This is a typical Happ quarter coin mech manufactured by Industrias-Lorenzo .  This is a quick tutorial on how to adjust this mech to accept 0.984" tokens.  Click on the pictures to get an enlarged better view.

I have pulled back the magnet swing arm gate assembly to give you a better view of the cradle and lever assemblies.

You will notice the red lever assembly that looks like a backwards "L".  As a token drops into the other red piece (the cradle), it pivots and drops the coin against the top of the lever which then swings back and hits a part of the main body of the mech and this keeps tokens that are too large from falling through the mech.

This area of the mech needs to be scraped away with an hobby  knife until it allows the lever to swing back far enough to accept a 0.984" token.

The area that needs to be scraped away is marked in this picture by the white line within the yellow circle.  Take the hobby knife and scrape away the plastic a little bit at a time taking breaks to test the token.  Care should be taken to test the coin continually and to only scrap away a little of the plastic.  Also, when testing the coin make sure the mech is level as it would hopefully be when installed in you coin door.
Eventually the plastic area will look more like the white line in this picture.
Once the token drops through the mech on a relatively consistent basis you will have a mech that looks like so.
This picture shows the lever as it swings back and will allow a larger coin than the quarter size the mech was originally intended for.

 

If there are any questions or anything needs clarification, email me at mperry@miavio.com and I'll update this page.  Thanks.