Lubing and torqueing studs

[Ryland3210]

My preferred pattern would be:

 

8 6 2 4 10

 

9 3 1 5 7

 

Hair splitting?

 

Yes, I considered that too, so I checked my Ford manual before sending the pattern, to see which it specified.

 

I do think my sequence has a slight advantage in that it avoids the twisting of the head from your 3 to 4. One could argue that the Ford pattern causes twisting when moving diagonally as well, but in that case, it first occurs after more screws are tighten, therefore has less of an effect. It probably is hair splitting, so long as the stages are observed. I believe the four stage process could be used with either pattern. If only two stages were used, I'd worry a lot about the pattern, and not getting a proper seal. At least three stages ought to be used, and the more the better.

[Guest Mattress]

– How much danger is there of stripping the threads in an aluminium engine block and when, roughly, is the danger point approached? Presumably oil in the threads makes that danger more likely.

 

I'm not sure how the Italians design engine components, but we work with a 2 to 2.5 times thread engagement as a rule of thumb for aluminum castings. So an M8 bolt should have an engagement length of 16 to 20mm for standard torque to ensure that the fastener fails before the threads strip.

 

We actually test the fasteners to make sure they fail from over-torquing. thread stripping can be progressive and may go undetected resulting in a sudden failure.

 

So hopefully you would bust a bolt on a guzzi before the threads strip, but I wouldn't advocate checking it out on a whim.

[belfastguzzi]

I'm not sure how the Italians design engine components, but we work with a 2 to 2.5 times thread engagement as a rule of thumb for aluminum castings. So an M8 bolt should have an engagement length of 16 to 20mm for standard torque to ensure that the fastener fails before the threads strip.

 

We actually test the fasteners to make sure they fail from over-torquing. thread stripping can be progressive and may go undetected resulting in a sudden failure.

 

So hopefully you would bust a bolt on a guzzi before the threads strip, but I wouldn't advocate checking it out on a whim.

Very interesting, thanks. I'm sure we've all seen loads of stripped threads in aluminium pieces, but probably due to cross-threading or else in items with very little thread depth. My original query was about a car engine block (French), so from what you say, selection of the bolt length there would be determined, in part, according to when thread damage could happen.

I.e. if an engine bolt is fully screwed in, and then tightened, something else would give, before the threads would start to strip?

[Guest Mattress]

Very interesting, thanks. I'm sure we've all seen loads of stripped threads in aluminium pieces, but probably due to cross-threading or else in items with very little thread depth. My original query was about a car engine block (French), so from what you say, selection of the bolt length there would be determined, in part, according to when thread damage could happen.

I.e. if an engine bolt is fully screwed in, and then tightened, something else would give, before the threads would start to strip?

 

I should have noted that this rule of thumb we use is for a steel bolt. I guess some motorcycles use nylon or aluminum bolts sometimes. This rule has stood by us well, but it assumes a decent quality aluminum casting. If there is a lot of porosity in the thread area (not uncommon) the threads might still strip.

 

But to your question, yes we design the joint so that once the screw is fully screwed in and then tightened, the bolt head will snap off before the threads begin to strip.

 

In a bolted joint the bottom 2-3 threads of a coarse thread bolt carry the majority of the load (like >80%). So you could imagine a progressive shearing of threads from the bottom towards the joint as the load shears the female thread, transferring the load to the next higher threads, etc etc. Everything might look okay until a load is applied to the joint in operation and then you could have a sudden failure.

 

If you snap the head off you will notice it right away and you should be able to extract the bolt and replace.

[Ryland3210]

Very interesting, thanks. I'm sure we've all seen loads of stripped threads in aluminium pieces, but probably due to cross-threading or else in items with very little thread depth. My original query was about a car engine block (French), so from what you say, selection of the bolt length there would be determined, in part, according to when thread damage could happen.

I.e. if an engine bolt is fully screwed in, and then tightened, something else would give, before the threads would start to strip?

 

Our practice on industrial machinery where steel screws are used in tool steel blocks, is to have at least one diameter of thread depth. At that point, the shear stress is low enough such that the screw will fail in tension before the threads strip.

 

I take exception to only one part of Mattress's commentary. I believe the maximum shear stress occurs nearest the surface of the block, rather than at the extreme end of the bolt. I agree with the domino effect failure mode, but believe it starts from the surface, not the bottom. The result is the same either way in a catastrophic failure.

 

Other evidence of the threads nearest the surface yielding first is the common practice of countersinking to avoid the deformation of those threads above the surface. We do not use gaskets in our applications, only O-Rings. Our goal is to achieve metal to metal contact between the mating surfaces, so to prevent any possibilty of thread yielding causing an interference, we counter bore about one or more thread pitches. That assumes one still has enough thread depth left in the hole. The counterbore also serves as a convenient pilot for the screw.

 

B&T, a machine builder of die casting machines in years past, had a patented tie bar nut design that compensated for the stress concentration on the threads nearest the tension end of the tie bar. These highly stressed, cyclicly loaded tie bars ranged in size from 2.5 inches to over 13 inches in diameter. The threads on the nuts were tapered to distribute the load over several threads. They were able to guarantee their machine's tie bars would never break as a result, a significant marketing advantage.

 

It is easy to cross thread aluminum female threads with a steel screw that has a buggered up end. It's good practice to inspect the screw, and file or grind the first thread if dented or otherwise defective. My practice is to always start the screw/nut by hand first.

[Guest Mattress]

snip

I take exception to only one part of Mattress's commentary. I believe the maximum shear stress occurs nearest the surface of the block, rather than at the extreme end of the bolt. I agree with the domino effect failure mode, but believe it starts from the surface, not the bottom. The result is the same either way in a catastrophic failure.

 

 

I might be wrong. I was thinking of a bolt and a smooth through hole with a nut on the other side. You could be right.

 

Anyway I'm glad I spent the money on a torque wrench. I replaced my valve cover and used factory torque which was a lot less than my ham fist approach.