module threaded_insert( s = undef, d = undef, l = undef, gap = 0) {
	     if( 3==s) threaded_insert( d =  4.6 , l =  4.6, gap = gap);
	else if( 4==s) threaded_insert( d =  6.3 , l =  6.35, gap = gap);
	else if( 5==s) threaded_insert( d =  7.1 , l =  9.5, gap = gap);
	else if( 6==s) threaded_insert( d =  8.7 , l = 12.7, gap = gap);
	else if( 8==s) threaded_insert( d = 10.24, l = 12.7, gap = gap);
	else {
		assert( is_undef(s), "Unknown size (expected 3 for M3, ...) or manual (d,l).");
		assert( is_num(d),   "Diameter d (numeric) or size expected.");
		assert( is_num(l),   "Length l (numeric) or size expected.");
		cylinder( d = d+2*gap, h = l+2*gap);
	}
}