Wombat Head Stud vs Head Nut

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JimKlaas - 06/29/12 at 11:13am

My 1972 Wombat 94 is leaking oil. The oil seems to be coming from the cylinder. This has been a known problem that I have been putting off. A year or two ago I replaced the gaskets but when I tightened the nuts down I noticed they never seemed to tighten. I suspect one or more of them are striped.
Questions:
1. Which strips first the Nut or the Stud?
2. If it is the Nut---I just have to replace them right?
3. If t is the Stud----what is the process?

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Bullfrog - 06/29/12 at 11:23am

There is another possibility to check. On more than one occasion I have bumped into Wombat head nuts which are not tapped deeply enough.
I fought a problem with head gasket leaks which kept returning (um, and an occasional seizure) over a couple of seasons. Other factors kept pointing me in the wrong direction. Things like a hard charge across a field on an 18 degree morning in February resulting in a "squeak" while I still had proper jetting for the previous August and 105 degrees in the carb convinced me that jetting caused the squeak.
BUT, I finally found that one of the head nuts was not tapped deeply enough. That head nut came down and "jammed", then the stud bottomed in the case hole . . . and I was torquing the nut to the stud and the stud to the case -- all BEFORE the nut came down on the head. Three nuts would seal things up to pass a pressure test . . . for a while . . . then the problem would return.
When I finally found the problem, cleaning/deepening the threads in the offending nut with an 8mm tap fixed the problem properly.
Ed

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dirty_rat - 06/30/12 at 11:08am

If one of the nuts/studs is truly stripped, it is probably the stud into the much softer aluminum case. As Ed suggested, I would check all the other possibilities first, to make sure it isn't an easier fix. If it is the case that is stripped out, you can always repair with a heli-coil.

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JimKlaas - 06/30/12 at 8:31pm

ok...Ed gets an extra Scooby snack today....there was one of the nuts that did not appear to be threaded deep enough... Great Job Ed!!! BUT...you aren't done yet...for extra credit answer the following two questions...
1. Are the studs standard 8mm 1.25 pitch? (both ends or ???)
2. When I tighten the studs into the case should I add a dab of locktite? If so what flavor?
That is all I can think of for now but I am sure I will have more question later. The piston and the dome where the spark plug sticks into the cylinder are currently soaking in Sea Foam.

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Bullfrog - 07/01/12 at 1:16am

Yes. 8mm X 1.25 thread pitch. This will be somewhat like, "You asked for the time and I described how to build a watch . . ." but . . .
For metric threads it is pretty easy to measure the distance between the crest of the threads which are 10 threads apart (even with a ruler). Divide that measurement for 10 threads by 10 and you've got the thread pitch (distance from one thread peak to the next). OR just take the stud with you to the Ace Hardware store and check to see what nut threads on.
If you break out a magnifier glass and examine the threads at each end of the stud, you'll see that the top end has threads which have pretty sharp peaks. The bottom end will have flat top peaks (because the threads haven't been cut to complete depth) -- so the base of each thread is wider than normal and fits tighter to the threads in the cases. It seems like the threads are cut for a shorter length on the bottom end too (that is from memory -- which is shaky). I definitely do use std flavor (blue) Loctite on the base end of the studs.
Ed

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JimKlaas - 07/01/12 at 1:08pm

Got it Ed....the threads at the base of the stud just seemed ever so slightly tighter than the top threads and I wondered if this was some sort of jamming thread by design. Blue Loctite...got it...thanks.

Jim.....now I have to find where the torque values are for a Wombat 94 early frame are? and more importantly keep in mind in/lbs vs ft/lbs.... man I hate when I make THAT mistake....

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Bullfrog - 07/02/12 at 1:11am

The "official" factory head nut torque specification for the 125 cc engines is 170 in-lbs. However, over the years Harry Taylor tested higher torques and he stopped using the factory spec years ago. I've forgotten exactly what spec HT recommends for all-out racers . . . but it is higher than the 250 in-lbs which is the max. limit on my torque wrench . . . so I've been tightening to 250 in-lbs.
Ed
PS: That would be INCH-pounds.

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JimKlaas - 07/02/12 at 11:01am

INCH pounds is KEY!!!
I am thinking a nice round number like 200 inch pounds.
Jim

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MTRob - 07/02/12 at 12:21pm

My torque wrench shows in ft Lbs, whats the different in ft/lb to inch/lbs. I have to torque my heads to 105 inch pounds whats that in insh/lbs?
Rob

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JImKlaas - 07/02/12 at 1:33pm

105 inch pounds = 8.75 foot pounds....12 TIMES the force...this is something you do once and never again...remember...drink the beer AFTER you torque your bolts and NOT before...this is also applies when truing or building wheels....
Jim

http://www.convertunits.com/from/1+foot ... unds+force

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Bullfrog - 07/02/12 at 2:51pm

Uh-oh - RED FLAG (semantics alert!) NO, not 12 times the force. OK, sort of, but . . . let's fill in the holes in the discussion.
105 in-lbs of torque means that you would apply 105 pounds of force at the end of a wrench which is 1 INCH long (a pretty short and impractical wrench). If you use a wrench which is, let's say, 12 inches long . . . you'll have 12 times the leverage, so you reduce the force applied at the end of that long wrench by a factor of 12. The result is the same TORQUE applied to the nut -- which is the important factor in this discussion.
Some torque wrenches indicate in-lbs, others indicate ft-lbs.
Divide an in-lb specification by 12 to calculate ft-lbs. Conversely, multiply a ft-lb specification by 12 to calculate in-lbs.
Ed

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MTRob - 7/02/12 at 3:07pm

Either way 105 in/lbs comes out to be 8.75 ft/lbs Right?
Don't want to get this wrong.
Rob

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Bullfrog - 07/03/12 at 12:46am

Rob:
See previous post.
Inch-pound specification is 105 in-lb.
Divide by 12.
. . . and you have 8.75 ft-lbs
Ed

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