Patent ID: 6948560

Claim:
A jar, for use in a downhole toolstring comprising: a hollow housing; a jar mandrel; a latch sub; at least one latch key; a cam surface; a chamber; a compression spring; and an adjuster, wherein: the hollow housing supports, moveably retained therein, the jar mandrel and the latch sub; the jar mandrel and the latch sub are releasably securable together by means of the at least one latch key, each said latch key being moveable between a latching position, in which the latching sub and the jar mandrel are connected together and a release position permitting separation thereof; the cam surface is engageable with each said latch key to move each said latch key from said latching position to said release position when the jar mandrel occupies a preselected position in the housing; the compression spring is constrained within the chamber and acts between the latch sub and the hollow housing to bias the jar mandrel when connected to the latch sub away from the preselected position; and the adjuster includes an adjuster mandrel that is rotatable relative to the hollow housing and has an external portion that is engageable from outside the hollow housing via a side thereof, and an adjuster portion that is threadedly connected to the jar mandrel such that rotation of the adjuster mandrel relative to the jar mandrel alters the length of the chamber and hence the degree of compression of the compression spring.