16 CFR §1505.51

1. (a)Test probe. Section 1505.6(g)(2) defines accessibility, for certain paragraphs, as the ability to reach a heated surface with a 1/4-inch-diameter rod 3 inches long. To test for accessibility using this test probe, it shall be inserted no more than 3 inches into any opening in the toy. Unless the probe contacts a surface within 3 inches of the plane of the toy's opening, that surface is not accessible.
2. (b)Accessibility of Type C and C-marked surfaces. Under § 1505.6(g)(2) (iii) and (iv), touching by casual contact or without employing the aid of a common household tool shall be determined by use of the accessibility test probe described in §§ 1505.6(g)(2) and 1505.51(a).