Official thermactor plug bolt size?

[Zero_chance]

My friend is convinced on plugging the thermactor system at the heads and wants to just use a bolt. I was just wondering if anyone that actually has plain bolts in there and, if so, what size are they? Looking at old posts, Ive found two sizes that are commonly thrown out there. One is 5/8 coarse thread, the other is 7/16 coarse thread. Does anyone that has them actually remember which size they are?

 

[the pony boy]

i believe its 5/8.i have mine plugged with old spark plugs!!!! phil

 

[DJTR]

no clue never tried

 

[89MustangGX]

5/8" sounds familiar -- they are pretty good size bolts. You will probably have to cut some down -- I had a hard time finding short ones.

 

[5spd GT]

Just use the plugs if you can (if the threads are clean enough)...If you are going to delete the smog pump then you will need a threaded insert known as a thermactor plug. They screw into the back and front of your heads. Put silicone on the threads to prevent any exhaust leak when installing them. The thermactor plugs you will use for the front (same as back) will go to make the head holes smaller so the accessories can bolt up like your power steering stud and one of your bigger alternator bolts that go through you alternator bracket. The back thermactor plugs you will use just block off and delete the smog equipment’s previous hookups. You can get them at a Ford Dealership - part # F4ZZ-6E086-A or you can buy some from Summitracing - part # TFS-51400265 and they run anywhere from $3 to $12 dollars from what I have seen. Summit has them cheaper.