Short summary: HR head bolts after 2011 (Tsunami f'kd up Nissan's factory) are no longer reliable/Suspect: Poor quality control in varied heat treatment creating some bolts softer and others harder in metallurgy/Solution: Stronger-than-HR, Juke head bolts with an additional washer to account for extra bolt length in DE's (not necessary for HR motor).

So during my engine's rebuild today, my mechanic calls me slightly distressed to tell me he broke a head bolt. My heart skips a beat, and then I realize it's not the end of the world (Fortunately he is able to back it out with a flat-head screwdriver because how the bolt snapped, and it really isn't torqued down at all because of it). The first thing he thinks is maybe the torque wrench was off, but checks it on a bolt nearby he knows the torque spec for, and it clicks normally without turning the bolt. He says he's never broken a head bolt before, and was following the procedures verbatim, but felt something was off while he was torquing it down. The second thing we think is maybe the machine shop took to much material off the bottom of the head (that doesn't make sense in hindsight to make the bolt snap how it did), we're both confused. These were brand new HR head bolts going into a DE motor.

So as I'm driving over there, I call up JWT to ask if they might know what might have caused this (I figured they're the VQ experts, and since I just ordered cams from them they should be happy to help; though these guys are just plain helpful and awesome to deal with in general). At first, they point me to the FSM online at nicoclub to compare the height of the cylinder heads; once we measured this, and examined the bolt thoroughly, we ruled this out as the reason why it broke. (I was driving at the time, and didn't have any real resources in front of me so was looking for a quick answer before working hours closed on a Friday)

After getting to my mechanic's house, he shows me the bolt. It made me think of a video I watched demonstrating how metals are tested for tensile strength. I'm not metallurgist, but the stretching and narrowing of the diameter where the bolt broke just made me think the metal was soft and weak.

Calling JWT a second time, my mechanic explains what happened, and while he hands me the phone and goes to measure the heads to confirm deck height - another tech of JWT comes on the phone (this guy must be an automotive-VQ-genius). He explains how they initially had great success with the HR head bolts & HR gaskets, and used them in all of their race engines, but after around 2011 (i.e. Japan Tsunami time) began to experience problems with the quality of these bolts. Some were too soft, and would break, but while torquing them down, during the first step to 77ft-lbs, they were able to tell the 'soft bolts' from the 'hard bolts'. He told me, they had to go through 3 sets (48 bolts) to get one complete set of 'hard bolts'. Apparently, after doing this though, while they could install the bolts properly, they still had problems from uneven pressure caused by differing hardness in these bolts.

So the solution: JWT (Clark) recommends for DE motors is to use Juke head bolts (slightly stronger than HR head bolts, and also slightly longer). To correct for the additional length of the Juke head bolts he recommends to either A) cut the old head bolts to free the washer, and use that washer on the Juke bolts in addition to the washer that's already there... or B) get the washer's from an SR20 head bolt (which are apparently separate from the bolt) and use that in addition to the washer that's already there. HR's shouldn't require the additional washer. I'm not sure on rev-up's; didn't ask. Apparently these Juke head bolts have not had the same quality control issues, and he believes it may be that they're created in a different factory or something.

If anyone has a machine to perform a Brinell and/or Rockwell hardness test, I will mail you the new HR head bolts if you are as interested in this issue as I am now. It would be interesting to see the results, and Nissan's response. Otherwise, I suppose I'll just return the defective bolts to Z1 soon.

Also, if JWT want's to eventually chime in on this first hand that would be awesome; I know I'm giving this information secondhand.





http://www.jimwolftechnology.com/wol...HEAD_BOLTS.pdf

PS... JWT told me some Nissan tech's are aware of the problem, having weekly repairs on different vehicles for the same issue, and it's not a 350z-specific problem. Nissan Murano's and other cars may likely have head bolts of varying hardness post-2011 which may cause coolant leaks on the side of the block, etc.

PSS... While disapointed with these head bolts, and a further delayed rebuild, I am now oddly entertained by JWT's extensive VQ knowledge. They are like the Brabus version of the Nissan world, screw Nismo - JWT all the way! haha

 

So many people in the industry bash JWT, but they're wizards and are pretty much OEM+ products. They know what they're doing.

 

Very good information. Thanks for sharing. You saved me some headaches.

 

Crazy

 

Pm sent

 

Why would anyone bash JWT? Their service and products are topnotch.

 

Juke head bolts:

 

I have JWT in my good graces…+1 to this post -

 

Good info! Why not shoot for ARP head studs like any other engine build?

 

Availability.

 

Based on the way those threads look on the bottom half of the bolt, and the mid-section diamater chage, it appears that it got tightened down to the point where the bolt stretched and snapped. I've seen OEM caliper bolts look the exact same when someone turned the wrench the wrong way, thinking that the bolt was seized while in reality, they were just turning the wrench the wrong way. I'm not saying that the mechanic isn't telling you what really went down, but I've seen identical breakage in bolts that occurred as a result of human error as opposed to manufacturing defects.

Is there a posted TSB regarding bad HR head bolts from Nissan?

EDIT:
I just read the JWT bulletin. Ugh. That sucks.

 

Initial testing is back from an independent test comparing some used 2002 DE head bolts from my engine to the new HR head bolts I had purchased from z1 recently. Shows the elemental composition of the bolts, and them magnified under an electron microscope at 2200x magnification.