I know there’s been a lot of discussion on here regarding the big end nuts on XK engines.
The original big end bolts on my car are so over-engineered I’m sure they will be sound enough to use again, especially given the difficulties that aftermarket ones seem to introduce. If I re-use the original bolts, what would be the best option for the nuts? I’m naturally a little cautious with the idea of split pins in the castellated nuts, particularly as on dismantling several of mine had lost their ‘tails’. Is there a decent locknut alternative? Or is there a really pressing case for using new nuts and bolts for the con rods?
I Have used these, but with the corresponding bolts.
I would not reuse the bolts if I were you.
I do not see why they are over engineered, and they are very old now.
They are stretched when used, therefore you should replace them.
Properly torqued, no rod bolt will loosen in any street engine. No locking nut need apply. You can always use the XJ6 bolts and 12 point nuts. I just do not use any split pins or soft iron washers in any critical fastener.
Peter, I don’t think the XK bolts are stretched, they are simple torque-to-spec bolts.
The reason I asked about using these nuts on the original bolts is that my old '67 Land Rover had similar, large-section rod bolts to the XK, and that just had flanged nuts very like these to hold the big end caps on. Never any problem on those, but they were designed that way whereas the XK has castellated nuts with split pins. That said, both vehicles use UNF threads here so the forces would be similar, I guess.
I’m sure I saw a thread that said there were all sorts of machining issues with the replacement bolts, which are designed for something else, not the XK engine.
Not wanting to get into another argument here, just gathering facts.
At the very minimum never use an old split cotter pin twice.
Spend more money if it makes you happy.
I once disassembled a Mark VII engine and found broken split cotter pins in the sump, but every single one of them had twists and plier marks, clear evidence that they had been used several times.
I also disassembled my 120, a 140 and my Mark V engines and all the split cotter pins were installed correctly and in perfect condition. I of course installed new, same size, same method.
Jaguar used them from the 1930s until 1968, about 316,000 engines or 3,700,000 pins. Properly sized and installed they don’t fall off.
The advantage of 12 point nuts is fast assembly; on the production line time is money.
Or peace of mind.
I 100% agree with Rob and concur with his experiences, and thus totally support staying with Jaguars original design, but you MUST use new split-pins. THey are simply not meant to be re-used.
But please note that there are two key aspects you must get 100% right.
You must use the correct size in both length and most importantly diameter. In North America I understand its not a problem still getting correct imperial (inch) diameter split pins as per Jaguars originals, but in Australia now most suppliers sell you metric diameter split pins either labelled correctly in their metric size, or indeed labelled deceptively as a soft-conversion approximate imperial size. If the split pin is a fraction too large it wont fit, but if it is too small and easily fits it will be too loose, thus can move, and in an application like a con-rod bolt can accelerate a fatigue failure. I actually purchased a whole selection of Imperial Size split pins out of USA to cover all my XK140 restoration needs, but regardless make sure what you are buying…
Fit. There is a right way and a wrong way to fit split pins. The right way is for the correct size split pin to be inserted so that the head slips sideways fully inside the groove in the slotted nut. So the neck of the head pushes into the mouth of the hole in the bolt. The legs protruding through the hole are now bent, one up the side of the thread and over the end of the bolt, and the other over the hexagon side of the nut facing towards the head of the bolt.
DONT fit them sideways, with the head flat and mostly outside the slot in the nut, and the legs bent outwards and around the nut… And yes, I see this done all the time with armature restorers/mechanics. It probably wont matter in a relatively static situation like a ball joint nut/split pin, but it does matter in a dynamic situation, especially such as con-rods or prop-shafts
I will give the Engineering Party, Shadow Cabinet opinion!
Most honorable gentlemen,
For me, this is the difference between pulling a plow with the reliable, old, grey mule, and pulling it with a modern John Deer diesel. The difference between the modern home and a nineteenth century stone castle. On street driven automobiles, all properly torqued critical threaded fasteners, capscrews, are self locking. In engineering land, all capscrews are are essentially springs. Adding a split ring spring washer is of no value. Critical capscrews, are stretched by design. 12 point nuts simply offer more splines to absorb the fastening torque. Because of this, they utilize smaller socket sizes than hex nuts. I use them all of the time. The split pin adds nothing to the mechanical integrity of the fastener. Nothing. How could it? They fracture under vibration and wind up in the oil pump and the bearings.
The ARP studs may not seat completely out of the box which is very problematic so if you use them, check again.
For any bolt there is a stretch value. You measure the bolt length before and after torquing, and the difference the micrometer reads tells you whether the bolt is still stretching to the desired value (bolts stretch when torqued) or needs to be replaced.
I believe the XJ6 S3 bolts are ovalized so they have their version of ‚thread-lock‘, either way, they‘re good but I can’t see an issue with the old nuts if you believe they are alright. But that’s all I know, no experience to back it up.
Isn’t that the reason for torqueing fasteners - to impart a prescribed amount of stretch?
12-point nuts and new bolts were used on my rebuild, but I don’t know what make he used.
I switched over to ARP bolts and mine actually fitted fine in the original XK 120 connecting rods (although the final XK 120 generation C.7017 if that would explain why these ARP bolts fitted nicely).
ARP specifies the following.
High Tensile ARP Connection Rod Bolt and Nut. With a 190,000 psi Tensile strength, made from 8740 Nickel Chrome Moly Steel. Thread rolled to MIL-S-8879 spec and finished in a black oxide finish against corrosion. These ARP Con Rod Bolt and Nut replace the Original Bolt C32344 and Nut C36030.Torqued nuts to 55 ft lbs with some 30W Oil
Thanks Bob,
Yes, the ARP bolts are good - I have used them in the Mustang, the Cobra and the Mini. I’ve already ordered a set of the XJ6 S3 bolts and the 12-point nuts, so hopefully these should be OK and fit straight in.
The torque is a surrogate for stretch, which imparts the clamping force. Measuring stretch by comparing free length with fastened length cuts out the ‘middleman’ of a torque number but requires that the fastener supplies specifies the required stretch figure.
I don’t follow this “if it exceeds 0,015” throw them away” comment. If the bolt stretches too far you have tightened it too much, so just tighten it correctly with less force. The only time excessive stretch would be meaningful in a damage context is if a part had been tightened beyond its elastic limit and had taken a ‘set’ instead of returning to its original length.
Since nobody has the pre-tightened length for bolts in a used engine, nobody can know if the fastener has taken a permanent stretch. Nor (I bet) are the bolts produced to a reliable length accurate to under ten thou variance since the rolled thread end is not machine finished.
Therefore, since one can never know the “correct” absolute free length for a given bolt, only the increase in length after tightening, the concept of rejecting the fasteners based on excess length doesn’t make sense. I could understand a grossly weakened overstressed bolt lengthening by more than expected when torqued to a given figure if it is a torque-to-yield fastener like long AJ6/AJ16 head bolts, which can only be used three times, but not a rod bolt.
Also important id to resize/check the circularity of any big end using bolts of higher tightening torque. The ARP bolt might love 55ft/lb but id the big end was sized by Jag to 34 then it can distort out of round.
Since the same paragraph says to never reuse rod bolts (then again it’s „power tuning the XK“) the stretch test applies to new bolts as well. A lengthening above the value given would indicate that the bolt was produced too soft or weak, or that it has lost its temper or strength. Such a bolt would become loose - maybe.
If it ‚passes‘ the stretch test it at least isn’t going to lengthen during use due to being stretched absurdly or soft.
There are (big) bolts with rods inside that indicate the lengthening, and bolts that are stretched with hydraulic pressure and only tightened by hand.
Good point about the ovality. I would stick with Jag hardware and values, it’s not as if they’re not obtainable.
That refers to permanent deformation. As you say, bolt stretch is actually what you are seeking when you apply torque to a treaded fastener. Torque is an educated guess, but bolt stretch is dead on. If you then subsequently remove the nut, the capscrew should return to its original length, if it does not, then it has yielded and will no longer safely produce the specified compression. It will then be subject to accelerated fatigue and eventually fracture.
I would disagree a bit with this. You install the capscrews, torqued to spec, before you resize the big end. You use higher torque for the ARP faster because it is made of superior steel, so stretching it to 90% of yield produces an immensely stronger connection.
Here are photos of the ARP bolts installed in two different rods, one from a 4.2L S3 XJ6 and the other from a 3.4L Mk2. The area where some folks reported a problem is circled in red. These rods are exactly as they came out of the engines, no prep or relieving. I don’t see an issue with the fit on either rod.
