Stuck cylinder head stud

[RobH]

"Studs that I have seen helicoiled have only had or needed the one helicoil fitted at the top"

 

The whole point of the long stud is that the load is taken lower down in the block. Surely using a short helicoil just at the top defeats the design intent ?

[Motorsport Mickey]

"Studs that I have seen helicoiled have only had or needed the one helicoil fitted at the top"

 

The whole point of the long stud is that the load is taken lower down in the block. Surely using a short helicoil just at the top defeats the design intent ?

Correct as regards the design intent of spreading the torque throughout the main frame of the block, this helps prevent the cylinder block top surface being lifted also.

However I think the comment by Chris regarding the mechanical retention is also correct as regards the ability of the stud to apply the full torque load even through a standard length short helicoil.

 

 

Personally I see no reason not to fit two helicoils in a tapped hole for a better fix as the coils will follow the TPI of the helicoil tap, I have done it many times. A well proven repair method, and theoretically stronger than original.

Best of luck

Paul

And Paul is nearly correct, but there is no "theoretically" stronger involved, the greater thread root of the larger dia Helicoil thread exposed to the tightening torque itself locked into position by the screwed in stud ensures that the torque failure point of the Helicoil is guaranteed to exceed that of the standard thread form in the block.

 

Mick Richards

[boxofbits]

Hi Pete,

they do not have to be. As long as the thread allows the stud to run down the odd small gaps make no difference.

The extra gripping effect of a helicoil more than compensates for any small gap.

 

Roger

I appreciate what Pete is saying.

 

I think he means if the spacing between the threads is not precise then it could jam the stud. I had personal experience of this when I inserted 2 UNC helicoils for a bolt through the transmission housing and into the side of the gearbox on my Mini. I had to take it out in the end to put a longer one in which I should've done in the first place, and if you've ever tried to get a helicoil out, it is an absolute ba$txxd! I realise in theory it should work, but my feeling is if it was put in to quite a depth in the block and the deeper insert thread then started to lock up and damage the stud thread, that could be a bit of a scenario.

 

I also wonder if an impact wrench is just going to end up snapping the stud off. I'd prefer to weld a large nut to it and use a long bar to tighten/loosen until you can see some movement so you can feel what's going on.

 

Kevin

Edited November 27, 2016 by boxofbits

[Charlie D]

Hello,

I was thinking about this problem the other day, before I removed my studs. (I always expect the worst, so it’s nice when it doesn’t happen.). It occurred to me that I could helicoil a stripped stud hole, but, in the case of the longer studs, I was wondering how many machine shops would have a long enough tap to go deep down into the block.

If I’m right, the helicoil comes with it’s own tap. But do they do extra long ones?


Charlie.

[Motorsport Mickey]

No you make your own like I did.

 

From memory use a small 1/4 drive socket welded (concentrically) onto a 1/4 dia bar of the correct length to reach the area to be threaded when a tap is inserted into the socket. I used a single wrap of tank tape around the joint of tap and socket (will go into the stud hole) to help keep the tap and socket engaged, but using it is no problem, you just ensure the tap has completed cutting the thread before reversing the rotation until the tap unscrews from the thread it's just cut, then just lift carefully out of the hole. In my case the tapped hole was of the standard thread form and diameter but the same fix will work for Helicoil sized taps.

 

Mick Richards

[Lebro]

+1 & to add to above, some taps cannot cut beyond the top end of their threads because the shank diameter is too large, cure is to grind it down to just below the root dia of the thread.

 

Bob.

Edited November 28, 2016 by Lebro

[RogerH]

Hi Kevin,

two or more helicoils down a common hole can only be in alignment because the external thread they are fitted into ensures that.

 

As for using one helicoil where two or three would fit.

The nut and stud at the head end are made from material that will easily take the load.

At the other end the material the block is made from is not as strong. A shortened thread section may well pull out and strip the threads.

The stud needs to be anchored down as much as possible.

 

Roger

[Charlie D]

Motorsport Mickey and Lebro,

Thanks for that solution. (Hope I never need to do it though.)
Like most things, it’s obvious --- once someone has explained it to you!

Charlie.

[Geoff C]

I had the same bolt just snap off with nut intact, so got my mates at the local body shop to weld a 4" bolt onto it and with air tool it came undone. We tried to undo it by hand but failed even with an extension on the spanner, then purchased a new stud of right length from Revingtons, sorted within 1/2 hours.

Hope that this helps if you are not sorted.

 

Geoff