Extension-type spark plug

1. An extension-type spark plug, comprising:

a tubular housing;

an upper insulator received at least in part in said housing, said upper insulator having an inner surface presenting a through cavity extending between a terminal end and a distal end and having an upper diameter portion and a lower diameter portion separated from one another by a radially extending shoulder, said upper diameter portion having a reduced diameter from said lower diameter portion;

a lower insulator constructed of a separate piece of material from said upper insulator received at least in part in said housing, said lower insulator having a through cavity extending between opposite ends;

a firing electrode fixed in said through cavity of said lower insulator and extending axially outwardly of one of said ends;

a lower terminal stud fixed in said through cavity of said lower insulator at the end opposite said firing electrode;

an upper terminal stud disposed in said through cavity and along said inner surface of said upper insulator;

said upper terminal stud extending between proximal and distal ends and having an enlarged head with one diameter at said proximal end and an elongate body extending from said enlarged head to said distal end and having a diameter less than said one diameter of said enlarged head at said distal end such that said enlarged head and said elongate body and said inner surface of said upper insulator provide a clearance region therebetween; and

a spring member engaging said distal end of said upper terminal stud and said lower terminal stud to provide electrical communication between said upper terminal stud and said lower terminal stud, said spring member having a bias force for biasing said enlarged head of said upper terminal stud into abutment with said shoulder of said upper insulator and allowing said enlarged head to move axially out of abutment with said shoulder into said clearance region under an external force applied on said upper terminal stud, said external force being sufficient to overcome said bias force of said spring member.

2. The extension-type spark plug of claim 1 further comprising a tubular outer insulator received in said housing, said tubular outer insulator having one end abutting said distal end of said upper insulator and another end receiving said lower insulator at least in part therein.

3. The extension-type spark plug of claim 2 further comprising a tubular inner insulator received in said tubular outer insulator.

4. The extension-type spark plug of claim 3 wherein said tubular inner insulator has one end receiving said lower terminal stud therein and another end spaced axially from said enlarged head of said upper temiinal stud to provide said clearance region between said enlarged head and said elongate body and said inner insulator.

5. The extension-type spark plug of claim 4 wherein said tubular inner insulator has a through cavity sized to receive said elongate body of said upper terminal stud at least partially therein.

6. The extension-type spark plug of claim 5 wherein said elongate body is received in a loose fit within said through cavity of said tubular inner insulator.

7. The extension-type spark plug of claim 5 wherein said spring member is received in said though cavity of said tubular inner insulator.

8. The extension-type spark plug of claim 3 wherein said tubular inner insulator is substantially fixed against axial movement relative to said tubular outer insulator.

9. The extension-type spark plug of claim 2 wherein said tubular outer insulator has an outer surface spaced from said housing to provide an annular gap between said tubular outer insulator and said housing.

10. The extension-type spark plug of claim 1 wherein said enlarged head of said upper terminal stud is received in a loose fit with said lower diameter portion of said through cavity of said upper insulator.