leaking base gasket ('75 Commando) center stud issue

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Sep 26, 2007
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Looking for a suggestion here. (Bike is a '75 Commando)

I was tracking down the source of oil seepage from the front of my clyinder block, at the gasket - and found the problem.

But now i have to figure out how to solve this particular issue.

After I lifted off my cylinders, peeled (carefully) away the old gasket, I looked at the base of the cylinders themselves - all OK.

But when i looked at the mating surface of the cases, I realized the port (hole?) for the center stud was kind of puckered up on the surface of the case. The stud was not 100% tight either. It looks like the stud may have been overtightened (prev owner, not me, I swear!), causing the stud to pull up the surface of the top thread of the aluminum case. (That's my theory). End result is that their is a protrusion upward from the surface, in a small ring around the stud port. It is barely visible, but is easy to feel with your finger. I'm sure it's enough to cause the leaking.

But the question is - how do i resolve this? I don't want to pull the engine out of the frame and split the cases, so that rules out planing the surface at a machine shop.

Is it possible that I could reverse the process? If I seat the stud firmly into its port, then put the cylinders back on (with proper gasket, etc.) and torque things down, is this likely to press the protrusion/puckering back where it came from? Or am i just dreaming?

Is it an option to flatten it down by placing a flat metal plate on the protrusion and tapping it with a hammer? Or is that just plain dumb?

Appreciate the insights from anyone who has ever had to deal with this - or has even a good theoretical fix.


Keith [/b]
The puckering you describe is quite common and is a major cause of leaks. I habitually lightly coutersink every threaded stud hole, alloy or iron, to prevent/remove this problem. You'll find most modern engines are countersunk from new.
Having said that look carefully at the thread to be sure it isn't about to fail.
If you do repair the thread countersink before you start. A bit of masking tape and grease with stop the swarf getting into your cases, any that does can be washed out with petrol through the sump drain.

countersink a threaded stud hole - how?

cash - thanks for the suggestion.

The masking tape and grease I follow. And "swarf" probably refers to nasty little bits of metal.

But just what do you mean by "countersink" the threaded stud hole? With what? How?

Don't mean to sound like an idiot, but i have no idea of the technique you propose. (though i definitely know what countersinking means when it refers to screw holes in wood, for example, and how to accomplish this)

Maybe the same technique - which in my case would involve using a large drill bit to remove the offending puckering, and actually taking off additional metal, just a bit wider than the diameter of the stud itself, to catch the "puckered" metal.

That it? sounds messy and risky. But if that's what it takes, that's what I'll have to do. I didn't check the threads yet.
Re: countersink a threaded stud hole - how?

pkeithkelly said:
The masking tape and grease I follow. And "swarf" probably refers to nasty little bits of metal.

Yes, swarf is the tech. term for the metal cuttings from drilling or machining metal.

pkeithkelly said:
But just what do you mean by "countersink" the threaded stud hole? With what? How?

Same technique as you would use for wood, but generally you would need to use a countersink bit for metal: http://www.thesitebox.com/Category/4159 ... k-bit.aspx

Or you could use a normal drill bit?
Just take it easy, and don't remove any more metal than you need to.
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