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XJ40 1986 - 1994

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  #1  
Old 01-30-2011, 04:31 PM
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Default Replace head bolts?

I am getting conflicting advice about re-using existing head bolts during a head gasket replacement. 148,000 miles, no visible corrosion ('92 xj40 4,0).
What does Jaguar recommend to dealers??
Also, is graphite paste or never seize on threads a good idea or would that affect torque readings or tend to have bolts back out?
Thanks in advance.
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Old 01-30-2011, 07:28 PM
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This is what we were issued way-back-when....

bob gauff
Attached Files
File Type: pdf 12-62AM Cylinder Head Bolts.pdf (30.1 KB, 125 views)
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Owning and repairing British and German cars since the early 70's

2001 XJ8 Mistral
1999 XJ8L Topaz
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1992 Sovereign (sold)
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Old 01-30-2011, 09:01 PM
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Shelby, You need to look in the repair manual for what they recommend for the bolts. Nomally they will specify whether lube is to be used or not and if lube is required, what to use. Yes, adding anything to the threads does affect the torque to which the bolts need to be torqued. Even something as simple as water can affect it. If the bolt is designed to be put in dry (ie, no lube), adding lube and then torquing it to the called out torque can lead to over-stressing the bolt. I just don't have the necessary information to make an educated calculation on this. If you can feed me the information about the required lubes and what you are wanting to use, I can do the calculations to make sure you get the proper compression of the head gasket.

Also, keep in mind that you need to know if the bolts are "torqued to yield" bolts or not. Torqued to yield bolts are only designed to be tightened one time. Normally you can tell these bolts from others by how they are torqued. In most cases, they will be called out to be torqued to a low value (around 40 ft-lbs) and then directed to be tightened an additional amount (1/2 turn or so), regardless of the torque required to achieve that distance. So, keep that in mind too. By using them a second time, you can stretch the bolt beyond what they are meant to be stretched and can lead to catastrophic failure upon starting the engine. Something about a 40 pound head lifting off the engine doesn't sound good.
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Old 01-31-2011, 08:47 PM
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Thanks to both Thermo and motorcarman. I am going to assume original bolts can probably be reused but any replacement bolts sold are of the TTY variety. I have a pint of graphite grease I like to use where two dissimilar metals are in contact, and may need to be taken off in the future. Only have a Haynes repair manual, doesn't spec "wet" or "dry" for head bolts, any word on Jaguar's recommendation?? Thanks again.
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Old 01-31-2011, 08:49 PM
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Shelby, if it is not specifying a lube, more than likely they are wanting you to install them dry.
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Old 01-31-2011, 09:00 PM
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The headbolts are REUSE TWICE. (see TSB)
In the 10 or 15 years I have re used the bolts I have snapped a bolt.
It was on the final torque and I did NOT want to remove the head and start over, so I put a dab of 2 part epoxy on the broken upper part and re inserted the bolt into the hole, turning it until the segments lined up. I waited until morning and slowly unscrewed the bolt and removed the bolt along with the broken lower threaded part. I installed another used bolt and finished the job.

Yes I got lucky.
Sometimes it's luck and sometimes it's just 'calm down and THINK'.

It's also good to have 30 years of collecting used Jaguar parts.

bob gauff
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Owning and repairing British and German cars since the early 70's

2001 XJ8 Mistral
1999 XJ8L Topaz
1985 XJ6 Belgian Import (Gray Market project car)
1985 XJ6 (Parts Car)
1983 XJ6 (Parts Car)
1992 Sovereign (sold)
2001 S-Type 4.0 (sold)
1997 XJ6L Black (sold)
1997 XJ6L White (sold)
1972 XJ6 (sold)
1966 'E'Type (first Jaguar) (sold)
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Old 01-31-2011, 10:25 PM
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Ok, here's the skinny.....

AJ6 headbolts are [were] always torque to yield bolts. That bulletin you'll notice is dated 1997. That is smack in the middle of the period of time the XJ40 headgaskets were failing due to a modification they made to the head gasket at an oil passage. Well, the gasket modification [a cost cutting move, BTW] caused an oil leak at the passage just ahead of the #1 cylinder exhaust port. Head gaskets were dropping like flies. About a year into all the gasket replacements, the bulletin about using torque to yield bolts twice, showed up. I questioned the technical folks repeatedly about the back peddling. One of the guys finally called me back and owned up to the fact that auditors "ran the numbers" and it was decided that given the large quantity of repairs they were buying, specifying a second use of the bolts saved them a lot of money. And secondly, any rare repeat repairs they had to assume due to a bolt failure, were an acceptable risk given the original savings.

Needless to say, if I do a head gasket on an XJ40, or and X300, it gets new bolts. I ain't likin' the risk.

Just my thoughts......
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Old 01-31-2011, 10:25 PM
 
 
 
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Tags
40, aj27, bolt, bolts, broken, cylinder, gasket, head, jaguar, readings, replace, replacing, studs, torque, xj40



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