Can torque be confirmed?... will threads survive 3 torques?

@Mendel Leisk has the test you can do for the headgasket bolts...

 

I think Mendel has the way to check the head bolts for reusability, when you have them out and you can measure them (it's not secret knowledge, it's in the repair manual).

I don't know that anybody has a good way to check the condition and torque of the installed bolts without disturbing them.

 

Torque-to-yield bolts ensure consistent clamping forces despite variations in thread condition/lubrication. After having followed the 3-step torque-angle installation procedure, if you happen to measure the torque required to remove each of the bolts, you will likely notice a significant difference in loosening torque between the bolts, just because the initial condition of each of the holes and threads was dissimilar. The clamping forces would be very close despite the differing final torques.

Therefore, loosening torque cannot be used to check whether the bolt has been correctly installed. If the bolt length excluding the head is shorter than 149.2 mm, and the diameter across the threads is greater than 9.4 mm throughout, you can reuse the bolt. At least that's what Toyota says.

 

That would be dangerous, because you would likely be taking some of those bolts to their ultimate tensile stress. Without the stress-strain curve for the specific material, it would be hard to say and therefore not advisable.

The good news is that the threads in the block are hardy, so you don't have to worry about those. If you reuse the old bolts make sure they're within spec. Use new ones if in doubt. And remember to use a paint marker to put a dot on the outer edge of each bolt at a consistent position once you've got it torqued down to 500 kgf·cm, so you will subsequently know for sure whether/by how much you have rotated it after the initial torque.

 

There need be no concern about the threads on the engine block... It's solid... Your concern about block threads being week is ridiculous. You could torque threads on an engine block ten times and it'd make no difference. It's a flipping engine block! The bolts on the other hand can get damaged from being torqued or over-torqued too many times.

 

I've had contrary experiences with tapped threads in aluminum components (which are what these engine blocks are, except for the thin sleeves of the cylinders).

I don't know what specific alloy the block is, or whether they made the threads by cutting or rolling (it seems to make a difference), but I don't think I would dismiss the OP's concern out of hand.

This seems to be the kind of thing @Georgina Rudkus knows about.

 

No matter how precise each hole is tapped and how accurate the tap is made, each hole and the head bolt will have minor imperfections, and will have different frictional resistance as the bolt is tightened into the hole. The varying frictional resistance is why almost all torque wrenches have a pus or minus three degrees with a total range of six degrees of inaccuracy.

In consideration of the six degrees range of inaccuracy, the industry went to angular torque which results in less inaccuracy in tightening fasteners. Imagine how much accurate how a micrometer is because applying angular torque uses the same principle. Inaccuracy only results in the sum of the initial low torque value applied to the bolt and the tolerance of the thread surfaces needed to allow the bolt threads to move into the threads of the hole.

A company called Jawco makes thread chasing and rethreading taps and dies. These are not ordinary taps and dies that cut threads. They reform threads in tapped holes that may be deformed from previous tightening of head bolts. When a bolt is tensioned, the peak of the thread bends the triangular section of the metal toward the top of the engine block. The thread chasing tap gently bends the metal back into its original position. Therefore, I would rechase each hole and clean it out before reassembly.

Used bolts when tensioned will have minor deformation of the threads resulting from tensioned. If reused, they can be reformed with thread chasing dies. New bolts; not so much, but it doesn't hurt, since the threads might have minor handling nicks from moving through the assembly line.

 

How about the effect of this repeated flexing on the threads in the hole, given that the block is an aluminum alloy?

My understanding has long been that steel and aluminum differ in an important way, that you can quantify a stress below which steel will endure essentially infinite cycles, while for aluminum there just is no such point.

I've had experiences reusing tapped aluminum castings where the bolt simply brought all the old threads right up out of the hole before reaching the original specified torque. Timeserts or Helicoils aren't very hard to install, but nobody really wants to have to.

 

When removed from the tapped hole, each bolt becomes a unique matching pair, If one was to reuse the bolt without any preparation, one should return the particular bolt to the particular hole it was removed from. I do this even with non critical assemblies mounted with steel bolts to aluminum threads to preserve the fragile aluminum threads in the hole.

There is an advantage to reusing the original bolts, if done properly.

Putting new bolts into a threaded hole means that the bolts move the aluminum threads into their original tapped position, and when tightened the new bolt will bend them into a new opposite position like bending a paper clip back and forth. This is where returning the same bolt to the same hole will preserve the "set" of the threads from the first assembly.

Therefore, new bolts should be used only is the originals are "overstretched."

 

Great info... I've always thought of re-forming threads as not much different than "cleaning up the threads" with the same size thread in a tap and die kit. No doubt that kind of thing with cheap metal is definitely different than something designed to work on threads in an engine block.

 

I have a Craftsman thread restorer kit that my dad gave me as a Christmas present nearly 20 years ago. It still works great. They are still made in USA, and are now sold on eBay with the same quality as the originals. Just search eBay for

Jawco Metric NF-SAE NC-USS Rethreading Set Thread Restore File Tap Dies 42pc USA

 

duplicate

 

Yep, I had someone with a leaky oil pan bolt right after I told everyone on Priuschat that in all my years I never once had a problem with a leaky oil pan bolt. My plan was to replace the whole oil pan because an oil pan cost like $27 for a gen2 engine and the replacement plug leaked just as much.

But then once I was under there the gasket holding the oil pan in place wouldn't budge and I began to imagine what kind of under car gymnastics I'd have to do to remove it and make a smooth surface for a new gasket, which I had yet to buy... So instead I got out my cheap Harbor Freight tap and die kit that I've never used and made sure I had oil pouring out and a magnet on it and very gently cleaned up the threads in a way that ensured I did everything possible to ensure the metal shavings wouldn't be left behind. Worked perfect! Have change the oil in that car several times in years since and it's still not leaking. Still got the oil pan on my parts rack.

 

Both the captive "nut" of the oil pan and the drain plug are made of mild steel. When over torqued, the apex of each thread is bent to the point in relation to the thicker root of the 60 degree equilateral triangle so that the hole is enlarged and the bolt is reduced in diameter.

A thread chasing tool bends and reforms the apexes of the threads and moves them back to the original perfect equilateral triangle cross section. Reforming actually work hardens the threads and makes them more resistant to deforming.

Using a new drain plug completes the repair. In this case, I prefer an aluminum drain plug, as the aluminum would be sacrificial rather that the oil pan's threads.

Too many Threadserts are installed or oil pan's are replaced at great cost to owners and customers by uninformed technicians.

 

Great news to hear.

Yeah, that's the specified torque for M12 bolts; 75-80 ft. lbs.