Originally Posted by Diesel911

The Fitting you are showing in the picture does not seal on the threads It seals in the inside tapered area. This means as long as the nut can be tightened down over the threads it well seal even if the outside threads are dinged up a little bit.
If you are having an extremely bad time of it removing the Fittng. I would just leave it alone. The valves are made of plastic and unless something has happend to the Spring inside like it rusting in two they is normally not much of a problem with them.

To remove the Roller and Tappet there is an external semicircular wire spring that Retains it. You have to without bending it remove that and ounce that is done it will slide out.

In the firs pic I point to the Retaining ring.
In the 2nd pic you see the retaing ring and the Groove that the Retaing ring goes into. (You also see that little O-ring that can cause the Diesel to Leak into your Engine.)
The little hook on the Wire Spring goes through a hole in the Pump body and that is what retains the little tappet and roller.
Note there is a lot of parts on the Tappet and Roller that just slide in and out to assemble; this means that they can easily slide out and get lost if you are not careful.