A compliant electrical contact having a closed coil with opposed, paraxial leads extending therefrom at an angle from the axis of the coil. A vial-mounting structure includes a vial-holding member and a locking member movably secured with respect to the vial-holding member. Other embodiments are disclosed. Generally, this spring attachment feature will be the D-ring that is used to attach the rebounding mat to the springs. A seventh crank arm includes a third counterweight, and connects the third main journal and the fifth crank pin.