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Old 07-31-2015, 08:48 AM
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Davezilla Davezilla is offline
Motorized Bicycle Elite Member
Join Date: Mar 2014
Location: San Antonio Texas
Posts: 2,710
Default Re: Cylinder Stud broken

I agree with Tom, in these cases the threads may pull out or strip before a stud stretches or breaks from overtorquing. This is also why I never use a bolt in the aluminum unless I absolutely have to. Studs go in hand tight, then the nut on the other end is where all the torquing friction is, this prevents premature wear or stripping of the holes in the aluminum, but the threads can still pull right out of the aluminum if excessively over torqued.
In places where a bolt has to be in the aluminum it's always best to helicoil or install steel inserts, especially if it's somewhere that a bolt will be installed and removed frequently.

I've also seen Fred's recommendations for torquing the heads at really low values and I know why he recommends such low values. His reasoning is because the gaskets can smash down when torqued too far, not the aluminum head gasket as much as the fiber base gaskets, the aluminum can smash a little too but pretty much a neglidgeable amount... unless you're running a really Really tight squish band, then the extra smashing of the gaskets could cause the piston to hit the head. It would be really rare that this would happen, but if your squish clearance is less than .010" and the paper fiber gasket can smash down by about .005" or more, and the aluminum gasket .0005" that's leaving you only .0045" or less of clearance between the piston and head, now warm up the engine and the rod stretches a little from heat expansion this distance becomes even closer... you pretty much know what's next.

The other reason he recommends the low torque values is because as the torque goes up, the clamping force of the studs goes up and can warp the head enough for the gasket to leak between the studs, but as thick as his heads are this is highly unlikely.

To combat both these issues I use copper base and head gaskets since they won't smash down as much from torquing, the copper is rather soft and will require a retorquing, but then it hardens up from the engine's vibration and won't smash any more but will have a perfect seal by this time. Copper work hardens and you can actually see this in action by bending a piece of copper a few times, you'll feel it bend really easy as it starts off super soft, then you'll notice each subsequent bend you do that it becomes more and more difficult to bend, and if you keep going it'll eventually crack, this won't happen between a head and cylinder, but the vibration of the engine, even a very smooth running engine will cause the copper to match up perfectly to the micro contours of the head and cylinder causing it to need to be re torqued, but then it hardens up to a certain point after it seals. Copper gaskets can be reused numerous times before they wear out too much, but they need to be annealed between each use by heating up to cherry red then quenched quickly. I've tried cooling it both ways, letting it cool on it's own, and dipping in cool water to cool rapidly and both methods leave the copper dead soft after being heated to cherry red.

The reason less torque is needed with the fine threaded studs is also in the clamping force applied as the finer threads will apply more clamping force at a lower torque value, you can also go into lubed vs dry torquing and how much it effects the clamping force and on and on as well.
I understand his reasoning for the low torque values, but on a street engine it's not going to be that much of an issue as on a race engine where tolerances need to be as accurate as possible and smashing a gasket a fraction of a thousandth could effect the engine in a bad way. It also distorts the jug ever so slightly as well but that's also going into a totally different area...
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