Can Lighter Wristpin Balance A Heavy Piston?

Discussion in 'Rotating Assemblies' started by Opie390, Mar 12, 2021.

  1. Opie390

    Opie390 Member

    If my new identical pistons are 1oz heavier each could i maintain balance by using a 1oz lighter wrist pin? (provided its strong enough) would that be an acceptable way to maintain engine balance
    Theoretically? in a stock low rpm engine That was only factory assmbly line balanced to begin with?
    Or A mildly built <400hp street engine?

    Im also probably not gonna find a 28gram lighter wristpin thats safe to use anyway so itsa moot point
    The silvolite identical H1131F30 replacement pistons i ordered are 1 oz heavier each so its gonna require rebalancing the entire crank assy for that much added weight (+28grams per piston) for sure, isnt it?
  2. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    you're correct in your idea that your very unlikely to find a 1 oz lighter weight wrist pin,
    that is suitable and fully functional thats directly interchangeable

    Id point out, that pressed in piston pins do tend to be a bit heavier than full floating pin designs,
    you might consider converting to,
    or buying full floating pistons and pins that are significantly lighter
    but theres dozens of piston and wrist pin and connecting rod options
    and manufacturers

    why not talk to a good local machine shop that does engine balancing work to assess your options
    new wrist pins and machine work that machines off a bit of weight from the piston skirts might get you to your goal
    and certainly theres a set of off the shelf components that may match,
    or can easily be machined and balanced to match your requirements

    and yeah. I'm fully aware that's very unlikely to save you a significant amount of cash anyway,
    as most machine shops are equally intent on remaining RATHER PROFITABLE as they are to doing the work,
    if your replacement components are significantly different in weight,
    you'll want to make the required changes, and machine work generally costs money, one reason I've constantly suggested careful planning on parts acquisition and engine assembly.
    as they (any machine shop)are surely very likely to charge for the piston machine work , new pins and rebalancing

    related threads
    Last edited: Mar 12, 2021
  3. Indycars

    Indycars Administrator Staff Member

    Last edited: Mar 12, 2021
  4. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    Good question Opie390. If you look at the formula used for engine balancing (100 percent of the rotating weight plus 50 percent of the reciprocating weight), and since each journal shares two connecting rods, you will find that the formula really means one piston and two rods. The connecting rod gets divided into the big end and the small end. Therefore, when trying to achieve the same bobweight number as a previous combination, it is much easier to make adjustments to the rotating mass (mostly the connecting rod big ends) then it is to the piston and pin. 28 grams is almost the weight of 5 nickels.
    I would look for lighter (and stronger) connecting rods.

    BTW - what do your current wrist pins weigh? Because I ran across these:
    Last edited: Mar 13, 2021

Share This Page