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KZ750H Restoration 22 Mar 2020 11:16 #821407

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OK, then definitely thanks to Kidkawie :) I just found his original post!

Hopefully it will warm up a bit this week so I can put the satin clear coat on them and get everything back together. Hoping that the clear coat will clean up the imperfections on the kill switch, too. If not, there's another one lying around here somewhere :)
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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KZ750H Restoration 23 Mar 2020 11:39 #821517

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So, it was one of those days today...
I got the cams back in - whatever cams they are :) - and then went to swap out the washers under the cylinder head nuts. They were soft stainless washers as I didn't have anything else at the time, I figured I needed to retorque the nuts anyway so I could just replace the washers one at a time and retighten to 40Nm as I go. One of the nuts on the 4 long studs in the middle was a bit harder to get off and I noticed the stud was longer. I figured it had backed out when I tried to get the nut off. There was no way to get a couple of nuts on it and tighten it back down as my 17mm socket doesn't fit in the gap. So... of with his head!
Once I got the head off I couldn't get the stud in any further. It was then I looked at the old photos and saw that it was always a bit further out :pinch: Should have realised there was no way taking that nut off was going to undo the stud with loc-tite on the other end!
Just to be sure I pulled the stud out and then tightened it back up to see how far it went into the threads: about 20 mm, so far enough.
Anyway, long story short, the head is back on, the cams are in and I now have 9 helicoils in the cam caps.

1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)
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KZ750H Restoration 23 Mar 2020 11:53 #821520

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When putting in the M8 cylinder head bolts under the cams, the second one just seemed to keep turning and I got that sick feeling in my stomach. Just as I was about to back off the torque wrench clicked. I figured it was a stretch bolt and it was supposed to feel awful. I went to double check the torque and the bolt wanted to turn again, so I pulled it out and did what I should have done at the start: cleaned all the oil off the threads! After that everything felt normal again.
I figure I probably tightened the bolt to the equivalent of about 38-40 Nm (instead of 30 Nm). I'm wondering if I've put so much stress on the threads, that they might fail under pressure. Does anyone know if these are torque to yield bolts? That would mean that the clamping load is greater than any forces applied during operation and if I can tighten the bolt to the proper torque, then it will always hold.

Sorry about the long posts - there's nobody around to unload all this on :)
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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KZ750H Restoration 23 Mar 2020 15:47 #821530

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Thinking about the state of the engine studs and I'm wondering whether the long one is actually the right height and someone has been cleaning the others out with a tap and gone too deep. Does anyone know what the stud height should be? I can't find it in the manual. Alternatively it would be nice to know how many threads I should see sticking out above the washers at the top (a photo would be fine). The threads on the long one start at about the same height as the washer.

The lack of experience is pretty obvious now :) kicking myself for not checking the height before I took the studs out - I just assumed they'd all be bottomed out.
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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KZ750H Restoration 23 Mar 2020 16:39 #821533

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Your post made me realize I only documented the length of stud, not the installed depth., on my 750 motor. The thread lengths on the APE studs aren't the same length as the stock studs, either. As long as there is enough thread engagement on both ends, this may be picking the fly poop out of the pepper. Having an odd stud sticking out more than others that are the same length is a bit disturbing, though.
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'81 KZ-750 E2
'86 Suzuki Savage 650

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KZ750H Restoration 23 Mar 2020 17:13 #821535

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Yeah, it was really doing my head in until I just got an e-mail from the old man. He suggested that someone may have broken/buggered a stud and just used whatever was lying around from another bike. Then I looked at the photo of the studs after getting them plated

Look at the stud on the far left - I don't think it would have stretched that much :ohmy:
I do wonder if it is supposed to go in the front left hole. If you look at the photo above the front left stud looks shorter than the right hand studs. Almost looks like I could swap the left hand studs around and they'd be the same length... Hard to tell from the photo though, will have to take the nuts off again and check the thread height.

1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)
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Last edit: by calum. Reason: Added some wild speculation

KZ750H Restoration 24 Mar 2020 05:27 #821563

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The head bolts that install down inside the cam chain tunnel area ARE stretch bolts. I bought brand new ones for my 750 and they would not reach the minimum torque as listed in the service manual before serious yielding occurred so I stopped short to avoid breaking them.

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KZ750H Restoration 24 Mar 2020 05:39 #821566

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Thanks Ed. I put new ones in yesterday as well. They hadn't arrived when I put the head on originally so I reused the old ones. When I put the head back on yesterday I though I might as well replace them. Interestingly, the 'neck' on the old bolts isn't as thin as the new ones (the ones you posted a pic of), maybe that's why I had no problems last time.
Anyway, I have decided to get one of the studs from the old engine I have at school and pull off the block and the head again. Then I know it's all fine. Bonus: I can replace the athena gasket :)
Any idea how much of the studs should be sticking out past the washers in the head?
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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KZ750H Restoration 24 Mar 2020 09:37 #821582

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It would be good to know what steel alloy was used to make the studs. If they come from the factory hardened, electroplating like zinc would require a hydrogen embrittlement relief bake, or the alloy will will become very brittle. Anything over 40RC on the Rockwell scale would require a 350 degree bake within 3 hours of plating to remove the hydrogen atoms from the material.
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'86 Suzuki Savage 650

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KZ750H Restoration 24 Mar 2020 10:27 #821588

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Wouldn't that only apply to hot-dip plating? I'm not sure if this is the right term, but it's what google translate spat out at me ;) Normal electroplating won't affect the hardness of the studs.
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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KZ750H Restoration 24 Mar 2020 11:13 #821596

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Any electroplating of zinc on steel alloys above ~ 40C hardness (Rockwell) would subject the metal to hydrogen. The only way to eliminate it is with a post plating bake. The hydrogen won't change the hardness level of the original material, but if the hydrogen bonds with the steel it becomes brittle. I spent many years in the metal fabrication industry and aerospace manufacturing. This was a huge lesson for our company. It only applies if the material you are plating is above that hardness level, though. Whether or not the OEM studs are hardened from the factory may be hard to determine without having one tested. The stock studs may not be hard enough for this to be an issue. I certainly wouldn't try to plate APE racing studs. Those do appear to be hardened.
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'86 Suzuki Savage 650

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KZ750H Restoration 24 Mar 2020 11:41 #821606

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OK, good to know. Thanks.
I'm assuming the engine mounting bolts won't be hard enough to be a problem. I'm assuming they're at most 8.8 but even 10.9 should be under 40, shouldn't it?
1980 KZ750H (kzrider.com/forum/11-projects/612054-kz750h-restoration)
2000 BMW F650GS
2012 Honda Hornet
1993 Mini Cooper :)

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