Cam Cap Stud Problems

[Greg Foster]

I was adjusting the valves on the TC. Having to remove the cam caps to allow for shim changes, the #4 intake cap stud on the outside of the engine unscrewed from the head. I put it aside and adjusted the valve clearance. I then removed the retaining nut and cleaned the threads prior to re-installation. Threads cleaned up fine on the nut side of the stud. The bottom threads were another story as i couldn't get a die to start on the threads. Just figured I didn't have the correct size (5/16 x18tpi) . Proceeding to reinstall the stud, it screwed in just fine and I had treated it with some locktite blue thread locker. When it finally bottomed out, the stud was not completely seated so I pressed it to turn a bit further, which it did and THEN PROCEEDED TO TURN THE HELI-COIL SOMEONE IN THE PAST HAD INSTALLED!! Unbeknownst to me it was there. From the attached foto, you can see there is a tweaked thread on the bottom stud, the one I removed from the head. I had an extra stud to look at for comparison. Question 1; How does the stud bottom out...or does it with the reduced shank turned on the base? Question 2; How closes is the water jacket or oil gallery to the bottom of this threaded aperture? What proceedure should I use to clean out and repair the stripped threads....of the heli-coil? I understand why the nut was bottomed out on the upper threads as the stud wasn't in properly in the first place.....

[Greg Foster]

https://www.threadkits.com/perma-kee/in ... ka5-16-18/
This is one option which looks promising in regards to the now stripped out enlarged threads....

or
https://www.threadkits.com/perma-coil/i ... /1208-105/

Question is...how long do the threads have to be to correctly hold?
Greg

[rgh0]

The studs are supposed to screw in until the extended nose bottoms out and locks the stud in place, you should not need to use Loctite on the stud. Having stripped out the Helicoil you need to find an insert with a larger O/D thread that fits the size of the hole you now have.

The outside stud holes are well away from the water jackets which are lower down but not that far from the oil drain passages around the valves and cams but a small break through into this area is not a problem. You need around 5 threads into the Aluminium at least. You also need to ensure the new insert is centred on the hollow dowel and you don't disturb this hollow dowel diameter location so the stud and cam cap still fits.

I would also get a new stud as the bottom of the current studs thread looks significantly damaged.

cheers
Rohan

[Greg Foster]

Rohan,
Thanks for your concise input. I will cautiously measure all components in consideration
1 - Hollow dowel boring diameter
2 - Depth needed for Hollow dowel and the depth at which the insert should be installed for dowel clearance
3 - Remaining stripped out hole diameter and comparison to the dowel os diam.
4 - Depth needed for the cap stud to bottom correctly
There is a good article I found defining differences in repair threads
https://dsportmag.com/the-tech/three-wa ... d-threads/
Also I found a website which has oversized inserts if needed...
https://www.mscdirect.com/product/details/86765252
Not just a simple hole to repair but what is ...on a lotus.
Makes me think that the overly large clearance on intake valve #4 could be related to the poor repair of this stud.....
Greg

[Hawksfield]

Greg

You may find that the previous fitting of the heli-coil the depth of the thread has been increased when tapping out for the heli-coil diameter thread
This will have bored out the hollow for the stud therefore allowing the stud to run deeper.

Hope its not the case but fear it maybe

Good Luck

Greg
Another thought is that the previous work was a snapped stud removal by drilling out, hope not

[2cams70]

Loctite threadlocker in combination with a Helicoil is a no no. The Loctite will lock the threads and can cause the insert to rotate out of the hole.

I faced the same problem with the cylinder head tensioner pivot bolt where I had to repair threads that had been previously Helicoiled. You need to carefully measure the enlarged hole diameter to determine the minimum OD of insert that will work. I settled on using "Lock N Stitch" Inserts in my case. They worked perfectly. I had to have inserts custom made with internal BSP threads. Your case should be an off the shelf insert however because 5/16 threads are much more common. I posted about it here:

viewtopic.php?f=39&t=40164&start=15

Be very careful about keeping things aligned when machining. You may be better off removing the head and taking it to a machinist. I bought the tooling and provided it to my machinist to do the job in my particular case. All depends how confident / how much of a gambler you are!!

http://www.locknstitch.com/index.html

[gjz30075]

I'm NOT a machinist, but just thinking about this: In the worse case, can he (or have it done) fill the hole
with an aluminum piece, welded in, and bore and thread the new hole?

[2cams70]

The other problem that will need to be considered in this case is the locating dowel for the cam cap. Boring the stud hole to accept an oversize insert will likely destroy the location for the dowel. Not sure how you would overcome this. Definitely appears to be a head off job to do properly however.

[2cams70]

Note I'd recommend getting a hardness test done on the head too. The thread should not have re-stripped with a Helicoil in it unless grossly over torqued. It may be worth considering a replacement head if this one has gone soft.

[Craven]

It?s quite possible to have a purpose made stud to utilise the thread made for the insert, very low torque requirement 12lbs ? does not need an over engineered repair but accuracy is important.

[Greg Foster]

You all have given me food for thought... this engine was rebuilt a number of years ago and the head was tested for hardness. Everything showed up within reasonable range. It sat mothballed and maintained in storage. So what I think happened is when the stud was installed, or reinstalled after the helicoil was done, the "mechanic" if so desired to be called, didn't clean out the bore and the stud bottomed on debris and didn't allow it to seat all the way. When the engine work was done, my guess is a young inexperienced person in the shop was given the job of adjusting the valves (that intake valve showed a clearance of .016" which is why I had removed the cap to make adjustments.) The stud may have backed out , as it did with me, nut jammed on the shank. Could be the shop foreman said put it back together and send it out the door. You never know.
Of course, when it came out with me, I blew out the threads with some cleaner (debris was still packed in the bottom), cleaned up the threads on the stud where I removed the retaining nut and put it back in. I noticed the stud was sitting higher than the others and thought to add another twist and that is when the whole thing popped loose.
It will take me a few days to research the situation and formulate a plan. I will keep you posted.
Greg

[rgh0]

Dimensioning, tolerancing, machining and then assembling a twink head these days is a real challenge given the variation you can face after 50 years of mistreatment combined with all the variability of standard parts which are not really standard or all the variations possible for custom performance builds.

Commercial builders cannot afford to quote the real cost to do it right as they would get no work. So they cut the quotes and hope it works out and if not they hope they can get it out the door at not to much of a loss by cutting corners.

I have spent a lot of time to work out how to measure what I have in terms of a cylinder head and how to specify to a machinist what I want done in both dimensions and tolerances and then I have worked out how to check its all been done right and then assemble it right. It all takes time and if i was doing it for a living I would make no money compared to the hours spent

All this does not help the average person wanting their head rebuilt right but maybe I can help with the questions they need to ask

cheers
Rohan

[Greg Foster]

Rohan,
Long distance advice may be heard clearly. The caveat will always be whether it is heeded. You have a vast knowledge of things Twin Cam, which was acquired through a few bruises and bashes, aha moments and listening to others to glean their experience. I also have had the 2x4 hit me upside the head....hopefully learning something.
Never will I nor most listers have gained anything near your experience. I find this web forum to be quite frank and knowledegable as compared to may others where the BS piles deep. Precise machining, setup problem solving and honesty by those who do the work is a shrinking art. I certainly respect those who are able to resolve issues with well thought out protocol and follow through. Not trying to swell anyone's head but thanks to the insights of those who have helped or are willing to assist.
To further the stud discussion, I sent McMaster Carr an inquiry into the possibility of using a Key Locking insert. https://www.mcmaster.com/thread-repair-inserts
It seems that a maximum hole diameter of .397" would fit the bill but recessing the insert and driving the pins may be problematic. I really don't want to remove the head but in the long term may be better off to initiate a proper repair. It doesn't seem the stud is a high stressed item.
If I had it to do again I would have just added another washer or installed a thicker washer under the retainer nut... you never know, failure may have still occurred.
Greg

[mbell]

For the positioning piece could you have a small disk machined up, deepen the current hole and insert it under the thread insert?

Would guess light inference would be enough to hold it in place or maybe some epoxy just to be sure.

It would need some very careful machining and may just be enough to fit a thread insert in the right spot and not bother.

[Mr.Gale]

A couple of years ago I had a similar problem when I tightened the nut on one of the front studs it pulled out. I told a Lotus enthusiast (Keath Frank) and he made up this jig to drill out the old threads and tap for a Hielicoil. I'm not sure this would fix your problem but if you are in the US I will loan it to you if you pay the postage both ways.
The aluminum block is attached with one stud than the small hole brass gizmo is inserted as a guide for the drill. After it is drilled out the small hole gizmo is removed and replaced with the large hole gizmo which is a guide for the tap.
Mr.Gale