How do I use WP_Query to get a range of posts with custom fields that have numbers with letters in them?

Intercept the query on the posts_where filter. In there you can do a replacement of the non numeric values:

preg_replace( "/[^0-9,.]/", "", $yourMeta );

Or in a (generic) code piece:

add_action( 'posts_where', 'wpse120784WhereDiameter', 100 );
function wpse120784WhereDiameter( $where )
{
    // If conditions here - only target the cases where we need it
    # if ( foo ) { etc.

    // Only run once for this single query
    remove_filter( current_filter(), __FUNCTION__ );

    $diameter = get_post_meta( ... );
    $where .= preg_replace( "/[^0-9,.]/", $diameter, $where );

    return $where;
}

When you manage to target the exact MM diameters that you’re searching for, then the Regex Search/Replace call will simply pop off the MM/Inch (whatever unit there is).

Of course you’ll need to somehow gain access to the post meta (get_post_meta()) values to target them in the Regex replacement task more accurately, but it should bring you to your goal.

Still I think that removing the MM from your entries would be much easier and better than what you’re trying to achieve.

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