Application

TLNSh releasable overshot externally engages and retrieves tubular fish, particularly sucker rods during fishing operations in oil, gas and exploratory wellbores. 

Design 

Top sub connects overshot bowl to the fishing string with sucker rod or tool joint thread. The guide, flat-bottom or cultip, is connected to the lower end of the bowl and leads the top of the fish inside the overshot. 
If the fish diameter is near the maximum catch size for the particular overshot, use spiral grapple together with spiral grapple control ring. If the fish diameter is considerably below the maximum catch size, use a basket grapple and a basket grapple control ring. 
The bowl of the overshot has a helically tapered spiral section in its ID. The grapple is threaded into this section of the bowl, as it has a helically tapered spiral on its OD that directly matches the ID of the bowl. When an upward pull is exerted against a fish, an expansion strain speads evenly over a long section of the fish. Trapezoidal multi-start thread in grapple ID facilitates fish release and improves engagement with the fish.

Note: 

Catch range of spiral and basket grapples is +0.2/-1.3mm against the nominal size.