I now have my D6 running after the great instructions on how and where to drill the starter hole. Since I repaired an engine miss on #6 with an injector tip. But now I have a fuel leak at the #6 injector pump, not at the line connection but right below that where the line connector is pressed through the cast housing. The part I think I need is 4F5019, that's the number right on it. The number in my D6 9U parts book shows #4F6921 and that number I see on ebay for $125.00. The one I removed 4F5019 had a large tapered seal ring for the fuel in and the one on ebay 4F6921 an oring to seal fuel in. I'm asking the experts what do you think? Does the number I have 4F5019 supersede to 4F6921?     

4F5019 is the old number superseded by 4F6921 and currently 8M9758. The seals start out as an o-ring but take on the cone shape when installed.

Thank you so much for quick response! I was hoping the new number would work. I ordered the one off ebay. Have you ever seen one leak/spray high pressure fuel at that position? It was squirting out where the center fitting was pressed into the main body. I was wondering if it had something to do with the injector tip I replaced because the tip was stuck open and compression was going up through the injection line back to the pump?

Unusual to have them leak there, that's a new one on me. I doubt it had anything to do with your injector problem.

M00051, Please make sure that you don't have a leak from the fuel line. Sometimes folks try to tighten the fuel line like it holds up the world and as a result the flare on the fuel injection line splits, causing a leak.
Elton

Good point!!! and much more common to have the ferrule in the fuel line split. Moving one of the other pumps into that location would prove the condition.

Probably easier yet to remove the injection line and very carefully examine the fuel line ferrules or flares for cracks. Use a magnifying glass and good light. At that kind of pressure, a crack that can't hardly be seen can move a lot of fuel. Elton

Hi Team,
also suggest whilst the fuel line is off that you closely inspect the tapered seat and end of the injection pump discharge connection as they are very hard and can be cracked due to over zealous tightening of a leaking injection line with split ferrule as others describe above. I have seen the line seat area of the injection pump outlet broken away in chunks before today and that is obvious--cracks are a bit harder to spot.
I do not recall encountering a leak at the injection pump body and inner parts--anything is possible-- the unit is deemed non-serviceable by Cat and so I have never dismantled one--looking at the sectioned views I posted earlier there would likely be an "O" Ring seal between said components but a new or used pump would be the way to go.
Cheers,
Eddie B.

on the point of being careful tightening fuel lines is there any special method on these old girls.

i know someone who had to replace the pump in his benz as the half wit mechanic who worked on it didnt RTFM and find out that the sealing washers have to be bedded in by a 3 stage tightening process backing off between stages. he just pulled em down to the final tension and it leaked so he nipped em a bit more and damn near snapped the fittings off.
$7500 grand for a second-hand one plus labour to fix. suffice to say the half wit didnt work on it again.