I've just snapped 2 bolts, why ?

Hi everyone, I was replacing the cylinder head today on my kz650 as I'd had the problem of the oil leaking from what I thought was just under the front left hand cylinder head nut.
Removed the whole top end, spent years removing the old gaskets, bought some 3 bond for the studs & yesterday put the barrels (jugs) back on.
Today I put the head on & torqued the 12 nuts up in the correct order, to 29ft lbs. Fine & no problems.
Next I did the 2 10mm bolts up with wrench set to 19 ft lbs. Both of the bolts have snapped.
What did I do wrong??
Will it be ok to run with the 12 nuts as I can't handle ripping it down again.

Quite possibly nothing. It depends on the torque wrench you are using and the quality of it's calibration. Also the lower 15% to 20% or so of the torque wrench is likely to be the farthest from specification. So if your wrench goes to 125 lb ft of torque as an example, Anything less than 25 lb ft is dubious. None of this will make you feel any better which is unfortunate.
Two other possibilities: 1) Did you chase the threads before installing the new bolts? Clean threads are necessary to provide proper torque values. With old bolts both the hole and the bolt threads should be chased. 2) Some bolts torque spec is dependent on using anti-seize on the threads. The factory service manual (on anything, not just bikes) should tell you if the thread friction modifier is needed. I'm guessing you used the stock bolts (I did) Someone else may have installed them improperly and stressed them. A very small amount of rust can create the start of a fracture that will cause them to break. Many things come into play. You may never know the answer for certain.

There is potential for oil leaks, I would think head warpage is possible. I've had that soul crushing "I don't have the heart to do this again" feeling many times. I recommend you fix it.

Thanks for the reply,
I've just had my dinner (it's 9pm in good old blighty)
I had totally degreased the head & all the threads were nice & clean , the bolts were still a nice black colour with no signs of damage & screwed in nice & easily. My torque wrench is a draper expert & not a Chinese rubbish type !
They just kept doing up & I was wondering when I was going to hear the click
Ok then, I'll take it apart again & see if I can remove the broken bits. Shouldn't need to remove the barrels. ?
I know I probably shouldn't, but can I use the head gasket again as the engine has not been run?
I love old motorbikes !!!

I definitely would replace those bolts. Otherwise you may end up with even more headaches. However, I would torque them and the other head bolts to the low end of the torque range. Also, keep in mind that the values are for dry, solvent cleaned threads. Ed

Thanks Ed,
I know that tomorrow morning I would wake up paranoid & go & remove the head .b
It's nice to be told to do it though. Cheers.

Edit: Ed posted while I was typing...

Original message...
Pretty sure those bolts are not 10mm. They are 6mm and 20 ft-lbs is too much for them. The manual says 16-20 and you should have stopped at 16 max. That's a good example were I don't like blind adhesion to what the manual says. On my 750 those bolts are 8mm but with a "stretch shank" where the bolt necks down some to facilitate stretch. Anyway, the manual says 22 ft-lbs but the bolts won't take that much. I tightened them down by feel and stopped when I could feel the bolt stretch. That's how I do most bolts; I trust my feel more than the wrench unless it's something like the head nuts or a large bolt like holding on the rotor or the sprocket.

Sorry to say it but I'd pull the head back off. Order new OEM bolts and this time keep the torque lower.

Edit 2: below is what the stretch bolt looks like for the 750. Again, they couldn't take the torque as shown in the manual without breaking either even though I bought brand new bolts from Kawasaki.

WHY? Why do people try and torque anything other than cylinder stud nuts??? and not know diff between inch and foot pounds?
Drill straight thru! Should be able to if you can't get them out. Drill thru and bolt and nut on bottom them.
I have never torqued anything other than head studs for 40 years!

Ok, just whipped off the head & the broken bits came out easily !
They are meant to be ft lbs & not inch.
Can I re-use the head gasket & do I need OEM bolts as they don't look like stretch ones to me.

Buxvan wrote: They just kept doing up & I was wondering when I was going to hear the click

Well, I suppose you did hear a click of sorts.

Nessism is right the bolts are Metric 6 mm.
The torque for the highest grad M6 ( 12.9 ) are 19,3 Nm = 14 ft lbs / M6 ( 8.8 ) are 11.3 Nm = 8 ft lbs.
The torque specs in the manual looks like more for M8 ( 8.8 ) = 27 Nm = 20 ft lbs.
I think this is why you snapped your bolts.

I would reuse the head gasket, if she was not heated up and clean.

Just to double check, the bolt shaft is the size driving the torque spec, not the head on the bolt.

They are 6mm for sure. Using a torque wrench is fine. I use one for all my motorcycle work, but you have to be careful of goofball torque specs. 6mm bolt is good for 5ft/lb to 15 at most. Lot's of errors in the Kawasaki manuals in torque specs can lead to lots of problems.