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Helicoil And Head Stud - Any Advice


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#1 MP Clegg

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Posted 12 April 2019 - 08:37 AM

Hi All,


I have a 1990 Cooper so 1275cc Carb engine which recently drank its coolant, I had hoped head gasket but on stripping back it was fine but a steel ruler on the head showed light through.


I had the head skimmed and then I rebuilt (i'm not a mechanic so just a Haynes manual follower) and all appeared ok in the garage and on a quick run for MOT but a longer run straight after left me with coolant dripping from the underside of the bonnet.


Running back up in the garage later after cleaning everything down I could see coolant from between 1 and 3 studs at the front between block and head and wondered if the first run had shifted anything so thought I should check all the torques.  The nut on stud one just turned freely as did three but all the rest click on the wrench as I expected.


Pulling the two studs out they look fine, no damage on the threads, they were in the right way around as well!  When I try to put them in again they just turn freely, I cant pull them back out upwards, I have to unwind them, so they are biting on something but that might be the gasket.  After taking number one back out again to show a garage it has a nice spiral of cast iron in the thread, not a helicoil but they feel the thread in the block has failed.


Long description sorry, question is if the block thread has gone has anyone any experience of fixing this, can it be done with Helicoils and is this a home garage job or is it engine out on and a bench essential?  There seem to be lots of kits for UNC 3/8" Helicoils but don't want to start if I'm just going to damage the block further.


Any advice would be great.  Martin

#2 Moke Spider

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Posted 12 April 2019 - 08:55 AM

Bummer !


Yes, you can Helicoil these threads quite OK and was in fact done at the factory from time to time.


Be aware. Generally, and it depends on the brand of Helicoil, there's 3 lengths available in 'Helicoils'. For this I would suggest the mid or longer type.

#3 DeadSquare


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Posted 12 April 2019 - 01:36 PM

It is best done with the engine in the car, as it holds the block steady which helps to eye up the tap to get it square.


I remove the head,  place a ring of cloth a bit away from the damaged stud hole,  replace the head to be a guide and drill through the head into the block with a 25/64th" drill, making sure to drill deep enough for the chosen length of helicoil.


I have a 2" steel cube, through which I have tapped the helicoil thread to make it easy for me to get the tap square, but if you are careful, it is not too difficult to eye it up against the other head studs.


Use something like WD40 to keep lubricating the tap, and back it off 1/4 turn for every 3/4 turn of cutting, again making sure that there is enough depth of thread for the helicoil.


Make sure that you dry the hole well with a tissue to help get all the swarf out before screwing in the insert.


Some people Locktite it all, but I usually don't, and have never (that I recall) had a problem.

#4 Steve220


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Posted 12 April 2019 - 02:08 PM

If you can hire a magdrill from somewhere, they work great with the head off!

#5 viz139


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Posted 12 April 2019 - 03:47 PM

I did one once using the head as a guide. Main thing is to block off all other holes to prevent swarf going into the boars or cooling system.

#6 Cooperman


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Posted 12 April 2019 - 10:08 PM

Heli-Coils work fine in the block.


As above, the only problem is getting the drill vertical in both planes. You really do need some sort of guide jig, but I have done it by eye with some success using a hand drill. 


Ideally the Heli-Coil should have a thread depth of 1.5 x the thread diameter.


Namrick Ltd sell kits which include drill, tap(s), insertion tool and several inserts. Buy on-line for next day delivery.

#7 mini13


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Posted 13 April 2019 - 03:18 PM

Mag drills are Good for this, but as said it can be done with a longer drill bit using the head as a guide, I've wrapped masking tape round the drill bit to make it a fairly snug fit on the head hole,

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