Here is a link for verification. Notice in the description "For standard non-sanwich adapter applications use GM # 25013759."

http://paceperformance.com/i-5134579...oler-only.html

BillK is correct that the center plug needs to be removed if you want to have the oil to the cooler flow out and back only through the two taped holes on the pan rail in front of the oil filter pad location. In a Gen 5 or 6 block such as late model 502's and 496" (8.1L) these taped holes are there. On the Mercruiser engines Mercury uses a filter adapter plate which routes the oil out of the plate area out to the remote filter, through the cooler and back into one of these taped holes which is a return to the block oil system which requires the center valve to be in place. The relief valve is there so that if their is a pressure drop of more than the valve is set for it will open and allow the oil to go straight to the block oiling system and not starve the system for oil and obviously damage the engine. If the oil were to do this all the time it would not get properly cooled and filtered by the Mercruiser system. On the Mercruiser engine you have, if both pressure valves are in as they are on most stock blocks you should leave both valves in place and if you are replacing them, use the 25013759-30lb) valves for both. The pressure valves are not shown on the mercruiserparts.com diagram shown on this thread because they are in the block and not in the actual remote oil system shown in the diagram. As has been said previously, one is in the flat filter pad area in the block and one is in the deep threaded hole behind the threaded filter adapter fitting the Mercruiser filter adapter center bolt threads into.
Hope this helps clarify the confusion.

Best Regards,
Ray @ Raylar

Thanks Ray, that helps me out, engine oil system ran fine before the build, same stock valves were removed and replaced after build, according to machine shop. Nothing about the oil system was changed so I plan to leave well enough alone. I will monitor it first couple of times out and change if needed.......:coolcowboy:

do they pry out or are they threaded in ? sorry for all the questions..just trying to make sure I get this right before putting the engine in...yes 98 merc set up

Just screw a 3/8 bolt into the valve and pull it out. They come right out.

Ok, if running a std merc oil cooler and remote filter set-up which valve should I be using ? thanks...

Gen V & Gen VI Big Block Chevy Engines & All GM Performance Parts 454 & 502 crate engines use two of these bypass valves. Location for oil filter bypass valve is adjacent to oil filter nipple on oil filter mounting pad, Location for second valve (For oil cooler bypass) is above oil filter nipple in block on filter mounting pad. If a cooler is not connected to the oil cooler ports on the oil pan rail, the second valve is not needed. Engines using a sandwich adapter between the block and oil filter to connect a remote oil cooler must use GM # 25161284 oil filter bypass valve (in the location adjacent to oil filter nipple) to compensate for higher system backpressure.
The valve #25013759 is only a 11 psi diff
leave out the center one then the oil will go right to the main galley and not the bypass galley gm used for there air cooled cooler set up. and replace the other one witch is the oil filter bypass to a 25161284 oil filter bypass valve

I read a thread on here once that suggested to block off the pad bypass not in the threaded hole. I forget the details about why but if I can find the thread again I'll link it. What would be the thinking behind that?

You can install a 30 psi bypass valve or plug the outer pad
"oil filter bypass" not the center one that would be called the gm's oem air cooled cooler bypass valve the center is the many oil galley that feeds the engine.

Well said and I will add to that: Use #12 hose with the least restrictive fittings possible for your application.