Are the flywheel bolts grade 8?

I have an other thread going in the V12 forum about a Fidanza flywheel, bolts lengths and dowels, but what I really want to know is: Are the flywheel bolts grade 8 or something completely different ?
This is a picture of one of the flexplate bolts I have from changing from automatic to manual. I can but flywheel bolts at $5-$8 a piece from the usuals or I can get grade 8 bolts from Bolt Depot for a fraction of that.

Thanks as always … Ole

No, they are not. They are considerably stronger. In fact there are no “'grade” bolts on an E-Type because British bolts used the British marking system, not the American, believe it or not.

“Considerably” stronger than grade 8 ?? I hope you’ll excuse my skepticism. Could we get actual numbers or references?

In fact there IS grades on E-type bolts, and as you would expect on a British made car, they use the British Grading system.
Why would a British car use the USA grading system as used in USA and pretty well no where else?

A basic Mild-Steel of 45-55 ton yield strength is Grade R.
Thus for those into XKs will find many bolts with BEES 45R55 embossed on the head - BEES being the Brand, and R being the Grade, but to be double identified also the 45-55.
Other brand bolts such as AUTO-R, and GKN-R just show their brand and their 45-55 ton mild-steel grade R.

The next highest grade commonly used was 55-65 ton, shown as Grade T
Then the next grade not often used was 65-75 ton, shown as Grade V
Then the top grade in Automotive use, 75-85 ton was shown as Grade X.

Now in the early 1960s, Jaguar actually marginally improved the strength of their mild-steel bolts to now be grade S instead of R.
Grade S is one increment stronger at 50-60 ton yield.
Anyone who has restored an E-type will find mostly Grade S bolts, a few T and a few X.
All the smaller fasteners less than 1/4" dia are invariably ungraded-mild-steel, less than R, and don’t need to be identified.

So the pictured flywheel bolt, shown as GKN (brand) and X, is a 75-85 ton yield setscrew.

Someone else can work out how that equates with GRADE 8

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Grade 8 is listed as 130,000 pound yield strength, so the GKN X must be 150,000 - 190,000 pound yield.
Can we trust that the replacements from the usuals are indeed that strong? That sounds pretty difficult to source.

A 12.9 rated metric fastener has a 176,900 pound yield if that’s any help. It would probably suffice.


I should have stated that GKN X would have a 75-85 ton yield, equating to 150,000 - 170,000 pounds.
HOWEVER - if we are talking Imperial long tons, they are 2240 pounds (thank you, Clive), resulting in a yield of 168,000 - 190,400 pounds. A 12.9 fastener would still probably suffice.


Bottom line.These flywheel setscrews are clearly VERY SPECIAL not just in strength, but also in dimensions.Best to locate and use second hand originals which are not that hard to find.Lot of risk in sourcing aftermarket, especially when in Australia at least, we are bombarded with Chinese rubbish that still puts steel grading markings on head, regardless of the rubbish material used to make the bolt.
Bolts and setscrews are best to reuse Jaguar originals, subject to a physical check only for obvious thread damage.Much preferable and safer than using new, regardless of source, and of course ORIGINAL so no Concours deductions, or more importantly self-satisfaction.
There is a cost in cleaning/refinishing original bolts, so cheaper to buy new, but if you are doing your own car, time is of no consequence.

From what I’ve seen of the original Jaguar supplied bolts they are also “close fitting” - that is the grip is slightly larger than on a shop bolt so as to more precisely fit the hole in the flywheel. Here is a most informative chart for torquing various strength bolts. Note the difference with lubricants. ARP lubricant is not shown but it is slipperier than most and must be used with caution if you are not torquing an ARP manufactured part. You can easily over tighten it.