cheat the tech guys during inspection! For a stock setup, usually the notches all face the water pump. Piston or pin slap is pretty easy to identify once the engine is running, if you have a good ear, though a hell of a time to find you got some in wrong. What (exactly) are the pistons you're installing and have you measured to see if there's actually is an offset?
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.