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TR2/3/3A Stripped threads - TR3A block RHS rear ead stud

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Don Elliott

Don Elliott

Obi Wan
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A good friend here in Montreal is re-building the engine for his TR3A and found that the threads in the top of the block where the rear RHS head stud is supposed to secure the head to a torque of 100 ft lbs has been stripped of its threads.

He would like to know what is the best option to solving this problem. He is concerned that a heli-coil may not take the torque. Also a larger stepped stud, specially manufactured means a larger threaded hole in the block (like the heli-coil as well) and he is concerned that he may hit a water passage with both of these options.

Any proven suggestions ?
 
While I have not had to repair that location, IMO a Helicoil is the best answer to both issues. It will be smaller than the next larger standard thread; and properly installed, actually stronger than the original threads. The insert has a higher tensile strength than the original cast iron, and spreads the load better. Should be plenty of metal left to take the threads.
 
I think the helicoil like randall suggested would be a good solution.... they have been used quite a lot on early porsche motors with the soft magnesium case that are quite common for them to pull head studs when over heated.

I never installed one though, thank god.
 
Having used both the helicoil and just recently the keensert, I must say (personal oppinion) I like the keensert better.
Why? Because of the two cam locks that hold the threaded insert in place, and the fact that it is a steel insert.
I'm sure the helicoil would be as good, just another option for you.
 
We are often asked about this at work for the products we make, some of which receive HeliCoils.

The HeliCoiled thread is actually stronger than the original tapped hole. This results from the load being spread out over more contact area and the fact that the insert is made of a (somewhat) hardened stainless steel.

The Keensert is also a good choice. We used them frequently in a firm that I worked at several years ago. Like the HeliCoil, it provides more contact area to spread the load out. It also has the advantage that the keys prevent the insert from backing out. This wouldn't apply to a head stud, but the advantage of the keys on the insert is that you can use Loctite if you want to. Using Loctite on a bolt in a HeliCoil can result in the unwanted surprise of an insert backing out while attached to the bolt/stud.

In this application I'd make the decision based on the size of each insert and which will have the least impact on the surrounding areas. (i.e. Will the insert break into water jackets or oil galleys and will the gasket surface be adversely effected by the diameter of the insert?) If you've got the room for an insert, either type will provide the strength you need. Select an insert that will support as much of the stud's thread length as possible to get the most benefit.
 
dklawson said:
Using Loctite on a bolt in a HeliCoil can result in the unwanted surprise of an insert backing out while attached to the bolt/stud.
Click to expand...
Personally, I put HeliCoils in with a dab of high strength Loctite, to avoid that problem. Since I started doing that, not one has ever come out with the bolt/stud/spark plug.
 
I'll keep that in mind for the future Randall. The only high-strength I have at home is 680 (green) but I would think 271 (red) would be a better choice for this. What are you using?
 
At my job we are using the keenserts more than the helicoils. I setup wireform machines and the bolt holes in the machines see a lot of abuse. The helicoils didn't hold up to the abuse as well as the keenserts. Granted your application won't see anything like the tonnage and use my machines see.
 
Randall - When you put in the dab, do you add it after the helicoil in in place ? Where do you add the dab ? If it's on the backside of the helicoil threads to secure it against the threads in the cast iron block, do to allow the Loctite to cure and then run a die into the threads to clean up the excess on that side to prevent the stud (or sparkplug) from becoming permanently "glued" into the threads of the helicoil ?
 
Had to go look it up; the Loctite I've been using is only medium-strength compound 609. Had to buy a big bottle of it years ago, still trying to use it up
grin.gif


I clean the new threads with solvent after tapping (especially since I always use tapping oil) as well as wiping the insert just in case it's oily. Make sure they are dry (compressed air if available, time otherwise). Then put a few drops of Loctite on the insert before turning it in and breaking the tang. I let it sit overnight, then use more solvent to flush away any un-cured Loctite.

Generally, I don't use Loctite on the new male fastener, I'm more likely to put anti-seize on it (especially for a spark plug). But it depends on circumstances, I have used lower strength Loctite with good results.

I would agree with the Keenserts for something that will be reassembled thousands of times. But every time I put a TR motor together, I hope it will be the last time.

FWIW, I just added a Helicoil to the upper starter bolt hole in my gearbox. The previous engine had one in the block, but I decided to put it in the gearbox this time. Makes changing the starter by yourself a whole lot easier. Of course, now that I've converted to the gear-drive starter I'll probably never have to R&R it again :laugh:
 
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