Application

TLNSH releasable overshot externally engages and retrieves tubular fish, particularly sucker rods, during fishing in oil, gas and exploratory wells.

Design

The overshot is composed of three outside parts: top sub, bowl and cutlip or flat-bottom beveled guide. The top sub connects overshot to the fishing string with tool joint or rod thread connection. Connected at the bottom, the guide leads the fish neck into the overshot.
The Bowl of the Overshot is designed with helically tapered spiral section on its inside diameter. The grapple (Spiral Grapple or Basket Grapple with multistart buttress thread on its ID), is fitted into this section. When an upward pull is exerted against a fish, an expansion strain is spread evenly over a long section of the bowl and the compression strain is spread evenly over a long section of the fish. 
Basket grapple is the best for retrieval of the fish, having small OD, while spiral grapple is used for the fish with large OD that is near maximum catch size of the specific overshot.
Note:
Catch size range of both spiral and basket grapples is +0.2/-0.3mm of the specific size.