[__G__]

Hi all,

I'm assembling my engine (first time) and just bolted the head down. On several of the M12 bolts I felt a distinct (but not dramatic) loosening of the bolts just before I achieved full torque. The final stage of tensioning is 180deg deflection, but around 150deg deflection it seemed to go just a bit loose.
With normal bolts I'd assume that I've damaged the threads. But given that these are torque to yield, I'm wondering if this feeling is the yielding part. It happened on maybe 6 or 7 out of 10 bolts, at almost the same point and it felt the same each time...
Any ideas whether I have a problem to fix or is this normal?

Thanks in advance

[yupetc]

I experience exactly the same. It's called stick-slip. Happens on tty bolts.

[__G__]

Stick-slip that I've experienced before is a sharp almost snap like release of the bolt like the bolt friction is too high. What I'm referring to was definitely not that. Normal stick-slip happens in what feels like only a few microseconds, whereas this happened much slower perhaps a few hundred microseconds giving it a plastic feel, hence why I thought it might be normal. But the consequences of being wrong are worth asking the question.

[yupetc]

Originally Posted by __G__

Stick-slip that I've experienced before is a sharp almost snap like release of the bolt like the bolt friction is too high. What I'm referring to was definitely not that. Normal stick-slip happens in what feels like only a few microseconds, whereas this happened much slower perhaps a few hundred microseconds giving it a plastic feel, hence why I thought it might be normal. But the consequences of being wrong are worth asking the question.

If you have such deep suspicion that something might be wrong with it, then take the bolts out and measure the bolts against the ones you removed from the engine in the first place. These tty bolts are supposed to operate fully under the plastic deformation range. If you have elongated bolts you have likely over-torqued them and they are no in the elastic deformation range (past the yield point of the bolt). That'll warrant a new set of head bolts again. No matter what, if I'm in your shoes, the first thing I'd do is order a new set of bolts and try it again anyways. If the female threaded holes are damaged you'll definitely be able to tell.

If that's the case, I highly doubt you should use a matching tap to clean the thread, but if it really is damaged, maybe get some engine shop to throw some ideas to repair it. One thing I can definitely tell you is the time-sert repair method will absolutely not work with the head bolts because the kind of beating these bolts are going through, the inserts will definitely start to walk and become loose.