Tapping for 1/2" head bolts

WABOOMWABOOM Posts: 61Member
My head gasket went bad again. One of the bolts wouldn't achieve full torque.  Now I know why... the 7/16 bolts threads pulled out on one hole. The diameter measures .428" (technically that's .020 oversize for the minor of a 1/2-13).
 Should I tap this hole for 1/2-13 as is,  or should I drill up and tap for Heli-Coil? That drill size is .531". Seems like a lot of material to lose, but I wanted to ask you guys.


  • ToddhToddh Posts: 336Member
    Because you need to torque the bolt, retapping the hole and opening up the corresponding head bolt hole for a 1/2” bolt would be the least expensive route.  Otherwise, have a machine shop install a sleeve in its place at the correct 7/16 bolt/thread size. 

    Others may say differently on using a heli-coil but I personally wouldn’t trust one for that application. 
  • ToddhToddh Posts: 336Member
    If it was me, I would go with the machine shop route.  One advantage of going the machine shop route is getting your head decked.  No one wants to pull their engine and roll the dice on “other issues” showing up but no one wants to be left on the side of the road either. 
  • KEL 39KEL 39 Posts: 112Member
    Use a 7/16 Heli coil kit for about $50
    Ivan L. 
  • 50C8DAN50C8DAN Posts: 2,688Senior Contributor
    Helicoils will work well, often stronger than the original threads.  Supertainum has been recommended to me but I have never tried them.  

  • WABOOMWABOOM Posts: 61Member
    Ok. I never even thought of a 7/16 heli coil. The head was skimmed for flatness latsummer when I did the head gasket. So I feel pretty good about the heads flatness. Also the deck of block looks straight too. Can't get a .005 shim under the 36" Starret ruler I brought home from work. 
  • 50C8DAN50C8DAN Posts: 2,688Senior Contributor
    I used Timeserts in all of my C8 Aluminum head spark plug holes a number of years ago and they have been great.


  • Ol racerOl racer Posts: 2,607Senior Contributor
    Either method would work, however I would prefer Tap 1/2" then drill that head hole and check all the other thread holes. Perhaps consider Dorman 7/16" studs in other holes. Dorman Website lists Stud dimensions to order at local Auto Store. Studs with large Grade 8 Washers give far more accurate torque. 
  • 50C8DAN50C8DAN Posts: 2,688Senior Contributor
    I often refer to H.A.M.B. for some misc tech stuff, lots of experience there.

    Here is a thread on stripped threads.

  • tigermothtigermoth Posts: 526Expert Adviser
    I have had complete success with thread inserts.
    regards, Tom
  • WABOOMWABOOM Posts: 61Member
    Thank you. 
  • cheyenne7271cheyenne7271 Posts: 492Member
    I used to be a Lawson distributor years ago and the Supertanium products are fantastic. A quality insert can be used be also you can drill and tap all of the holes to accept 1/2 bolts or studs 
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  • HansHans Posts: 256Senior Contributor
    Have you considered Step Studs | McMaster-Carr ?

    Larger threads in the block and normal threads for head nuts.

    could save the problems of several items of  concern. 

  • 54coupe54coupe Posts: 1,027Member
    Hans said:
    Have you considered Step Studs | McMaster-Carr ?

    Larger threads in the block and normal threads for head nuts.

    could save the problems of several items of  concern. 

    Those step studs look like they just have different lengths of thread on each end. The diameter seems to be the same on both ends.
  • 50C8DAN50C8DAN Posts: 2,688Senior Contributor
    If it were mine I would just change up all the threads to 1/2" and be done with it.
  • WABOOMWABOOM Posts: 61Member
    Installed the Time-Sert thread repair sleeve.
  • ajetajet Posts: 24Member
    30 or so years ago I "cleaned" the head bolt holes in my 308 ,

    unknowing with an oversize 1/2 " tap....

     Inserted all the  holes !   Hasn't leaked...

     use the CORRECT tap !

  • WABOOMWABOOM Posts: 61Member
    Update on this thread. After installing the Time-Sert thread repair insert. I made head studs by cutting lengths of Grade 8 All-thread. I installed the homemade studs with Loc-Tite Head Bolt water jacket sealant. I installed a Best Gasket Graph-tite 722G head gasket. I sprayed the head gasket with Copper Kote and installed while tacky. Fastened with grade 8 nuts and washers lubricated with copper anti-seize. Torqued in proper sequence pattern to half of spec'd torque. Then the sequence pattern to spec'd torque of 60-65 lb/ft. 
    For the intake/exhaust gasket I used the Remflex Graphite gasket.
    Very pleased with the results. 
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