I am sure that also locates it.
Tom
I can’t agree with some of the statements in this thread. For a start, there’s no shear force on the flywheel bolts because after the clamping force of the ten very high tensile bolts, it’s taken care of by the two closely fitted dowels. I have never renewed the flywheel bolts in my '62 3.8 in the 40 years I owned it even though I had the flywheel out several times, and once even ran the car for a while with a different crankshaft while I cleaned up the original. And I never bothered too much about the rev limit when driving the car either. The XK engine was designed to be driven hard. I did have the crank/flywheel/clutch/damper assembly properly balanced however.
The ten bolts are very high quality and very high tensile. They are marked with the British strength designation “X” which equates to about 195,000 psi and they have a shallower head than a regular automotive.hex bolt. I’m sure I’d trust them more than any aftermarket supplied part, especially after the ARP con-rod bolt debacle.
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Jerry, I cannot presume to answer for Jaguar in particular, but my understanding is the dowels are for locating very precisely. There can of course be various ways to do so, but to “simply” hang it on some bolts would leave room for movement, thus imprecise positioning. Some manufacturers also balance their flywheel with the crankshaft, so now if the flywheel were reinstalled a bolt hole off, that would cause imbalance. I believe I have seen some go on in only one position, due to dowel position, but my memory may be faulty on that. And I believe sometimes things are done because of tradition, even though a new procedure comes into play. I never heard why the special bolts. As said before, sometimes stretch bolts are use, and of course they must be replace. What I do know is if special bolts are suggested, I will use them!
Peter and Pekka, yes, apparently bolts used to be very over engineered. Largely because it is very hard to accurately tighten bolts. Even the most accurate torque wrench is extremely inaccurate because they measure torque, and what we want to measure is clamping force as a result of stretching the bolt. As manufacturers are trying to save money and weight, they are downsizing bolts, but that requires more precise tightening methods, thus the advent of low torque, then degree turn to tight. Still not the best method, better much better than a torque wrench only.
Tom
Clive, I was typing as you posted, but I already agreed with your first part, and although I am not in a position to disagree with the dowels taking any shear at all, my understanding is the friction would take it.
For those interested, here is an informative article:
I believe the joint we are discussing here is on page 7, paragraph 2, friction-type or slip-critical joint, although I am always subject to error!
Mine are “BEES 75X85” .
The problem now days is everyone is selling inferior products and sourcing (and knowing their source) is questionable.
Right. X is the strength rating, 75, 85 is the tons/ sq. inch. 1 ton=2240 lbs.
FYI here are some photos of the two sets of bolts from my XK140 previously referenced:
First (sorry photo is not the best) are the bolts I removed in 2005. These are Bees grade X bolts. You can see the markings “BEES 75X85” on the heads. These are in all likelyhood the original bolts from 1956.
Secondly, below are the bolts I replaced them with in 2005. They are marked “ARB X”, and were purchased from XKs Unlimited in 2005 as part #C.4855. The current illustration on XKs web site for this part number appears to be different than this (there is a distinct circular depressed area in the centre of the head of the illustrated head which is not present on my 2005 bolts - the markings on the head of the illustrated bolt aren’t completely clear but do appear to be something like “OSB X”.
The bolt illustrated on the SNGB web site for this part number appears to only have the marking “X” on it’s head, and does not appear to have a “grip” (unthreaded) section. It would be interesting to know what markings (if any) are on the Original Poster’s bolts.
-David
You need to be careful because an “X” is also very commonly used on the head of American aircraft bolts and means something entirely different, simply that the particular bolt is made from non-corrosion resistant steel. The proper Jaguar flywheel bolts always had a maker’s mark in addition.
I note that this article is limited to static situations. No consideration of stress cycles and fatigue.
Jerry
…which is as good a reason as any for me to bow out 
Jerry, good point, I wonder how that plays into this. I would have thought that even though these bolted items turn, they would be static to each other and with the clamping force between them, the only “movement” would be thermally caused. But I do not know, do you think it would fall under another category, and how do you think it would affect it?
Tom
Has one ever been known to fail? On an XK engine?
I’ve never had one fail, including 15 years of use on my race car, but as years go by fatigue can build up. I’ve always thought it good practice to replace bolts that are under stress in critical areas, provided, and it’s a big proviso, the new stuff is at least as good as the old. Short of getting it from the car manufacturer or a supplier with a recognized reputation for quality (such as John Deere) I worry.
Tom,
No direct information but the Porsche recommendation and materials science classes of many years ago make me wonder… I’ll still be changing mine (as I did last year) but I’ll be checking the hardness of the new bolts.
Jerry
Thanks to all for the replies.
I decided to reuse the original bolts for several reasons:
1: Don’t trust the new ones supplied.
2: There are only 4 bolts at each end of the drive shaft , they are smaller and subject to very similar forces and stress.
3. The pressure plate is held in place with only 6 5/16 bolts and 3 very small dowels all of which are basically subject to same abuse as the flywheel bolts ( although with better leverage due to their distance from center).
4. I believe the crank/flywheel dowels are taking shear force because of their size and very tight fit. Locaters would not be so tightly machined and large.
Wish me luck.
Best The depth of the heads on XK flywheel bolts changed earlier engine had a deeper head and,I think, were better quality.
From Series 3 approximately, the bolts were shallower headed. We’re VERY easy to round off when tightening if you didn’t you a good single hex socket! Good old Leyland Cars!!!
They are subject to cyclical shear at high temperatures and that’s the sort of punishment that changes steel. The number of cycles a bolt has experienced is proportional to its likelihood of fatigue failure. (Steel eventually settles out at about half its original stress fatigue limit)
Jerry
The temperatures that these bolt experience is nowhere near the level that will cause any modification to the material specification.
Steel is unique in that is has an “endurance limit” below which it will not fail in fatigue…no matter how many stress cycles it experiences…so if you specify a tensioning torque that stresses the bolt below the endurance limit then it will not fail in fatigue…provided the stress in the bolt is below this magic level.
It’s not quite as simple as that because you also have to take into account the magnitude of any stress fluctuations …but these…I think…will be very small compared to the base stress level. Some clamping force to modify the tension in the bolts maybe…but a minor change in stress level I would think
My opinion is that the connection to the flywheel is a friction grip connection with the dowel as additional shear capacity in case something goes wrong…but the bolts are torqued and locked in position so the clamping force is locked in. So the stress in the bolt is purely tensile without having to combine tension and shear…which is the case on …say…a propeller shaft. So the original designers had a rather simple connection to analyse…and I suspect that the bolts are good for repeated use…because I have not noticed any warnings on their re-use in the tech manuals.
Regards
Matt