Just purchased a Cat fuel filter kit for my 2003 5.9. Website says it should be mounted before the stock filter. Sounds backwards to me. Does this mean I need to buy another, different kit to engine mount to install after the stock? Really looking for some answers. I don't understand mounting a 4 micron filter before a 5-7 micron. Thanks for your help. I asked this question on another site, but couldn't quite get the answer.

 

there is another post this site current that gives the explalation. will paste part of it. the cost of the parts and filters is $75 your cat filter is probabally a 1r0750 the 33674 is a larger version of cat 1r0749, same base thread. use your cat base and add the w/s and base.
we mount the filters just forward of the fuel tank. we run a fleet of dodges and recently installed one on a 06. there is two bolts through the floor board for a rear seat option at this location. some of our trucks use them others do not. get a 2" x 3" piece of angle iron 6" long. make a card board template that the two bolts and keeper nuts will go through hold the template on the inside against the 2" part and drill the two holes they will be about 5/8" i think and about 3 7/8 " between. center the holes on the 2" and near the edge on the 2" hold the angle iron over the bolts with the 3"part the furtherest away from the tank and drill a 1/4" hole between the two holes, keep drilling through the floor board. now after you mount the filters to the angle iron just simply mount it with a 1/4 grade 8 bolt. this is the only hard part the other items is easy. the total time including building the bracket is about 1 1/2 hours. i will give you napa numbers but they can cross reference to wix or car quest. the two bases are 4770 the water separator pre filter is 3406 the 2 micron filter is 3674. use a 3/8" nipple with about 1/2" space in the middle connect the two bases together observing the in and out. because of this the mounting plate on one filter base is about 1" further out than the other. install two 90* 3/8" barbed fittings on the bases and temporary bolt the angle iron up and hold the filters up to it. make sure everything clears and mark the one base that touches the plate to drill the three holes to mount the filters. next cut the feed fuel line at this location and bend the lines out a little and install 3/8 hose over the lines and to the barbed fittings. i put a small amount of flair on the steel lines. but i do not think it is necessary. Bleed the air out through the stock filter before trying to start it. use a gauge and do not change the filters until you see a four pound drop this should be about every 150,000 miles and the stock filter will never need to be changed. Cheapest place is http://www.fleetfilter.com/filters/w...ter-bases.html

 

found the link
https://www.dieseltruckresource.com/...r-t310660.html