JClark, drool inducer for ya.

[1320-20V]

@Boxersix

Piston development is moving right along. Core design is 95% complete with structural testing complete on the current core as well. Finite changes will be made by the manufacture to suit their processes and design parameters, but overall slug design will remain mostly the same as it’s a BMW replicant with necessary changes to suit use in this motor setup(deck height, rings land widths, pin boss offset, etc, etc)

I received a few quotes from various piston manufactures and am still unsure as to who I will be going through with that though it’s looking more like CP vs Mahle due to not meeting quantitiy minimums for Mahle.

More updates later on.

Who is actually doing the design and engineering work on your rods and pistons? I’m thinking you made that model in SW yourself? Or was it sent to you by someone else.

There are aspects of the design that I would change:

1. On the rod, the rate of change of your cross sectional area is not reflective of the applied stresses. It should be. Your cross sectional area should be proportional to your design stress. Your section modulus should change as a function of distance along the length of the rod. The big end of the rod is exposed to a constant angular velocity, and the small end only a linear velocity. You should have a section modulus that is appropriate for your buckling loads at the beginning of the exhaust stroke, BDC. This obviously depends on piston mass. Section modulus should change to reflect the transition from purely rotary to reciprocating.

2. Stress concentrations, I see you have not found the correct values for your fillets. Sure you have things filleted, but there is a science to the geometry here. Sharp corners and intersections can multiply the actual stress by 3x or more. For your particular application on the big end of the rod, your fillet radius should be:

Fillet radius/minimum thickness of rod = > .30

AND your big end size to your main rod thickness should be as small as possible. A ratio of two here is the difference between a Kt of 1.25 and 1.85. You can essentially double your rods tensile strength here, I’m not nit picking.

(Values from Design of Machine Elements, Spotts, Shoup, Hornberger. 8th edition, Pg 144.)

A custom rod design is nothing but calculus. Only calculus will provide the most efficient solution. Your primary goal as a part designer should be to establish the maximum applied loads and their directions. Then find out their frequencies and relative phases. Then establish the highest combined stress. Then find your design area, and then manipulate the section modulus to fit the varying load in the axial direction. Your answer will be very complex, hollow, and expensive to manufacture but you’re doing one-off work so it doesn’t really matter.

2. You should pay careful attention to the centroid of your rod as well. Think about that, and then think why your answer that lighter is not always better is wrong.

I only post this because It’s a cool project, and I like to see peoples home engineered parts. But if your actually going to pay for them to be made to your specs, you have TONS of areas where you could make improvements. I’d be glad to help.