Torque settings

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Tintin
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Torque settings

Postby Tintin » Thu Nov 16, 2017 2:40 pm

A scattering of torque figures for the more important fastenings is available from the W/S manual, Haynes and other publications but from time to time there seems to be a gap in published information. Is there a comprehensive torque setting table out there somewhere for the series XJs or even the XK engines? This has come to the fore again for me as I search for an authorative figure for exhaust manifold to cylinder head nuts (brass). For the AJ16 engine the manual specifies 18 lb. ft. but that feels quite high for the XK engine, and I'm sure the old ones weren't that tight when I took the manifolds off.

Thanks.

Robbie
1996 4 litre XJS Celebration 2+2
Daimler Sovereign S1 4.2 1971

mike020150
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Re: Torque settings

Postby mike020150 » Fri Nov 17, 2017 12:30 pm

I think you'll find that unless Jaguar published a figure in the workshop manual, you are limited to common sense. Sometimes, you can find a torque figure in the series 3 manual which doesn't appear in earlier manuals. Certainly, my series 1 and 2 manuals don't list a torque setting for the exhaust to head nuts but sadly, I don't have a copy of the series 3 manual to hand. Mike.
2014 XK Dynamic R convertible; 1977 Daimler Double Six Coupe; XE S

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Tintin
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Re: Torque settings

Postby Tintin » Fri Nov 17, 2017 2:02 pm

I found this on the e type forum, which whilst not component specific does provide some measure of guidance.
The majority of bolts on Jaguars are UNF, or UNC where they go into alloy . 1/4 UNF takes a 7/16 AF spanner ; 5/16 is 1/2 AF ; 3/8 is 9/16 AF ; 7/16 is 5/8 AF . If you take the following as dry settings you will not go far wrong , and take 20 to 25 % less if for any reason you are lubricating the threads ,

7/16 AF 12 - 15 lbs.ft
1/2 AF 17-20
9/16 AF 30- 35
5/8 Af 55
The manifold nuts on my engine are 1/2 AF spanner size and using the above chart that gives 17-20 lbs ft, but that appears to make no particular allowance for the metal concerned - brass nut/carbon steel stud/alloy head. I would have thought that a lower torque was appropriate for a softer metal.

The downpipe to exhaust manifold nuts are 9/16 AF spanner size (also brass) and following the table above should be torqued to 30-35 lbs ft. However, the manual gives an actual figure for these nuts and it is significantly less at 25 lbs ft. (presumably to protect the ears of the brittle casting of the manifold to pipe flange). Again, the metal involved would appear to be a factor in selecting the appropriate torque figure. Perhaps a reduction of 25% would also be appropriate for softer and brittle metals.

Given the number of queries raised (and not or only partially resolved) on the internet on this subject there would appear to be scope for someone with an engineering background to produce a component specific guide. I for one would welcome it!

Robbie
1996 4 litre XJS Celebration 2+2
Daimler Sovereign S1 4.2 1971

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Tintin
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Re: Torque settings

Postby Tintin » Fri Nov 17, 2017 4:26 pm

Well, in the end I tightened the nuts until it felt "right". Putting the torque wrench on it afterwards indicated that "right" in this case was about 10-11lbs.ft. Started the engine and all was well - no leaks from the manifolds or downpipes - nevertheless I will re-check tightness after a couple of hundred miles.

I must say it was great to hear the engine spark into life again and purr away (first time since returning from a JEC Normandy trip in late September). Also good to have the engine start and idle easily at the correct rpm on the adjusted AED (no throttle pedal necessary). The repainted manifolds are a big improvement too. That's another job to tick off the list!

Robbie
1996 4 litre XJS Celebration 2+2
Daimler Sovereign S1 4.2 1971

PaulGover
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Joined: Mon Jul 19, 2010 8:06 pm

Re: Torque settings

Postby PaulGover » Fri Nov 17, 2017 5:41 pm

I suspect, but haven't checked, that in the majority of cases the workshop manual torque setting is the same for all nuts/bolts of the same size thread, exceptions being where the material is not the normal steel used for >90% of the fixings.

Some time ago I read about the heavy engineering for big ship engines; the book commented that for small engines, an experienced engineer would know when a nut was properly tightened because he/she could feel the transition. If I remember correctly, it starts by closing the fitting gap, then the bolt stretches, the torque rising in proportion, reaching the yield point where the bolt would lengthen but the torque remain static. The correct torque is the yield point. Exceeding it causes the bolt to permanently weaken. That's what's happening with cylinder head bolts that say "so many ft-lb torque plus so-many-degrees turn", which is why you have to throw them away if you take the head off; they've permanently stretched.

The point about this is the yield point is a function of the steel used to make the bolt, its cross-sectional area, and the pitch of the thread, hence my comment at the start. Getting the required clamping force is a matter of using the correct number and size of bolts, not of tweaking the torque used. Actually the friction between the nut/bolt face and the surface being fastened makes a big variable, which means mucking about with torque wrenches is much less scientific than it appears, since the use of oil or copperslip changes everything.

It went on to say that modern big ship engines this was no longer possible, as no human could turn the spanner, and they usually had a hydraulic engine specifically for turning the nuts. (These beasts were 4ft bore by 12ft stroke V24, turning at 180 rpm or somesuch megafauna).

There's are informative write-ups at http://www.boltscience.com/pages/tighten.htm and http://www.appliedbolting.com/pdf/yielding.pdf
Last edited by PaulGover on Fri Nov 17, 2017 5:47 pm, edited 1 time in total.
1981 XJ6 Series III basket case
1991 XJS V12 "facelift"
2008 XJ TDVi

mike020150
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Re: Torque settings

Postby mike020150 » Fri Nov 17, 2017 5:43 pm

All that remains is to wash and polish her and she will be ready for XJ 50! :) Mike.
2014 XK Dynamic R convertible; 1977 Daimler Double Six Coupe; XE S

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Tintin
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Re: Torque settings

Postby Tintin » Sat Nov 18, 2017 10:33 am

What you say is useful and very interesting, Paul. My concern however is not so much with the carbon steel bolt/stud but the non carbon steel component it is threaded into or clamping. In my youth I have cracked more than one alloy casting and stripped more than one alloy tapping with the over enthusiastic use of a long handled spanner - a lesson learned the hard way!

Another common query on line is the appropriate setting for the alloy cam cover nuts - it is tempting to over-tighten to seal the cover/head gaskets against oil leaks. The carbon steel studs and nuts will take it but I imagine it would be easy to crack the covers. Personally I use 7 lbs. ft (10Nm) or thereabouts - again significantly less than the table I copied earlier form the e type forum.

Robbie
1996 4 litre XJS Celebration 2+2
Daimler Sovereign S1 4.2 1971


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