Jarring tool with micro adjustment

1. A jarring tool comprising:

a segment of sub housing containing an upper stop proximate a first end thereof;

an outer latch piece connected to an upper end of the lower shaft inside the sub housing;

an upper shaft passing through the stop and connecting on a first end thereof to an inner latch piece;

a cap on a second end of the upper shaft retaining a washer stack against the upper stop; and

a two-piece retainer inside the sub housing, the retainer having a first piece providing a restraining region and an open region and a second piece attached to the sub housing and to the first piece;

wherein the retainer retains the inner and outer latch pieces in a latched position in the restraining region until the latch pieces are displaced toward the second end of the sub housing through the retainer to the open region as a result of a tensile force on the outer latch piece.

2. The tool of claim 1 , further comprising a lower shaft attached to the outer latch piece on a first end thereof and providing a sub end on a second end thereof

3. The tool of claim 2 , further comprising a lower stop proximate the second end of the sub housing, the lower stop having a passage sized to permit extension of the lower shaft therethrough and away from the sub housing and to stop the extension of the shaft by contact with a shoulder of the shaft at a predetermined extension.

4. The tool of claim 1 , further comprising a second segment of sub housing attached to the upper stop and having an externally visible indicator that displaces and remains displaced in response to contact from the upper shaft cap resulting from unlatching of the inner and outer latch pieces.

5. The tool of claim 1 , further comprising an intermediate shaft arranged in a sliding and concentric relationship to the inner and outer latch pieces to define a protected passageway.

6. The tool of claim 1 , further comprising an access port defined in the sub housing, and an attached cover plate defining an opening of a predetermined size to control fluid flow into and out of the sub housing.

7. The tool of claim 1 , wherein the tensile force required to unlatch the first and second latch pieces being may be controlled by moving the first retainer piece relative to the second retainer piece by a threading engagement.

8. The tool of claim 1 wherein the outer latch piece is a collet that grasps the inner latch piece when latched.

9. A latch for a downhole jarring tool, comprising:

a retainer defining a restrictive region and an open region therein, the retainer having cooperating first and second pieces whereby the restrictive region is axially adjustable by moving the first and second pieces relative to one another;

an outer latch piece; and

an inner latch piece that is biased against movement through the retainer;

wherein the retainer retains the inner and outer latch pieces in a latched configuration in the restrictive region and allows the inner and outer latch pieces to unlatch when tensile forces applied to the outer latch piece move the latch pieces into the open region; and

wherein the first and second retainer pieces are in an adjustable fixed relationship such that the first and second pieces remain fixed during unlatching of the inner and outer latch pieces and are manually adjustable relative to one another to move the restrictive region.

10. The latch of claim 9 , wherein the retainer is anchored by a sub segment.

11. The latch of claim 9 , wherein the inner latch piece is biased against movement through the retainer by a shaft attached to a spring washer stack.

12. The latch of claim 9 , wherein the outer latch piece is connected to a sub end.

13. The latch of claim 9 , further comprising an intermediate shaft passing through the inner and outer latch pieces both in the latched and unlatched configuration.

14. The latch of claim 9 , further comprising:

a substantially cylindrical housing circumscribing the retainer and having a stop in one end thereof; and

a shaft passing through the stop and connected to the outer latch piece inside the housing;

wherein the shaft provides a shoulder for impacting the stop to create a jarring impact in response to a tensile force applied to the shaft that unlatches the first and second latch piece.

15. The latch of claim 9 , wherein the cooperating first and second pieces each define cooperatively threaded cylinders.

16. A jarring tool comprising:

a lower sub housing;

a center sub housing;

an upper sub housing;

an upper connector attaching the upper and center sub housings;

a lower connector attaching the lower and center sub housings;

a lower stop fitted into a bottom of the lower sub housing;

a lower shaft fitted through the lower stop and attached to a lower sub end, the lower shaft providing a shoulder inside the lower sub housing for contacting the lower stop when extended from the lower sub housing;

an outer latch piece attached to the lower shaft inside the lower sub housing;

an upper shaft passing through an upper stop in the upper connector and having a lower end attached to an inner latch piece and an upper end attached to a cap;

a spring washer stack arranged concentrically around the upper shaft between the cap and the upper stop; and

a retainer in the lower sub housing having a restraining region;

wherein the inner and outer latch pieces are restrained by the restraining region in a latched position until displaced through the restraining region due to a tensile force on the upper sub housing and lower sub end; and

wherein the restraining region is in a fixed relationship with the lower sub housing, the fixed relationship being adjustable by rotation of the restraining region about a central axis of the jarring tool.

17. The tool of claim 16 , further comprising an intermediate shaft attaching the upper shaft to the inner latch piece and being arranged concentrically within the inner latch piece, the intermediate shaft passing through the outer latch piece and into a sliding arrangement within the lower shaft such that a protected passageway is defined through the inner and outer latch pieces.

18. The tool of claim 16 further comprising:

an activation rod retained in the upper connector and proceeding partially out of the connector toward the cap; and

a spring loaded indicator cooperatively engaged with the activation rod and externally visible from the tool;

wherein the indicator is retained in a first position by the activation rod until the activation rod is displaced into the upper connector by the cap whereupon the indicator is released to move into a second position indicating activation of the tool.

19. The tool of claim 16 , wherein the outer latch piece is a collet that grasps the inner latch piece when in a latched position.

20. The tool of claim 16 , further comprising a bias spring interposing the upper stop and the inner latch piece urging the inner latch piece into the restrictive region.