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I thought I would share my repair tips regarding the head gasket failure on the Northstar... 1997 Concours DeVille

What I have done is modified the procedure to eliminate any future failures of pulled threads in the block, and add clamping force by using studs instead of one time Allen bolts.

You will need to remove the engine and disassemble the heads. Once down to the bare block you will need to make a deck plate to mount a Mag drill on. My plate was 3/16 X 6" x 9". You will have to drill 2 holes for the Allen bolts to fit through to bolt the deck plate to the block. I used 2 deep well sockets as spacers. You will also need to drill 2 11/16ths holes and use 5/8 x 11 bolts for the deck plate.

The Inserts:

The inserts I used are NS300L from Norm Huhn. They feature a 5/8 x 11 external thread with a M11 x 1.5 internal thread. Height is 1.75 inches. Theses inserts add twice the thread engagement over the timeserts.

The Block:

Once the deck plate is bolted onto the head surface, center the bit and drill out the hole to a depth of 3.125 with a 17/32nd dill bit. Tap the hole with the starter tap and then finish with a bottom tap. My suggestion is that if you do one hole you might as well do all 20.

Clean all holes out with brake cleaner thoroughly!! Blow all the chips out of the water jacket, bores and anywhere else. Use permanent thread lock on the inserts and install to the bottom of the bore. Make sure they bottom out. Make sure the tapped holes are clean!!!!!!!! The reason I stated you needed 2 bolts at 5/8 x 11 is now you tapped the holes to this size and need to hold the deck plate on for the remainder holes for each bank respectively.

Once the inserts are in the block with thread lock I let it set up for a day.

Stud Kit:

I used a stud kit for the Audi/Volkswagon from ARP which is M11 x 1.5 x 6". This will work by using the old washer pressed off the original Allen bolt. Be careful, the threads will shoot off and could injure you. The ground washers with the kit are not as thick as the originals. You will have to open the I.D. of the washer up. The Allen bolts are necked down past the thread and the stud is not. Part number is 204-4204. I used the thicker washer because there is approximately 1/16th of non-threaded area on the stud above the head surface.

Once the studs are threaded into the inserts with moly paste you can install the gaskets and heads. Torque the head in 3 steps to 65 ft.lbs. Use the same torque pattern per GM. I used a cross hatch pattern that most can figure out.

You use the deck plate and Mag drill to insure 90 degrees to the block surface for the studs. If you hand drill it.... good luck with the studs being straight.

I have had the car running for several weeks and have not had any problems at all. I pushed the limit after the rebuild and had the Caddy to 122mph. I figured if it was going to fail it will fail now.


20 NS300L inserts
5/8 x 11 starter tap
5/8 x 11 bottom tap
17/32 drill bit
deck plate from 3/16 steel 6" x 9"
permanent thread lock
ARP stud kit 204-4204 Audi/Volkswagon x 2 (one kit per side)
FelPro gasket kit for your car.
Mag drill rental.
2 cases of your favorite beverage???? :alchi:

The inserts were bought on Ebay and the guy has been super to deal with. You can also try a google search for the insert kit. Much better than the time-serts. I have photo's if someone wants to see the results.

I then went on to resolving the several fault codes. All in all not a bad car for 1500.00.

Hope this helps,


2006 STS 1SG AWD, 1997 Concours Deville (sold)
94 Posts
Great write-up. I'm right in the middle of my head gasket repair job. I’m also using Norm's inserts, but not planning to use studs (although they would provide extra insurance.)

Just one note of clarification for others: "Mag Drill" is a Magnetic Base Drill - kind of a short little portable drill press with an electromagnet on the bottom. It is set up to drill holes in steel surfaces by attaching itself with an electro-magnet on its' base. Then it works just like a little drill press and can drill holes down through the steel that it’s attached to.

I tried to rent a magnetic drill, but couldn’t find one small enough. I opted to make a drill guild that bolts to the block surface and keeps the drill perpendicular. The magnetic drill would be much simpler though.

I think I'm going to need more than two cases to get through this job...

1997 ETC (GAVE TO STEPSON 2011), 2000 DTS (RIP)
1,702 Posts
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