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Helicoiling A Head Stud In The Block


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#1 Chris M

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Posted 12 July 2019 - 08:17 PM

I have found one of my cylinder head studs is not holding in the block fully. As tightening up the cylinder head it works up. The thread is badly worn/damaged. I need to helicoil it. Has anyone done this themselves & what size is the thread ?
Any help or advice would be great 👍

#2 Moke Spider

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Posted 12 July 2019 - 08:32 PM

The most important thing in doing the job, is to get it dead square.

 

One of the easiest ways to do this is to use a cylinder head as a drilling guide, but you wn't be able to do the same where it comes to tapping the thread in the block. One way is to check it against a square after you've cut the first thread can help and check again on every thread until you've done about 4 threads, then you should be away. Be sure to chamfer the hole after tapping.

 

Thread size is 3/8" UNC.

 

When buying a Helicoil Kit, be aware the inserts come in a few lengths. I'd suggest you want some that are at least 1/2" long or longer.

 

When fitting the insert in to the block, you really want it to end up about 1 thread down from the deck.



#3 DeadSquare

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Posted 12 July 2019 - 10:56 PM

I don't know if it is still the case, but 50 years ago, the proper Helicoil tap had a long enough shank to go through a Mini cylinder head.

 

Having used the head as a drill guide like Moke suggests. with the head removed, slide a washer down the shank of the tap and inserted it through the face of the head.  Carefully placed the head over the studs and start tapping.

 

If it is a stud at either end of the block, a tapered wedge under the other end of the head, helps to keep things level



#4 Moke Spider

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Posted 12 July 2019 - 11:24 PM

I don't know if it is still the case, but 50 years ago, the proper Helicoil tap had a long enough shank to go through a Mini cylinder head.

 

I'm pretty sure it would be too fat to fit through a stock cylinder head stud hole.



#5 DeadSquare

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Posted 13 July 2019 - 07:02 AM

 

I don't know if it is still the case, but 50 years ago, the proper Helicoil tap had a long enough shank to go through a Mini cylinder head.

 

I'm pretty sure it would be too fat to fit through a stock cylinder head stud hole.

 

 

It did in 1964.  With almost all of the square showing, I half believed that they had made it specially.

 

Later in life, I developed the idea and made a  "thick walled, countersunk tube" to square up the tap when repairing damaged Mini sump plugs.



#6 ACDodd

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Posted 13 July 2019 - 01:31 PM

Rocker posts are ideal for this sort of thing.

Ac

Edited by ACDodd, 13 July 2019 - 01:31 PM.


#7 Curley

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Posted 13 July 2019 - 06:15 PM

The most important thing in doing the job, is to get it dead square.

 

One of the easiest ways to do this is to use a cylinder head as a drilling guide, but you wn't be able to do the same where it comes to tapping the thread in the block. One way is to check it against a square after you've cut the first thread can help and check again on every thread until you've done about 4 threads, then you should be away. Be sure to chamfer the hole after tapping.

 

I thought only the number 1 stud is exact size, all the others are a loose fit. Does this have any affect on getting the tap square?



#8 Moke Spider

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Posted 13 July 2019 - 08:16 PM

 

The most important thing in doing the job, is to get it dead square.

 

One of the easiest ways to do this is to use a cylinder head as a drilling guide, but you wn't be able to do the same where it comes to tapping the thread in the block. One way is to check it against a square after you've cut the first thread can help and check again on every thread until you've done about 4 threads, then you should be away. Be sure to chamfer the hole after tapping.

 

I thought only the number 1 stud is exact size, all the others are a loose fit. Does this have any affect on getting the tap square?

 

 

No. 1 in the front row and no. 3 in the back row are a more neat fit, yes.

 

I probably should have included a bit more info in that as you eventually need to drill them over the size of these holes (which ever one is used), to start off with a drill bit that is a neat fit in which ever head hole is used. The hole would then need to be opened up further to the final tapping size.

 

Ideally, one would do the job with the block in either the drill press or mill to get it smack on, but if the engine is assembled and in car, I doubt there'd be few who would remove and strip the engine just to sort this minor issue.

 

It's just a practical method of achieving an acceptable result.






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