ARP 625 Head Stud Install

ARP Head Studs

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Well, I finally took the time to give my truck a little preventive maintenance TLC yesterday. I’ve long since known the installing head studs in place of the factory bolts is a great first major mod for the 6.7 Cummins. I decided it was time to practice what I preach.

In my humble opinion, ARP 625’s are the ultimate in head studs. I used them in my install. They are not required for a successful install, but why not if they are in the budget.

ARP 625 Head Install Double R Diesel picture 1arp-625-head-install-double-r-diesel-2

Check the inventory of the box and make sure you have what is supposed to be there. You should have:

  • 20 regular length studs
  • 6 extra-long studs
  • 26 washers
  • 26 nuts


If your truck is lifted like mine, get it low as possible to make access easier.


  • Using a crankshaft-bearing tool, insert it into a hole in the flywheel housing that is covered by a thin metal cover.
  • Rotate the engine and get the TDC mark in the 12 o’clock position.
  • Make a mark 180 degrees opposite of the TDC to allow you to know where to stop while working under the truck.


  • Remove the oil cap and the crankcase ventilation filter cover.
  • Now disconnect the crankcase pressure sensor.
  • Remove the crankcase ventilation filter.


  • Loosen the six bolts that hold the valve cover down and carefully remove the valve cover.

NOTE: The injector wiring harness and cover gasket is all one piece so be careful not to damage it removing the valve cover.


Unplug the 2 plug ins for the injector harness/gasket and remove it. From this point and on, it’s a great idea to take pictures and also lay your parts out on a table in the order and orientation that come off the engine.

Pay close intention to how the injector wires connect and their colors just in case the wires don’t lay back in their original positions when putting the harness back in. You don’t want to cross the wires wrong. Also, keep in mind the wire post on top of each injector are small and very fragile. Break one and you’re buying a new injector.


Remove the small bolts that hold the aluminum rocker box and remove it. There is a rubber gasket on the bottom side, be careful not to damage it, so you can reuse it. Mine came off, still in place inside the groove underneath the rocker box.


Now remove the bolts that hold the rockers down and remove each assembly as a pair, being careful too not lose any parts. Everything including the rocker pair and pedestal should come off together. Lay them on a table labeled or marked for its corresponding cylinder. You can remove the valve cross bridges now or leave them in place. It doesn’t really matter, just make sure they go back in their original positions if they get moved.

Once the rockers are out of the way, you can start removing the head bolts one at a time and replacing it with a new stud torquing them to 100lb/ft of torque. Put a drop of oil on the threads that go into the block. Screw the studs down until they bottom out and back them out half a turn.

The idea here is to make sure the stud only receives vertical stress when torquing them down and no torsional stress. I was able to turn most of my studs completely by finger. There was only a couple that required a little turn with an Allen wrench. If any stud doesn’t want to go in fairly easy, then you may need to buy a thread chaser and chase the threads.

Be sure to, liberally coat the exposed stud threads and washers with ARP assembly lube. This is essential for proper torque and preload on the studs. Start with the center head bolts are work your way to both ends of the head. The 6 extra-long studs go in the place of the 6 outboard bolts that run alongside the head next to the exhaust manifold. The ARP instructions will show how to do this.


This is a long process so take your time and have patience. Once all 26 studs are in and torqued to 100 lb-ft, go back thru the same process again while increasing the torque applied to each stud in 10-15 lb-ft increments until you have them all too final specs. I torqued my 625s to 155 lb-ft of torque.


Now put the rocker assemblies back in place just like came out and check your valve lash with a feeler gauge between the rocker tips and the valve bridges. Make sure the engine is top dead center before doing this.

You will check your lash as follows on the top dead center of the compression stroke
Intake 1-2-4 Exhaust 1-3-5

Now rotate the engine 360 degrees and check the following valve lash
Intake 3-5-6 Exhaust 2-4-6

My intake valves were still in spec but I did end up setting my exhaust valves with a .020 lash.


Now it’s time to reinstall the rocker box but you have modify it ever so slightly to clear the number 26 stud in the rear. I forgot to take a picture of this on mine, but here is a screenshot of the GDP head stud install instructions that are available on their website.


Now button everything back up…

  • Put the valve cover back on
  • Put in a new crankcase ventilation filter
  • Hook injector wiring plugins and crankcase pressure sensor plug in back up
  • Reconnect the oil drains hoses and the crankcase breather hose back up
  • Hook the batteries up
  • Fire it up!

Check for any leaks and proper engine operation and you’re done!

This is definitely an all day job, but it’s worth the peace of mind knowing the cylinder head is now clamped down securely.