In Part 1 of this article, this writer recounts his efforts to diagnose the cause of a misfire on a recently rebuilt 3.0L V6 Nissan Maxima engine that resisted all his initial efforts at diagnosing the root cause of the problem. Briefly, the problem was this: a group of DIY engine builders had rebuilt the engine, and somewhat surprisingly, the engine ran well at first. However, after a few thousand kilometres of use, the engine began misfiring on cylinder #2 after cold starts but then diminished as the engine warmed up, and disappeared altogether when the engine was fully hot.
After an extensive round of testing and investigating the issue, it turned out that none of the usual suspects seemed to be causing the misfire, at which point this writer realised that only two possible causes of the misfire remained. The first was something exotic, like a mismatched piston, or something simple, like a head gasket or mating surface that was somehow damaged before the DIY engine rebuilders torqued down the cylinder head on bank 2. Of course, investigating these possible causes of the misfire required the disassembly of the engine, but since the vehicle's owner was on a tight budget, this writer called him to explain the situation to him and to get his authorisation.
The vehicle’s owner was understandably taken aback by this conversation, so he insisted on being present when this writer stripped the engine down, which was fair enough. In Part 2 of this article, we continue the diagnostic process (with the vehicle’s owner looking over this writer’s shoulder), but before we get to specifics, let us recount the short version of the story of the engine’s assembly. Here is-
According to the vehicle's owner, one of their group once assembled a lawn mower engine, which ran fine for most of one season before seizing up for unknown reasons. However, a car engine is somewhat different, but since the owner of the car could not afford to have the engine assembled by a professional engine rebuilder, the group of friends thought they had better research the process of assembling a modern engine lest it too, seize up for unknown reasons.
On a practical level, their research included reading some online "How to..." articles, watching a few dozen YouTube videos, and working their way through a repair manual published by a company whose name we prefer not to disclose here. Not that this manual is bad or inferior in any way, but reading through such a manual is not a substitute for the hands-on experience of assembling engines gained under the supervision and guidance of an experienced and knowledgeable engine rebuilder.
Also, all the parts used in the engine were new and sourced from a reputable supplier of engine parts, and they checked the dimensions of all parts against the dimensions provided in their manual. Everything checked out, so what could go wrong?
What could go wrong, indeed? Well, plenty of things could have gone wrong, as anybody (such as this writer) who has ever assembled engines, including high-performance competition engines, for a living would know. Nonetheless, this writer was inclined to admire this young man and his friends for their gumption in attempting to do something they had no business attempting, but still, they did a pretty good job at building an engine that ran well for just over three thousand kilometres.
Long story short, though: this writer needed to know a few things about the assembly process. For instance, he wanted to know if they dropped and perhaps damaged any parts during the process. He also wanted to know if they had checked and verified things like the-
-and most importantly, if they had followed the recommended torque settings, or had trouble at any point during the assembly process to make something fit or rotate freely.
Of course, expecting the car’s young and inexperienced owner to fully appreciate the importance of the above checks and verifications was perhaps too much to ask, but then again, getting truthful answers to the above questions would speed up the process of pinpointing the cause of the misfire considerably.
As it turned out, the DIY engine builders did not drop any parts during the assembly process, so they could not have damaged any parts in that particular way. However, they did not check or verify any of the above, because the engine rebuilder who had done the machining on the engine, and whom this writer knew well, assured them that “everything will fit as it should”. The only caveat to this was the requirement that they (the DIY engine rebuilders), buy the exact parts (including a set of new cylinder head bolts and specified valve lifter shims) on the list he gave them, which they made sure they did. Moreover; yes, they tightened everything in the correct order, and to the torque settings that the manual specified using a borrowed torque wrench- which they had calibrated (for a fee) by a specialist tool vendor.
However, something did happen when they torqued the cylinder head on bank 2. One of the new head bolts stripped out the threads in the aluminium cylinder block when they applied the final torque setting. Unfortunately, neither the manual, nor any of the DIY engine builders’ online sources provided a remedy for this particular eventuality, but knowing they were in trouble, they sought out the advice of an acquaintance of theirs who worked in a general machine shop. Not being an automotive machinist, the acquaintance told them to bring the engine in and he would see what he could do to fix the problem.
When the DFIY engine builders picked up the engine, the acquaintance assured them he had fixed the problem, but he also gave them four new bolts and washers to use on the four holes around cylinder #2 in place of the bolts they had bought. Happy that the problem had been solved, the DIY engine builders assembled the rest of the engine and installed it in the vehicle. According to the car’s owner, the engine started “on the first turn of the key”, and ran fine until the misfire appeared, which brings us to-
The story about the four new cylinder head bolts was intriguing for two reasons. The first was that there are no reliable ways to repair stripped-out or severely damaged cylinder head bolt threads in aluminium engine blocks, and the second was that it is exceedingly uncommon for OEM-equivalent head bolts to strip the threads out of holes in aluminium blocks when the bolts are lubricated and torqued correctly. It was, of course, possible that the threads in this particular hole suffered some form of damage during a previous repair, but there was no way of determining this one way or the other.
There was just one more question to ask at this point. Did the DIY engine builders apply the same torque settings to all the cylinder bolts, and especially to the four new bolts the acquaintance at the general machine shop gave them? We will get to the importance of this question and its answer a bit later on but it turned out that, yes, the DIY engine builders applied the same torque settings to all the bolts. This was an interesting answer because it could potentially explain the root cause of the misfire, but whether or not it did, remained to be seen at that point.
Based on his experience with turning a great many standard aluminium LS1 V8 engines into high-performance competition-grade power plants, this writer was inclined to the idea that the four different cylinder head bolts were the key to solving this problem. We can skip over most of the details of the conversation with the car's owner that followed but suffice it to say that what this writer found when he removed the engine's valve covers went halfway towards confirming his suspicions.
Like all V6 engines, the cylinder heads on the VQ30DE engine are held down with eight bolts. On this engine, the head bolts are of the torque-to-yield (stretch) variety that requires tightening in a very specific way to achieve the required clamping force. On this engine, the M11 head bolts are 123mm long and if one tightens them correctly with an 8mm Allen key-tipped socket, they will stretch by about 0.2mm, and in this state, they do a good job of holding the cylinder heads down.
However, on bank 2 of this engine, the four bolts around cylinder #2 required a 10mm Allen key-tipped socket, which meant that these four bolts were M12s. However, the key difference between the required M11 bolts and the M12 substituted bolts involves the pitch of their threads; the M11 bolts have a thread pitch of 1.25mm, while the M12 bolts usually have a thread pitch of 1.5mm. The difference might not seem like much, but here is why it is, in fact, crucially important-
If you screw an M11 bolt with a pitch of 1.25mm into a suitably threaded hole, and you rotate the bolt clockwise by 360 degrees, the bolt will descend into the hole by a distance of 1.25mm. Now, if you do the same with an M12 bolt with a pitch of 1.5mm, the bolt will descend into the hole by a distance of 1.5mm. Therefore, if the two bolts are used to clamp down something like a cylinder head and one applies the same stretch-to-yield torque values to both bolts, the M12 bolts will generate a vastly greater clamping force than the M11 bolt simply because the M12 bolt moves further into its threaded hole per degree of rotation than the M11 bolts do.
When this writer raised this point with the car's owner, he expressed surprise because, at the time he and his mates were building the engine, none of them thought that using different-sized bolts was important and besides, the machinist that gave them the bigger bolts did not say anything about different clamping forces. Ergo, the DIY engine builders did not think that using different bolts would make any difference to anything.
It gets worse though: this writer removed one of the M12 bolts, and sure enough, it had a 1.5mm pitch, and there was only one way to screw this bolt into the engine securely. Consider the image below-
If you have never seen objects like the ones in this image, they are called threaded inserts that have legitimate uses in engineering. In practice, inserts are used to effectively replace damaged internal threads in some steel and cast iron components. Here is how this works-
The existing hole is enlarged with a special drill bit to remove the damaged or stripped threads in the hole. This new hole is then also enlarged to accept the shank of the new bolt, but only to a depth that creates a step in the hole. The stepped part is then tapped with a special tap, and an insert (like the ones shown above) is then screwed into the newly-tapped step and secured with a thread-locking compound. The replacement bolt is then screwed into the insert's internal thread, and in circumstances where such a repair is both warranted and possible, the repair is effective and reliable.
In this case, though, the repair might have been warranted, but it was in no way possible to perform this kind of repair on an aluminium cylinder block. The main reason for this is that drilling out the stripped thread also removed much of the material around the hole which resists deformation of the area around the hole when the head bolts are torqued.
Moreover, the general machine shop almost certainly did not stock inserts with M11 internal threads, since M11 threads are very seldom (if ever) used in general engineering. Therefore, the machine shop did the next best thing, which was to use inserts with M12 internal threads, which have even bigger outside diameters than inserts with M11 internal threads.
One final thing worth mentioning about the non-standard bolts is that although their length was acceptable at 124mm, they were made from a material known as EN24, an extremely hard and high-tensile steel that did not stretch under the modest torque values that caused the standard bolts to stretch. The practical effect of this was that since the four non-standard bolts did not stretch under the applied torque, they exerted a vastly greater clamping force on the cylinder head and gasket than the standard bolts did, thus almost certainly preventing the cylinder head from expanding evenly, which brings us to-
Image source: https://www.ms-motorservice.com.tr/fileadmin/media/MAM/PDF_Assets/Piston-Rings-for-Combustion-Engines_53094.pdf
This schematic shows some details of the kinds of deformation(s) that affect a cylinder in an aluminium cylinder block when head bolts are over-tightened, such as in the case of the Maxima engine that features in this article. Note, though, that this schematic is not drawn to scale, and that the deformations seen here are greatly exaggerated for clarity: in most cases, deformations like this can only be detected and measured with laser-based measuring equipment. Let us look at this schematic in some detail-
The yellow arrows and lines indicated the space between the threaded holes and the water jacket, which in the Maxima engine’s case, were greatly reduced to accommodate the threaded steel inserts. This part of the casting plays a critical role in allowing the cylinder head to expand evenly while resisting deformation caused by tightening the cylinder head bolts to their recommended torque values, at the same time. Therefore, reducing this distance by removing material has the effect of weakening this part of the cylinder head casting, thus causing the cylinder head to expand in unpredictable ways.
The red arrows are largely self-explanatory since they clearly indicate the opposing forces that force different parts of the cylinder block into opposing directions under excessive torque loads.
After the removal of the cylinder head, it became clear that the deformed cylinder block created a leak path between the head gasket and the cylinder block when the engine was cold, but since aluminium is highly elastic, the leak path into the water jacket closed up as the engine expanded when it warmed up, which explained the misfire’s odd behaviour. Nonetheless, no head gasket can withstand this kind of abuse for long, and this gasket would have failed completely within the next few hundred kilometres of use. In fact, that the gasket lasted for as long as it did seemed to be some kind of miracle in itself.
Note, also, the “4-leaf clover” stress patterns; these are caused by deformations in the cylinder walls as a result of over-tightening cylinder head bolts. While these patterns are always present when head bolts are over-tightened, the degree of over-tightening of the head bolts determines the degree to which the cylinder walls will deform.
As a practical matter, the only way to determine to which degree the cylinder walls had deformed on this engine, was to remove piston #2 to look for the wear pattern (on the piston skirt) that is characteristic of deformed cylinder walls. Consider the image below-
Image source: https://www.memoparts.com/img/cms/Documents/Piston%20Failue.pdf
Although this image does not show the actual piston from the Maxima engine, the kind of wave-like wear pattern shown here (circled in red) was also present on the Maxima's piston. Note that in the case of the piston shown here, the uneven stresses were severe enough to cause a stress fracture in the piston's skirt, thus causing a small piece of the skirt to break off, as indicated by the yellow arrow.
Nonetheless, this wave-like wear pattern on pistons is only ever caused by deformed cylinder walls in general and by a narrowing of the cylinder at about the midpoint along its height, in particular. The mechanics of this wear pattern is rather complicated, but essentially, the piston’s weight, height, and diameter all cause opposite sides of the piston to “bounce” off the narrowest part of the cylinder wall. On down-strokes, one side of the piston will bounce off one side of the narrowed cylinder, while on up-stokes, the opposite side of the piston will bounce off the opposite side of the narrowed cylinder.
However, changes in engine speed and load cause the piston to bounce off the narrowed cylinder wall at different points along the piston skirt's height but over time, a unique wear pattern emerges that imprints a unique set of “waves” on the piston’s skirt. Thus, while these kinds of wear patterns may appear to be similar, the number of “waves”, and the distances between “waves” are always different because of the unique circumstances (in affected engines) that create such wear patterns on pistons.
As a practical matter, the Maxima's engine was not rendered totally useless, and restoring it to a usable condition was, in fact, possible, but this was likely to be a good deal more expensive than simply replacing it with a professionally rebuilt engine. With some extensive machining, including installing inserts in all the cylinder head bolt holes to equalise the load on the block and cylinder heads, and, of course, calculating a suitable torque value for the larger bolts. In short, the engine could be made to work again, but seeing that this was certainly not a reliable long-term solution, this writer refrained from mentioning the possibility that this engine could be "saved".
Instead, this writer simply advised the car’s owner that the best possible course of action, in this case, would be to cut his losses, and replace the engine with one that had been built by a professional engine rebuilder, which leaves us with this-
Overall, it would perhaps be unfair to expect non-professional mechanics, and even general machinists, to know and/or appreciate just how susceptible aluminium engines are to damage caused by things like unevenly torqued cylinder head bolts.
Nonetheless, we hope this article has given you some new insights into the critical importance of observing recommended torque settings and, more importantly, into what could happen if we do not follow these recommendations exactly. In the real world, where we, as mechanics, live, assembling engines is not an art- it is an exact science that requires attention to myriad details if the engines we build are to conform to accepted industry standards in all respects.
Of course, none of us would intentionally ignore any of the details that make rebuilt engines last (nearly) as long as new engines, but then again, we are all human and since making mistakes is easy, the trick to assembling reliable engines is to make sure the mistakes are never yours.
