Why is Google showing threads in printthread?

[Crazycat]

The only way is to add a rule for Googlebot to force the 301 redirection.
If you're using apache, add in your .htaccess something like:

RewriteEngine On 
RewriteCond %{HTTP_HOST} http://www.forum.com/ [NC]
RewriteCond %{HTTP_USER_AGENT} Googlebot
RewriteRule ^/printthread.php(.*)$ http://www.forum.com/showthread.php$1 [L,R=301]

[Ashley1]

Well, if I were to do the templates way, does that mean Google will then only crawl showthread URLs? I have some posts that are on Google ranked on the first page, but they are in printthread.php templates. I understand it may be lost, but is it possible to simply convert all printthread.php links to their proper Google SEO url / showthread.php url?

Not without a hack way of going about things. You should decide if you want printthread pages in the index or not, and then simply apply meta noindex, and leave it at that.

Disallow directives should not be used in general. The only thing they are really good for is optimising crawl budget.

I said a 301 redirect could be used earlier in my post, but it is not necessary or appropriate in your case.