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Nut fatigue - failing cylinder head nuts


Waynebaby
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Hi All,

I've attached a photo showing one of my cylinder head studs and ~1/3 of the nut that used to be screwed onto it. This particular nut, which decided to drop to pieces this week whilst the car was stood minding it's own business on axle stands, is the second stud nut to fail like this since I fitted them 3 years and roughly 4000 miles ago. The nuts, and their associated studs are described as "up-rated" by a well known supplier whose name rhymes with a walking frame for the elderly.

As they're "up-rated" I've increased the torque on the studs to 80 lbf ft (using a calibrated wrench) from the standard 65-70 lbf ft specified in the WSM. The only other thing I've done differently is to use ARP fastener assembly lube on the threads and contact faces, as this is supposed to assist in getting an accurate torque value. Has anybody else experienced similar problems? Is it likely that my ~18% higher torque setting is causing my "up-rated" nuts that much stress that they give up the ghost like this?

Wayne

 

IMG_1431.JPG

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Not great is it.

But re the ARP lube. I used ARP rod bolts, and it gave at least 2 torque settings. The lowest was using the ARP lube, but using 30weight oil was a higher setting. There may have been another figure, but I can't remember. What I am saying is that with teh lube you may have got a rather higher stress on the nut than you anticipate.

I think some new nuts may be a good idea, though the "best" may be hard to find. Maybe try some of the Mini racing places as they use the same size.

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Thanks for all the replies. I can’t remember now why I equated up rated studs with a higher torque setting. It must have seemed like a good idea at the time! I’ll get some fresh nuts/washers and reduce things back to spec without the ARP gunge. Fingers crossed that the head gasket doesn’t object too much.

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Yes, Minispares nuts with integral washers are the business for Mk1 and 4 cylinder engines. The standard offerings are usually trouble free for the Mk2  6s with their sensibly sized studs provided the correct deep nuts and hardened washers are used.

Never seen a nut crack in half like that before - thread pulling out is normal failure mode.  Mind you, with the torques used AND special lobe you're lucky you haven't pulled the studs out of the block.  That would be messy!

Nick

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1 hour ago, Nick Jones said:

 

Never seen a nut crack in half like that before - thread pulling out is normal failure mode.  Mind you, with the torques used AND special lobe you're lucky you haven't pulled the studs out of the block.  That would be messy!

Nick

...the block top surface may well have been pulled up. Which may cause future problems.

 

Roger

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ive seen nuts crack like that due to the plating process gives hydrogen embrittlement if over or excess plating has taken place,

these look like an oil quenched finish so still reckon they have been  hardened to excess  like left  carburising  over lunch break ?   friday  nuts  ???

where our metallurgist when you need him  ????

Pete

 

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Having learned this week that the use of assembly lube on threads can lead to inadvertent over-torquing of fasteners it's got me wondering if we are systematically over tightening various components on our cars by using copper grease? I'd bet that most of us will slap this on all those nice shiny new suspension bolts before we tighten them up as per the manual. I wonder if there is a rule of thumb somewhere about how much to reduce a torque setting when using lube?

On the subject of metallurgy, when I worked in the nuclear industry copper grease was banned because it was found to encourage inter-granular corrosion on stainless components. Probably not quite so critical on a 1973 GT6!

Wayne

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on the plus side copper grease is an anti seize compound rather than a straight lubricant grease 

on truck audits any problem fitting we used a torque to yield meter which measures the climbing torque against the angular rotation, it blew a whistle   when  any one of the two

function got out of kilt,  brilliant for solving problems on funny fixings or oiled or dry threads but i dont remember the % changes now,  

i have a meter in the cupboard that does this , but its never used these days .....your a bit too far away to organise a trial on your nuts ( not the best phrase to use ) 

Pete

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When I had my Land Rover, it came back from the garage a couple of times with copper slipped wheel nuts - I always cleaned it off and re-torqued correctly as failure is well known as the increased effective torque. The only place for grease on brakes is on the mating faces between the wheel and hub.

 

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I'll be honest, the ONLY fasteners I use a torque wrench for are for engine assembly  (Rods, caps, head etc)

And remember, the torque is the turning force, and the lager the torque the greater the clamping force. But lubricants etc will massively affect the relationship between the 2.

I expect there is a table somewhere to compare clean/dry threads against various lubricants. But the ARB figures give some hefty clues.

And yes, if the head comes off, it may well need attention around the stud holes with careful use of a file.

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A quick trawl on the internet shows that the reduction in torque values when using lube is an inexact science because of all the variables involved, but seems to suggest a reduction in the range of 25-40%. I re-torqued my head nuts after 500 miles to the maximum specified for a Mk3 GT6 (80lbf ft). It seems that I should have been aiming for somewhere between 48 and 60lbf ft. 

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22 minutes ago, Waynebaby said:

A quick trawl on the internet shows that the reduction in torque values when using lube is an inexact science because of all the variables involved, but seems to suggest a reduction in the range of 25-40%. I re-torqued my head nuts after 500 miles to the maximum specified for a Mk3 GT6 (80lbf ft). It seems that I should have been aiming for somewhere between 48 and 60lbf ft. 

But did you remove the nuts and re-lube them when re-torquing? If you didn't - if you just backed off a quarter turn and re-tightened - then there would be hardly any lube on there so you probably want nearer book figure. Maybe.

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52 minutes ago, NonMember said:

But did you remove the nuts and re-lube them when re-torquing? If you didn't - if you just backed off a quarter turn and re-tightened - then there would be hardly any lube on there so you probably want nearer book figure. Maybe.

I agree with your thinking. At the moment I've replaced the failed nut with a new one and treated it as "dry" with a torque setting of 80lbf ft. (i.e. the top end of the specified range but closest to what the other studs may be subjected to) The rest I've left alone for now as I'm weighing up the options. At the moment I've got 13 o.o.14 head nuts which are probably over tightened on an engine which has run sweetly and without any head gasket leaks in this state for over four thousand miles. If I start de-tensioning all the studs am I going to end up with a gasket leak I wonder? If I do provoke a leak then it's head off time and facing the possibility of a block issue which will need addressing before I can refit a new gasket.  At the moment I'm tempted to let sleeping nuts lie until the next engine rebuild. 

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I think I'd also be tempted to leave well alone until/unless problems arise. That said, I don't think I did my GT6's head nuts up to top-of-range - more likely mid-range - and it's done five Round Britain Reliability Runs and a fair bit of other mileage in the 25 years since, without any hint of head gasket issues. Unlike the Mk1 Vitesse, before I switched to the Mini nuts.

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On 23/01/2019 at 15:48, Waynebaby said:

I'd bet that most of us will slap this on all those nice shiny new suspension bolts before we tighten them up as per the manual.

Never use it on the threads, myself. The shank, yes, where it goes through whatever tube or component that it's meant to move about in, but not the threads.

 

On 23/01/2019 at 23:18, Anglefire said:

The only place for grease on brakes is on the mating faces between the wheel and hub.

 

...and on the rear of the brake pads, or where the rear shoes touch the backplate.

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