Board Thread:Friend Code Exchange/@comment-104.3.30.15-20150420020628/@comment-24668482-20150420230135

R - the game level you need to reach to unlock every combining element as potential free gifts is listed on the combining elements page here. Screw nuts are unlocked at level 100, Small screws are unlocked at level 105, Washers are unlocked at level 110 and Split washers are unlocked at level 115.