I refitted the flywheel , the manual calls for 67ft/lb. of torque for the bolts.
Can anyone confirm this torque setting or recommend otherwise ?
Thank you.
Yep. 67 lb/ft. for the flywheel.
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New bolts, I hope?
Jerry
OK Jerry. Iâll take the bait. Why would you use new bolts on the flywheel? Is there something special about this application that requires new bolts? They are subject to cycles in shear [edit], but the tensile load is constant (just the stretch from tightening to recommended torque). The dowels probably take the bulk of the shear load, so I wonder how stressed they really are. Isnât the threaded hole in the crankshaft flange that they screw into subject to just the same level of tensile stress, and no-one suggests throwing away the crank every time you remove the flywheel? I think Iâve seen the same advice before, but never an explanation. If this is just because of âan abundance of cautionâ I think I might worry more about the quality of new replacements more than I would about re-using sound-looking originals. Now Iâve blown it! My flywheel will fall off the next time I start the carâŚ
-David (who doesnât always replace his flywheel bolts 
)
I did not replace the bolts (did not give it any thought 'till now) , but David makes a valid point.
All bolts at 67 ft./lbs. but I did not bend the tabsâŚ
Iâll wait to see if anyone else weighs in.
Thanks for the replies .
Mark.
I did not replace the bolts (did not give it any thought 'till now) , but David makes a valid point.
All bolts at 67 ft./lbs. but I did not bend the tabsâŚ
Iâll wait to see if anyone else weighs in.
Thanks for the replies .
Mark.
David,
They are subject to cyclical shear at high temperatures and thatâs 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) Consider how many times the clutch is released with great vigor! I donât think there is any other bolt on the car that suffers as much as these.
Porsche requires replacement of these bolts every time they are loosened; of course these are modern stretch bolts that really will fail if they are not changed, not true of the XK bolts.
As far as bolt quality, do you still have the original flywheel bolts? Or are they replacements from a more recent date? If old, they have undergone a whole lot of cycles in 50 years; if new, why would the current replacements be less likely to be good quality than the ones on there now?
I think itâs the bolt shaft that is going to be affected by the cyclical stress rather than the threads, plus the flange is a large forged steel part.
I donât think itâs likely that your flywheel will fall off! But me, Iâm changing those bolts every time I loosen them (every 10-15 years or so).
YMMV
Jerry
Jerry,
Thanks for the detailed response. Hopefully it will allow the OP to decide whether to get new bolts or not. Can anyone recall whether the âboltsâ are actually âboltsâ or âsetscrewsâ in Jaguar terminology? By that, I mean do they have an unthreaded âgripâ area adjacent to the head (this would be the part that passes through the flywheel), or are they threaded all the way to the head? It they are threaded all the way to the head, I would have thought that the tight-fitting dowels are taking most if not all of the shear load, and the bolts are mostly there to clamp the flywheel to the crankshaft, and loaded mostly in tension, and not subject to cyclical loads. If they have a grip section, I would agree that they may see more shear, as they will be a tighter fit in the flywheel holes.
As for my personal cars, I checked my records and find that I did indeed use new bolts when I removed my flywheel on the XK140 in 2005. I think I may have been suspicious of the quality of the bolts I removed. I did not replace them when I removed it again in 2016. Iâve never removed the flywheel on my E-Type. I bought my new XK140 flywheel bolts from XKs (C.4855). The web site illustration seems to show them as having an unthreaded section, though given that there are 10 of them, I suspect they are not a particularly tight fit in the flywheel holes as this would require very tight manufacturing tolerances. If you do replace the bolts I would recommend getting the âspecialâ bolts from a reputable supplier. I wouldnât substitute a grade 5 or grade 8 standard bolt. If fatigue is the main issue with these bolts, a âhigherâ grade is not necessarily better.
As a final cautionary note: you do NOT want to have your flywheel part company with the crank when the engine is running. Google for horror stories. 30+ lbs of spinning steel can inflict a lot of damage, and your bellhousing will not necessarily protect your legsâŚ
-David
In fastener texts, David, the term is âshearâ. In lingerie catalogues, not so muchâŚ
The edit function works a treat BTW
[Edit: It does indeed, as I now see your second post conforms to the Queenâs English wot we woz both brought up with.]
OK Jerry, Iâll bite the bit that David spat outâŚ
Someone should ask Norman (whereâs Ginger when you need her?) but I canât imagine Frank Rainbow et al carrying spare new bolts around when they were doing pit work at Le Mans, (and 24 hrs is temp and cyclical test enough for me). I donât doubt your argument in todayâs terms, but that was then and this is now.
An advantage of old-school engineering was they left more wriggle room, perhaps, through not knowing how close to material limits they sometimes were? Kinda âunknown unknownsâ? [(c) D Rumsfeldt] Today they might design closer to material and usage limits, but then?
An interesting data point is that on the unburstable 1984-1997 Jag sixes (including the 322 hp/4100lb supercharged cars), the German (?) transmission designers felt they only needed 8 bolts not ten, and no/nada/zero dowels.
To continue along what Jerry started, IMO, the bolts and dowels should take no shear force. The bolts simply clamp the two parts together. The friction alone prevents slip. The dowels locate the flywheel. The shoulder of the bolt probably never touches the bore of the hole.
In terms of shear and torque how does this apply to the 4 bolts at each of the drive shaft ?
There are only 4 , smaller and no dowel .
Iâm not expert , just trying to understand.
Thank you.
The back of the flywheel has a recess that matches the flange of the crank, locating the two parts.
Thanks Pete,
sheer stupidity on my partâŚ
-David
Tommd55[quote=âTommd55, post:11, topic:351330â]
The bolts simple clamp the two parts together. The friction alone prevents slip.
[/quote]
I tend to agree, as long as the bolts are tightâŚ
Same issue as far as I can see. If the drive shaft bolts are tight, the flanges will be clamped together, and friction will prevent the bolts seeing significant shear forces. If these bolts get loose, the prop shaft will thrash about and the bolts will fail in shear in short order. Usually the accompanying cacophony will result in investigation before anything breaks looseâŚ
-David
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The flywheel bolts are a unique item - they have a part number in the catalogue, and look unlike anything on a jobbers shelf. So if you buy new ones where do they come from? If from Jaguar Iâd think they would be of good quality. If from an Asian source through a jobber Iâd be worried. Bolt counterfeiting is a very real and common problem, the problem going to the quality of the steel used. Itâs serious enough that suppliers to the aircraft industry need to certify the bolts they sell as to origin and quality. Not trying to keep you sleepless!
Hi,
In general terms, thatâs what I would call âoverkillâ just like the standard driveshaft and U-joints on the E-type, the XJ12 with more torque and more weight managed fine with finer and thinner items.
I think the only British engineer not to use some âsafetyâ factor in his calculations was Colin Chapman. He instead thought of something that just might work. And in many cases they did, but not always, mind you. 
Cheers,
Pekka T. - 1S20183
Fin.
Pete,
Itâs number of cycles that brings on fatigue, and I donât think any of those engines would have had very many.
Jerry
They why have he dowels and special bolts for this application?
Jerry
I agree with David, the bolts clamp, friction prevents slippage.