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ET: Engine (Twin) stripped crankcase threads


Gary Gittleson

Well Known and Active Website User
VOC Member
Yesterday, I noticed that the lower of the two studs below the center of the V on the drive side was loose. That is, the nut was but finger tight. I tried to tighten it but the stud turned with the nut. So I removed the nut and was able to easily turn the stud with a screwdriver. The stud has a slot for that.

The stud came out of the case very easily. Not good of course. The inner threads were neatly plugged with aluminum. After cleaning them up, I was able to re-install the stud and turn it in a couple of turns beyond its original position. I put the washer and nut back on and was able to tighten it a bit but stopped before more damage was done.

Short of a complete tear down, are there any suggestions for a repair? Or should I just keep riding it as is until another cause for tear down appears?

Gary
 

Gary Gittleson

Well Known and Active Website User
VOC Member
Of course, helicoil. But that's a bit of a challenge without splitting the cases. With the cases assembled, the threads that need repairing are some two inches down the hole.

Gary
 

Cyborg

Well Known and Active Website User
VOC Member
You could probably get an idea of how deep the hole is and roughly how much is left in the way of threads. The threaded section, assuming your cases are the same as mine is definitely longer than the threaded section on that stud, at least the hole is. You could try cleaning all the crap out of the bottom of that hole. Once you are sure everything is out of there (and it can be packed in there so it feels quite solid) ... if you feel lucky you could run a bottoming tap in there and see if you can gain a few more threads. Might be worth a try making up a new stud if things appear “ok”. I would make the threaded portion longer than what the original was and try to make the threads slightly larger in diameter. You can probably only gain another .002” in diameter or so given the size of the stud. After cleaning out the threads and other crap, rinse it out a few times with acetone and let it dry. Apply some appropriate Loctite and let it set before you do up the nut. If you have a decent bore scope (which I obviously do not) you could take a peek. Personally I would want to have a better excuse to tear the engine apart, but if you did take it apart, no doubt you’d end up with a long expensive list of unrelated reasons why it was a good idea.

Ps there are types of Loctite that aren’t bothered as much by oil, but based on my personal experience clean is better... and absolutely spotless is better yet.

Getting a long enough tap will be a challenge, but a TIG can fix that.

61143BD5-1550-4161-89EA-A9A5848A9E8A.jpeg
 

Robert Watson

Well Known and Active Website User
VOC Member
Helicoil is also a problem here for more than the depth of the hole. The drill size for a 5/16 bsf helicoil is 21/64 and the OD of the insert is bigger than that by a bunch so you would have to drill out the good side of the case a way oversize. If it was me I would agree with cyborg and take my best shot with loctite and put off splitting the cases until a real need arises!
 

Bill Thomas

Well Known and Active Website User
VOC Member
Little drop of Locite and leave well alone !, Till the Engine comes apart.
They are not very strong studs, Can't see the harm.
Cheers Bill.
 

Vincent Brake

Well Known and Active Website User
VOC Member
Yust drill it deeper with 6.8 mm

Some one here will measure how deep u can go

Than tap with 5/16 unc.

Only thing you will have to weld or braze solder a 7 mm on the tap.
Be sure to do that hot not over 500 degree (at the thread).

Than tap and longer stud.

Cheers.
 

Marcus Bowden

Well Known and Active Website User
VOC Member
Gary my handsome,
The thread in the c/case is 5/16" Whitworth. UNC will also do it, tapping size drill is 1/4" for BSW. You will be safe drilling another half-inch, then either get a second stage tap (suitable for tapping aluminium) and grind the shank to 1/4" dia then some 5/16" steel rod and drill 1/4". Grind a flat on the tap, hold the steel rod in a vice cut a short length of silver solder and drop it into the rod hole with some easy-flow flux and tap on top use a propane torch to heat on the area at the base of the hole keeping pressure on tap till it sinks into rod and you see silver come up. Squirt kerosene/paraffin into the hole and gently tap the hole further in, make up a longer stud with Whit one end and BSF the other or a length of 5/16" Whitworth stud bare if no threading facilities. Job done, bar the shouting which it essential.
 

peter holmes

Well Known and Active Website User
VOC Member
For us non engineering types, why would you use a UNC tap when it should be a BSW tap, are they identical thread forms?
 

bmetcalf

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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......
 
Last edited:

Gary Gittleson

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
'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

Well Known and Active Website User
VOC Member
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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