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How bad is it to reuse m42 head bolts?

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I’m putting together an m42 and just this morning I put the head on and torqued it down but on the last two bolts right at the end of the final torque they felt like they got easier to turn.. So I thought that They maybe they stripped. It took me until 7:00 this evening to go back and even look at the engine after thinking that the threads might be messed up. So I took out the two bolts that felt like they might’ve stripped and one that didn’t to compare them, and I don’t see anything wrong with them. All three came out feeling the same, and there are no obvious signs of stripped threads anywhere so I’m thinking that everything might be okay. 
 

So not understanding why the bolts felt the way they did, and knowing that you aren’t supposed to reuse those bolts because they stretch, and keeping in mind that they are brand new and were only in for like eight hours, can I reuse those three bolts or should I replace them?

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They are stretch to yield bolts, use once, then throw away      Get some new ones

 

Thanks, Rick 

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So, you say 'torqued down'.  Does that include the two angle increments that you add after the torque?

 

My experience was that, once I added the final angle, they ALL felt 'squidgy', as they were in elastic deformation.

As in, once they start to stretch, that little muscle down deep in your gut that tenses when you feel a bolt go 'soft' should tense.

 

Because they're DESIGNED to 'go soft', but unlike most fasteners, are designed to be ok that way.  It's just... odd.

 

The worst was the VW watercooled 8v Diesel heads- you turn those things a LOT once they go elastic, and it seems

like a recipe for disaster.  But it works...

 

And while, yes, you aren't supposed to reuse stretch bolts, you can- unless ALL the stretch is used up, and then, whoops!

I'd replace the 2- but I'd also check your process.

 

t

it might be a bit lower than your gut, that muscle...

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Alright, cool. Getting a few new ones seemed like the right thing to do but I’m just too cheap to pass up the opportunity to save money. 
 

Yes that feeling of the bolts going soft was right at the end of the final angle torque. I only felt it on the last two bolts and not all of them, don’t know why that is. 
Guess I’ll have to get used to that feeling 

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On 9/9/2020 at 9:15 AM, TobyB said:

So, you say 'torqued down'.  Does that include the two angle increments that you add after the torque?

 

My experience was that, once I added the final angle, they ALL felt 'squidgy', as they were in elastic deformation.

As in, once they start to stretch, that little muscle down deep in your gut that tenses when you feel a bolt go 'soft' should tense.

 

Because they're DESIGNED to 'go soft', but unlike most fasteners, are designed to be ok that way.  It's just... odd.

 

The worst was the VW watercooled 8v Diesel heads- you turn those things a LOT once they go elastic, and it seems

like a recipe for disaster.  But it works...

 

And while, yes, you aren't supposed to reuse stretch bolts, you can- unless ALL the stretch is used up, and then, whoops!

I'd replace the 2- but I'd also check your process.

 

t

it might be a bit lower than your gut, that muscle...

 

Are M42 head bolts likely to have the same sudden failure as the M20 head bolts of the day? Everyone remembers that crap, right?

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I've never heard of them losing their heads like French royals at a revolution....

 

The M42 development came after the M20, so by then, BMW had moved on to other mistakes.

 

Like 'profile gaskets'...

 

t

is building a guillotine for the current crop of 'royals'....

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Don't get me started on "profile gaskets."

 

Sing, oh choirs of cacophony...

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To reiterate: when you feel the head bolts stretch and get soft, that’s fine, right?

Just because it doesn’t feel right in my soul. 

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Nope, it's a really bad feeling- gets you right in the 'DON'T DO THAT' response you've developed over

years and decades of pulling aluminum threads out of castings with steel bolts.

 

Hate 'em.  But I get why they work better...

 

t

 

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Wait, @TobyB it isn’t okay? It kinda sounds like you’re saying it is okay, but the sentence starts with “nope.”
Just trying to make sure I’m not putting an engine together with loose head bolts 

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(edited)

Just wonderin'.... 

 

If you've already 'stretched' the bolts to plastic deformation, (i.e., they're now a tad longer and have internal strain?), to remove them and then go back in and torque a now longer bolt, you're inducing even more stretch than the first pass to reach the same torque... Plus the angle you'll add after that. 

 

Ergo, aren't you playing with fire to reuse them? I guess it all depends on how hot the flame. 

Edited by 2002Scoob

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3 hours ago, 2002Scoob said:

Just wonderin'.... 

 

If you've already 'stretched' the bolts to plastic deformation, (i.e., they're now a tad longer and have internal strain?), to remove them and then go back in and torque a now longer bolt, you're inducing even more stretch than the first pass to reach the same torque... Plus the angle you'll add after that. 

 

Ergo, aren't you playing with fire to reuse them? I guess it all depends on how hot the flame. 

+1, because once the new permanent length happens, the diameter reduces.  On the next torquing the bolt reaches plastic deformation at a lower torque so in the end the joint (the head to block joint) is not as tight with possible blowing the head gasket later on.

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(edited)

Theoretically worse than that...

 

If you've reached plastic deformation, it's going to be more willing to continue to stretch, potentially requiring more turns and making it harder to reach it's final torque because of it.... Potentially stretching the the hardware beyond elastic deformation and closer to failure, as Jimk refers to, reducing it's ability to maintain the clamping force needed to seal. 

 

Theoretically speaking, of course. *I'm no engineer/physicist. But I watch a lotta youtube on such things, haha. 

 

As a semi-paranoid person, I'd be tempted to just buy a few new bolts now, as it's much cheaper than having to replace ALL the bolts in the future if you have HG issues. 

Edited by 2002Scoob

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Yeah I’m passed that question now @2002Scoob I have new bolts. Just double checking that when I feel the new bolts stretch, that feeling is okay and doesn’t mean that they should be replaced. 

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Yup, feeling them 'give' is right.  You should.

 

I mis- noped above. 

 

Sorry.

 

As to the reusability, different bolts do different things.  Some are designed

to give and give and give until they're half an inch longer.  THEN they suddenly

fail.

Others are designed to only give a bit, as Jim describes, and they have a relatively

short 'plastic- elastic' phase, then they suddenly fail.

 

The VW head bolts are in the first category, but I don't know much more than that.

 

t

 

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