ET: Engine (Twin) stripped crankcase threads

bmetcalf

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When I stripped that spot, I did go ahead and split the cases to repair it. Can’t remember if I tried to leave either side’s internals in place to save some work or not.
 

vibrac

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For us non engineering types, why would you use a UNC tap when it should be a BSW tap, are they identical thread forms?
Whit thread has a 55 deg form UNC has a 60 deg at 1/4 dia is same and tpi is the same after 70 years and with the original taps not I suppose brand new a blind man would be pleased to see the difference in soft alloy. To say nothing of which taps or dies were used on the stud -perhaps a replacement
 

Peter Holmes

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A little off topic, but interesting (to me anyway) I have noticed that some threads used on plastic receptacles, toothpaste tubes etc. have a flat faced thread at 90 degrees for the working part of the thread with a conventional thread on the rear section, so I suppose you could describe the thread as 55/90 degree thread, I looked at it and thought it must be a far stronger thread and less likely to strip if over tightened, has this thread form been adopted in engineering yet?
 

Sakura

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A little off topic, but interesting (to me anyway) I have noticed that some threads used on plastic receptacles, toothpaste tubes etc. have a flat faced thread at 90 degrees for the working part of the thread with a conventional thread on the rear section, so I suppose you could describe the thread as 55/90 degree thread, I looked at it and thought it must be a far stronger thread and less likely to strip if over tightened, has this thread form been adopted in engineering yet?
There's nothing new where threads are concerned. There are square threads, both sides vertical, often used on feed screws for machines.
There are variants with both sloping sides called acme and others with one side square and the other at about 45 degrees used on vices with a quick release, which only need pressure on the closing side, which sounds like the thread you are seeing, the list goes on......
 
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Gary Gittleson

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Whew! More replies than I had expected. Many of these ideas had already run through my spinning head. Knowing the depth I could go to is very helpful. Without having it apart, I had no way of assessing the details of the situation.

I think I'll try a bit of loctite as per Mr. Cyborg, tightening very gingerly once dry and leaving it for the next tear down, assuming my own 78 year old body lasts that long.

It doesn't seem to be leaking by the way. Thanks to all. Great forum.

Cheers,
Gary
 

Cyborg

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I'm sure you know this, but I'll stick it on here anyway.

Just a matter of choosing the right version of Loctite. I don't think you want anything nuclear that will make a lot of heat necessary to get it out when and if you do split the cases. I think its just a matter of getting something in there to consolidate whats left of the threads.. as in just making sure its anchored. I suppose aerosol brake cleaner would do, I just use acetone in case whatever I'm cleaning subsequently get exposed to high heat and argon. I'm not one to be a worry wart about chemicals, but no brake cleaner for me. I have learned to check the Loctite web page, because they do come out with new and improved from time to time.
Although not for this application the newer 648 is a good example. Works better for interference fit when sleeving etc.

I think most of the failures using this stuff stem from using whatever is laying around and lack of cleanliness. I'm surprised how well the low strength stuff works when the threads are squeaky clean. Beats trying to stake those little nuts on the ET68 valve lifter gubbins.

 

kettlrj

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'There is nothing new in threads' the man said. I am a fan of a thread form called 'Spiralock'. It is only applied to the internal thread and can be applied to any related male thread. It is all to do with the resultant force that is applied by the male thread to the female thread, which in the case of a 55deg. Whit form will be at an angle of 27.5 deg. to the thread axis. This force loads up the first thread by up to 70% of the clamp load with the subsequent threads taking less and less load until by about the forth or fifth threads no load is taken. So what happens in practice is that the first threads fails and passes the load on to the second thread, by which time the clamp force has been lost or at least reduced. Any vibration or loading at the joint face will then create even more load on the second thread and that will soon fail, and so it goes on until all the threads are stripped. The Spiralock thread has a double flank which catches the tip of the male thread and causes a resultant force at 60 deg. to the thread axis. This creates a far better distribution of forces from thread to thread and in doing so creates a self locking thread in the process.
Take a look at
https://www.stanleyengineeredfastening.com/brands/optia/spiralock

Regards Richard.
 

vibrac

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Spiralock
Not a busting selection in fractional 20TPI is there (or 26) it would be nice to hold that clutch and similar nuts on.
Mind you some threads have to be seen to be believed 23/32" CEI on a speedo drive body! where did that come from what was wrong with 3/4"?
 
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