Con rod bolts - ARP won't fit ---- Solved

Yesterday I received a set of ARP rod bolts from one of the usuals (3.4 120)

They don’t even fit into the holes. I tried 2, even a few light taps with a hammer won’t get them even close to seating. These should set in place with the fingers.

Edit: I went back and gave each a couple of solid whacks. Won’t go. I measured 3 of Jag’s bolts and 3 of ARP’s. ARP bolt diameter 1/8" below the cap is .002" larger than Jag’s.


So… where do I get a set that fit? Thanks.

Also, on the E-type forum someone noted that the ARP bolts has a slight radius under the cap (mine do) preventing the bolt from making proper contact with the cap. He measured the gap at .008.

This has to mean that as the radius is cut by the square-cut cap, the bolt will settle more deeply into the cap reducing the nut’s force. I looked at my rods and there is just a small radius there (3.4). Is this missing on the 3.8 and 4.2? I don’t ever remember looking when I did my 3.8 years ago.

It was noted on the E-Type forum that ARP doesn’t list Jag as a marker they produce parts for.


Hmmm…the ARP set I bought actually had LESS of an interference fit than the originals…and…the major diameter of the shank ended “sooner” than the Jag parts. The sharp edge of the bolts did have to be beveled to clear the radius of the rod. I did NOT have a problem with the underside of the head seating properly…perhaps the bolt hole in the rod was already suitably beveled. In any case, we received different ARP bolts for the same application!

I thought I posted this photo recently? Taken around 2012.

The dimension written in the paper towel in the photo is the width across the flats of the bolt cap.

It has always been my understanding that the ARP bolt sold for XK connecting rods is not actually manufactured for the XK rod, but for a Ford V8 ( 351 400 Cleveland or Windsor, not sure), and they required fitting by a competent machine shop who would be expected to modify the rod cap to suit the bolts. And looking at the old photo, (EDIT: LOOKING AT THEM MORE CLOSELY, I NO LONGER THINK THE FOLLOWING IS TRUE) I would say that they are only intended for installation in the S3 XK6 rod. ARP themselves do not list a bolt kit for any Jaguar.

S3 XJ6 rod left, early rod right. ARP bolts side by side in both.

Underhead diameter XJ6 0.3740 (used), ARP 0.3750 (new)
Digital caliper, your MMV.

The correct ARP bolt is a 400 Cleveland, the 351C is to short, and they do require “fitting”.
I normally put them in a lathe or drill press and just give the cross hatch area a “touch” with a file to make them a perfect fit.
Never had a problem with head interference on the later S3 rods.

1 Like

I used the original bolts and nuts. I didn’t see anything wrong with them. New split cotter pins of course, and bent only once and clipped short. They’ve been fine for 11,000 miles.

1 Like

That’s your prerogative, but the only good place for used Jaguar rod bolts and nuts with split pins is the scrap bin.
The first engine rebuild I ever did was for my S1 E. Car was low mileage but was burning oil like no tomorrow, typical of early E’s, so just replaced rings. Big mistake, threw a rod a few K later, very expensive.
Did redline car regularly, but that should never have been a problem,
Went looking for an ARP bolt and found FORD 400C fitted, used them ever since.
4.2 Race car pulled >7500rpm. without problem.

1 Like

Hmm, experience leads to decision paths such as Norman outlines. Wonder what the bolt torque was just prior to the time the rod was thrown. I suppose it was unknown and hence the desire for a different approach if uncertainty of bolt retaining torque setting is in the mind.

On reusing rod bolts, I’ve followed a fairly simple path. If the bolt torque coming free is known, if the bolt is not stretched in length when lying free on the table, and if the tightening torque process leads to similar elongation (and feel using the wrench as a qualitative sense) as previous, then I reuse bolts. A calibrated torque wrench is crucial if stretch is not being monitored.

Scientifically, if a bolt has only been used within its elastic response range (and bolt torque settings typically are 75% to 90% of the maximum in the elastic range), then the bolt may be expected to perform similarly on reuse unless some other damage to the bolt has occurred.

I would love to hear additional engineering, scientific, or experience helping guide this decision process for people. Thanks for further opinions.

…keeping in mind that the part of the bolt that is the weakest is likely to do most if not all of the stretching. That will be the unused part of the threads.

I’d like to find a study on this, but I’m thinking that if a bolt stretches at all, it’s got to do it first where the diameter is smallest, at the threads.

Has anyone seen a rod bolt break at any other place? (except a shear break of course as those will happen independent of torque).

1 Like

That is exactly what mechanical engineering books and bolt test failure studies say.

1 Like

Yea, but the second word is the important one. In a testing facility they have 100% control over the part.

Out here we have to assume the last guy had no idea of the torque he’s applied, and that bolt may have been used at the factory and more than once on the first rebuild because the guy torqued to check with Plastigage, again when he mixed up a cap, again to finally button up.

I’ll sleep at night having spent a paltry $130.00 or so on new bolts/nuts.

‘If’ doesn’t do it for me and makes reading the rest a waste of time.

You may be interested to search for posts in the Archive from the late Jerry Mouton about this topic, his summary, after blowing up his ARP bolt engine

( as I recall)…the ARP bolts have to be fitted very specially, as outlined, but new OEM S3 bolts were entirely adequate


I have a quandary with a pushrod 3.5 litre MkV engine that I have fitted 3.4 litre conrods from a XK engine to. The conrod bolts fitted are new but I do not know the brand, they look like the ARP bolts shown in Mikes paper towel photo, black with a tapered shank.

They were a very tight fit in the conrod & I had to torque them past the torque setting to get the heads of the bolt to sit flat on the conrod. I then loosened them off & retorqued to the correct tension. I didn’t record the torque to seat the bolts initially but as I remember it was a lot.

I haven’t run the engine, so what do all the experts think, drop the sump, buy new bolts that fit correctly? If so who sells the correct bolts that fit with no modifications?

Cheers Peter

Yes…if you just want to hold a constant load with the bolt.
If the bolt experiences a fluctuating load then the answer is…unfortunately…no.
There is a phenomenon called “fatigue failure” which is responsible for a lot of failure in bolted connections.
It’s moderately complicated but staying below the elastic limit is not the determining factor.
There is an engineering factor called “endurance limit” available to Engineers when using steel for fatigue design. For any “brew” of steel …ie the specification of the steel…there may be known the “endurance limit”…which is a stress level in the steel …below which the steel will not fail in fatigue. If you keep the stresses below THIS level then the bolt will not fail regardless of the number of stress cycles it experiences. Unfortunately…this stress level is so low that the fastener will be too big in modern applications to be acceptable. Generalisation of course…but there are ways of calculating the number of fatigue cycles a bolt will withstand. It is largely based on the mean stress in the bolt and the magnitude of the fluctuating stress in the bolt in use. And…it is statistical…so not absolute numbers…but a probability of failure at x million cycles.
As I said…it’s complicated and hard to get right. It is compounded by the fact that threads themselves can cause stress concentrations because of their sharp cut shape…more complications…
So rolled thread forms are better than cut threads…and so on…a complex field.

Bottom line Roger, is if you don’t fully understand the engineering, and are not prepared to risk your chances with vendors and back yard mechanics who think they know better, stay with what the extremely competent and fully tested in service solutions that Jaguars engineers decided on when they were making our XKs, and taking responsibility for their design and manufacturing. Modern engineering is rarely better overall, as goals these days are for cheaper solutions, with no conceen re longevity in life of mechanical components. For con rod bolts, best to stay with original old bolts, but realistically checking condition for damage/corrosion if bolts are sourced loose. If you are pulling down a sound but worn engine with no bottom end failures the bolts will still be within original design parameters. Same comment really slotted nuts, but of course you must use correct and new split pins.

A problem in Australia is modern new split pins are invariably metric, or typical Chinese marketing also called an imperial ‘near enough’. They are not. You MUST acquire correct diameter imperial size split pins, and then know correct way to install them. Head sits inside slot, and legs fold back over end of bolt, and forward against side of hex nut.

A little bit of commonsense and care, and you will get a perfectly adequate and long life result. If you choose to modify, or listen to the vendors and the back yard mechanics trying g to sell you non original bolts and nuts, ask them to show you why they are better engineers than Jaguar employed in the 1950s.

1 Like

Not really a valid statement. We “modern” Engineers have access to extremely powerful analysis tools that our forebears simply didn’t have access to. Slide rules versus Finite Element Stress Analysis where a computer can carry out hundreds of millions in simultaneous equations to determine stresses in complex shaped components that were simply not even contemplated by the slide rule Engineers of the ‘60’s.
Apples with oranges.

1 Like

Matt, you miss my point.
I too am a qualified professional automotive engineer, and yes, more than capable of designing a con rod bolt equal to or better than Jaguar did in the 1950s.

But this conversation was all about using con rod bolts designed and made for applications other than an XK con rod. I say, stay with the original, and not something designed and made for another application, and adapted to possibly fit and function ok

1 Like

Having said that, I certainly wouldn’t use XK con rod bolts in a modern Jaguar engine, where bolt design has very different priorities

1 Like

I hold this truth to be self evident: When an engineer sees a bolt and nut assembly with a castle nut and cotter pin, the very first thing he thinks is that design review committee thought the bolt was not good enough. It’s a vote of no confidence. They needed an eighty cent bolt, but accounting gave them 35 cents. Use cheaper steel, eliminate the heat treat.

When you torque a threaded fastener with castle nut, do you stop short of the specified torque, or exceed it to make the slot and hole line up?

I remember when it was virtually impossible calculate the determinant of a matrix of equations with more than 4 variables. Now it’s child’s play for the right software. Not dying to go back to log tables and slide rules.