davco fuel filter and spin on

which ever gets the fuel first from the tank will be the primary, (should be the Davco).

I'm sure the 10 micron will go on the Davco, which I still think should be the primary.

 

Davco is the primary. I assume this truck is older or changed prior to you. 'MOST' newer DD60s run only a Davco with a 7 micron cartridge (Fleetguard
FS 19624 or similar) if you have the correct secondary filter on now it is a 4 micron IIRC. if you want to continue with the current set up ,run the 4 mic in sec position and a 10 or 25 mic in Davco. if it were my truck I would bypass the secondary and run only a 7 mic in the Davco. I own 3 DDEC 4s running with only the Davco and it works great

 

I believe the davco is the primary. when I had hey 60 series detroit I always use the upgraded fuel filters with a lower micron rating. The standard fuel filter generally is 10 micron if it is a detroit brand. The upgrade is 7 micron. in my opinion it's always best to get the upgraded fuel filter which will extend the injector life

 

I only have the davco on my truck and I found that I have to switch from the 7 micron to the 25 micron in the winter. In the sub zero Temps I could put a new 7 micron in and the fuel would be to the top of the glass by the end of the day and I was not running bio diesel and I have coolant lines to it so I have heat there so this kinda surprises me but if I go with the 25 micron there's no problem.

 

You can run anything you want in the davco, but the 2 or 7 would be overkill since you have another filter down the line. I have only the davco and the 7 micron filters I put in there seem to plug quite often especially in winter.