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MG MGB Technical - Stripped thread

I have a stripped manifold stud hole on my cylinder head. What's the best way to fix this? Is this something you could just tap to the next size up (if so, what size) or do I need to take it to a machine shop to fix? I believe it's a 5/16 24 UNF thread. Do I have to take the head off to do this? I have never tapped a hole before but am otherwise very handy at doing my own car repairs. Please advise.

Joseph Maggiore

Here's what I would try first: get an appropriately-sized bottom tap and run that in the hole all the way. There should be good threads deeper in the hole than the stripped part. Then you can either find a stud with longer threads that screw into the head or use a bolt that reaches the good threads (instead of the stud).

Wayne
Wayne Pearson

Joseph,

I believe the proper repair would be a Heli-coil. It involves drilling and tapping to the next size, and then screwing in the Heli-coil, which has male threads of the larger size, and female threads of the original threads.

If you can fit a drill in with the head on the car, there is no need to remove it. It might even be worth buying a close quarters drill or a 90-degree drill if that will allow you to do it in place.

If you don't want to do it yourself, by might get a friendly automotive machinist to make a house call.

I am reasonably confident that that area can be drilled without breaching any of the water jacket or ports, but I have not actually done it myself.

Charley
C R Huff

Heli-coil would be the way to go.I have had great luck with them.
rich osterhout

Put a stop collar on the drill!
Art Pearse

A point of clarification - Helicoils have the same thread both internal and external (they are just a spiral wire of the correct shape). The kit contains a drill, a tap, an insertion tool and the coils. The stripped hole is drilled over sized then tapped to the same thread count as the original size and finally the coil is installed. This is a relatively easy procedure, but there are two reasons that you might want to take the car to a good garage to have the job done. Number 1 is that the mechanics there should have done the procedure and have the kits on and (check this out ahead of time) and #2, the kit is quite expensive, so the cost of having it done by the professionals probably wouldn't cost much more than the kit. The argument for doing it yourself is that you would then have the kit on hand if another bolt hole gets stripped. A 5/16 - 24 thread is quite common on the MGBs, so the second option might be preferable. Finally Art's admonition of of using a collar on the drill is one to be followed. Cheers - Dave
David DuBois

Great point Dave.
rich osterhout

Add me to the list - Helicoil is the correct way to go.
David Overington

Over the years I have worked with a few different "Brands"and types of thread repair I personaly don't like the coil type rather I like the threaded "tube" type, they seem to be easier to install and work great. SOooo before you buy check out your alterntives. Let us know what you do. RIC
RIC LLOYD

Why not just tap it 3/8 NF and use a 3/8 stud? No need to go to the expense of helicoil.
Art Pearse

The helicoil is not the way to go as you have to buy a special tap and the matching drill size which is too expensive for a once of job.
The new and better way is to buy a 5/16" tube type threaded insert. The tube taps its own hole. They come in thick wall and thin wall. Get the thin wall; thick walls are for soft material. Enlarged the stripped hole with a drill to suit the external tapping size of the tube. You should easily feel when you reach the bottom of the hole. Use a 5/16" bolt. First put a nut as a lock nut on the bolt. Screw the threaded tube on with the slots facing out, but below the bolt end. Bring the nut back to lock the tube. The slots are the cutting edge of the threaded tube which cut its own threads. It needs the gap at the end of the bolt for the shavings to collect. The nut is used to screw the tube in as it locks against the tube and the bolt is used to unscrew itself to clean shavings. When the nut reaches the hole, the tube is in sufficiently for the stud or bolt. If the tube bottoms out before it is flush with the top, the exposed excess can be ground flat.
The head need not be removed. You should be able to do the job yourself. The drill will guide itself down the hole. You will need to remove the manifold, tape the ports and cover areas where shavings are not to enter.

Regards,


Richard.
RH Davidson

you never mentioned which side of the stud was stripped...I had the same problem with the side I place the nut onstripped, I just replaced the stud with as new one.
P. T. Murray

This thread was discussed between 30/06/2009 and 04/07/2009

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