ET: Engine (Twin) stripped crankcase threads

Gary Gittleson

Well Known and Active Forum 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

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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

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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.

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Robert Watson

Well Known and Active Forum 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 Forum 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 Forum 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

VOC Hon. Overseas Representative
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.
 
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