The 'E' Type Forum

On the big end bolt/nuts....as I am not replacing the split pin version and using the later type from the XJ6 I believe...(am told the split pin types ultimately fail and bits of the pins end up in the oil pump...)

Is Loctitie 243 enough or should it be a stronger compound...

Thoughts please

Regards
Jonathan

Jonathon, could you try chatting to engine overhaul companies and ask what they use?

Geoff

I stand to be corrected on this but I vaguely recall big end nuts are 'special' and not re-usable because they compress slightly when torqued correctly and so dont come undone.

If we assume that I am actually wrong - I use Loctite 601 to fix studs etc which takes around 25 minutes to cure and is supposed to be permanent but I have found I can undo the stud (or nut) with a bit of additional leverage :)

Loctite 270 is also pretty good - its stronger than 243

Hi Johnathan

I would use a high strength Loctite on big end bolts. Preferably 270 as it would probably be easier to dismantle the assembly when necessary. 601 is an excellent stud lock but might be a little too difficult to disassemble without using heat.

Regards

You shouldn't be using any sort of thread locking compound on these bolts.

If you have replacement bolts they should have come with the correct self locking nuts.

just do them up to the correct torque and forget them.

I also agree with Andrew . These are paradoxically low torque fixings ( the factory gives 37 lb-ft as the correct setting ) and personally I have never used a thread locker on them . Quite apart from anything else, the wet loctite will provide a degree of lubrication during torquing up which will affect quite significantly the actual ( as opposed to indicated ) torque you achieve and may result in over-torquing the bolts

If you intend to use the stuff I would ask Loctite for their advice. Their products differ in heat resistance as well as shear strength. probably a few other factors also.

Thanks guys, interesting varying views...having spoken with Ken Jenkins in the meantime at the JEC he was advocating a loctite compound...
I intend to re-use the original bolts and just use the new nuts from SNG

Regards
Jonathan

Hi Jonatha,

Why don't you change the both ????
It's not really expensive and difficult to change if you have a problem after ...

Mich

Hi Mich

I certainly could change both...but I have not read that new bolts are recommended...just new nuts

But would be interested in whether folks on here would change the bolts as a matter of course....?

This would still not answer the question though whether to use or not to use Loctite was preferred....

Regards
Jonathan

Jonathan,

Andrew, Christopher and I had rebuilt engines and we didn't used any Loctite for these bolts.
It's 3 answers for one question...

Mich

Hi Johnathan

I thought the question was about which grade of Loctite to use, not whether to use it or not. That is the question I answered on the basis that you were not going to use any means of mechanical locking.

Personally, I wouldn't use Loctite but would use new nuts and bolts and torque them up properly. Penny pinching on critical items is inviting problems, in my view.

Regards

Bear in mind also that chemical locking compounds were not available in the 60's to the extent they are today. Modern Harley Davidsons, for example, specify Loctite all over their engines (indeed over the whole bike) where in the old days they would probably have used split-pins. Might as well use a modern solution if one is available.

265bhp wrote: I intend to re-use the original bolts and just use the new nuts from SNG

Regards
Jonathan

Don't. The big end bolts take a pounding.
Replace them with new proper Jaguar ones. Don't use ARP ones as they have a shoulder on them that fits the rod and the ARP ones are not shaped correctly.

Big end bolts - any bolt for that matter - act like a spring. They stretch when you torque the nuts up & pull everything together like a strong tension spring. They will retract when slackened but at some point they will lose their 'springiness' & need to be replaced. This will vary according to specification, metallurgy and all sorts of things. No idea what Jaguar policy is on this.
The old Triumph motorcycles never had a torque setting for big-end bolts. You had to measure the length & tighten them till they stretched 1/8".

Thanks chaps...no loctite and new bolts then..
Best
Jonathan

Hugo wrote:The old Triumph motorcycles never had a torque setting for big-end bolts. You had to measure the length & tighten them till they stretched 1/8".

Can I have some of what you're smoking Hugo? :-)

I've built every type of post-war Meriden Triumph engine except the Tigress Scooter. AFAIK Triumph manuals gave rod bolt torque specs for all of them with shell big ends, i.e. except roller bearing pressed cranks like the Sports Tiger Cub.

I agree with the stuff about torque vs stretch, except that anyone trying to extend a roughly 2.5" bolt by 1/8" is going to strip or snap every one, as that is orders of magnitude too much. The stretch is a typically about 2 thou per inch - i.e. 4-5 thou total(ish), not an eighth (125 thou).

No idea where I got the 1/8" from. If I'd thought about it for a second I would have realised it was wrong. Thank you for picking me up on it. But the principle is correct - I found these interesting articles on a British bike forum while checking it out;

"... there is a process where you heat the metal to a critcal temperature. At this temperature the metal hardens almost glass hard. Now a glass hard rod bolt would not be a good thing so we have to anneal (sometimes called tempering) the metal which will soften it to the hardness (and/or toughness) we are looking for.
cool NOW HERE IS THE COOL STUFF:
Now there is another process that will harden the metal without heat. It involves bending or stretching the metal. We have all broken a coat hanger wire by bending it. It is said we work hardened the wire to a point it was glass hard and it broke.
Well, rod bolts (spokes or anything that is stretched in tension with a rapidly changing cyclical load) will work harden in use from the repeated stretching and releasing. A rod bolt is streched at TDC (in tension) and pulls itself back to original length at BDC (compression).
This constant changing of the molecular structure will harden the rod bolt (spoke, etc) until it approaches being glass hard, at which point it will fail or break.
All this mayhem can be avoided by pre-stretching the rod bolt (spoke, etc.) more than it will be stretched at maximum load in tension (stretching). The proper rod bolt stretch is determined by calculating the expected stretch from the load of the piston and rod trying to launch themsleves to the moon at TDC and tightening the rod bolt nut so that the bolt has been pre-streched more than the rod and piston can stretch it. This keeps the molecules in the bolt stable and the cyclical stretching is avoided. This is why it is so important to tighten critical hardware to the manufacturer's specifications. When you torque a bolt you are pre-stretching it based upon any expected cyclical tension load calculated by the engineers who designed the motor.
A typical Triumph rod bolt should be pre-stretched between .004 to .005."
So if we do this right, if we torque the rod bolt to 28 ft. lbs the bolt should stretch between .004" and .005."
All I can say about the 22 ft. pound figure for the 750 twin is Brian Jones must have recalculated the loads at TDC and determined that the pre-stretch at 22 lbs was enough to keep the rod bolt from hardening. (where is panic when you need him?)
ARP (American Racing Products) who make rod bolts for 7,000 hp nitro-burning drag motors doesn't even give a torque figure for their bolts. All they give you is bolt stretch figures. ARP has a web site and if you poke around the back of the catalog you will find more than you will ever want to know about rod bolts. If your serious about your engine building it will convince you to throw away your torque wrench."

And this;
"Threaded fasteners have two different strengths by which they are measured. Yield and Ultimate.
The Yield strength is the point at which the fastener will not return to it original length after being tensioned.
If the fastener has elongated past its original length it has reached at least 100% of yield and is no longer "elastic" . It will be unable to maintain a constant gripping force.
The ultimate strength is the point at which the fastener will fail and break.
Torque is just an a approximate amount of rotational force required to achieve a given percentage of Yield or Ultimate strength.
Most everything on our bikes is set up for yield strength, no break away bridge spans or Space Shuttle applications on the Britbikes.
In heavy industry most vessels and mechanical assemblies are comfortably engineered and sized with fasteners operating at about 50% of yield. Some more, some less. This allows for the range of thermal and dynamic expansion that may/will occur. If you get too close to 100% of yield the faster will be garbage and unable to sustain the preload.

Simple formula for amount of torque required is...
KxDxPxYSTxAS
Where....
K is the "nut" factor or friction coefficient
D is the diameter of the fastener
P is the percentage of yield you wish to achieve
YST is the engineered yield strength of the grade of fastener being used
AS is the surface area of the threaded fastener calculated by root, pitch and threads
per inch.

There are tables and charts for all these variables, you just need to look them up and plug 'em in.
Calculating a target elongation is something I'd have to look up, been a while, but involves TPI and the length of the captured load - Nut face to nut face or nut face to flange
Does this make any sense?
I assembled bridges, steelmills and powerplants thruout North America for Westinghouse PGSD in the 80's using an Ultrasonic Extensometer along with 10,000psi hydraulic tooling.
We measured "stretch" to the .0000 in order to achieve the perfect load on critical applications. Ft Lbs in excess of 125,000.
Surface prep is everything, clean fasteners that will spin up snug by hand are essential and an evenly applied load is fundamental.
Don in Nipomo bigt

Hmmm....some of the theory is ok, but there is a fatal elementary mistake which does not give confidence that the writer of the first extract really knew much about it. The maximum tensile load on a connecting rod bolt does not occur at TDC, but at the point in its travel on the exhaust stroke where the maximum point of linear deceleration occurs ( which produces the highest g force on the bolt/piston/rod ) .This of course depends on a number of variables , such as stroke, ratio of con-rod length to stroke, and engine speed .Similarly the compressive load is at a maximum when the rate of expansion of the burning mixture is maximised ( I think ) and this also depends on a number of factors including ignition timing, valve overlap and the other structural factors I have mentioned.