How much force is holding my head down?

abowie · Post by **abowie** » Mon Dec 08, 2014 9:51 pm

If my head studs are tightened to 56ft lb, how many psi is that?
I guess what I am trying to calculate is how much tension there is per stud, but I can't work it out.

norman m. macleod · Post by **norman m. macleod** » Tue Dec 09, 2014 2:31 pm

Hi Andrew,
Greetings from the West.....Can't imagine what you'll do with such information, but here goes anyway.....At 56 lb.ft. applied torque on a 7/16" XK engine head stud, there would be an axial force applied of some 7500 lb. This, however, is for dry thread conditions, and with a light oiling of 30 grade oil, the axial force could be up to 10 - 15% more, say around 8400lb.
I am unsure what you mean by your reference to psi, as if one sums the total of the 14 bolts and uses the total contact area between the head and the block to get some sort of average psi value, it is not very useful, as there is a higher loading concentration around the studs, it is far from even. It can be improved some by using thicker hardened and ground washers, but the standard tuppence-ha'penny chromed items from the usual suppliers these days are not really up to it.........

Best regards,
Norman.

D COUPE · Post by **D COUPE** » Tue Dec 09, 2014 3:31 pm

Top answer Norman that will teach him to ask questions!! Do also bear in mind You are upside down........

DC

mooney1el · Post by **mooney1el** » Tue Dec 09, 2014 5:36 pm

Does that mean that the block is pushing up on the head rather than the head pushing down on the block. :? :)

Post by **Heuer** » Tue Dec 09, 2014 5:43 pm

abowie · Post by **abowie** » Tue Dec 09, 2014 9:18 pm

?ɐlnɯɹoɟ ɐ ǝɹǝɥʇ sı ?ʇɐɥʇ ǝʇɐlnɔlɐɔ noʎ pıp ʍoɥ ˙uɐɯɹou sʞuɐɥʇ

˙ʎuunɟ oooos ǝɹɐ ɥsılƃuǝ noʎ

PeterCrespin · Post by **PeterCrespin** » Thu Dec 11, 2014 10:43 pm

I assume it is a function of the angle of the threads along the stud multiplying the rotational force applied less the various frictions? You can measure the clamping load directly if you know the stretch of the stud and its tensile strength.

On the other hand, as one of my teachers once told me, "If you can't do anything useful with the answer, it may be better not to bother asking the question..."

abowie · Post by **abowie** » Fri Dec 12, 2014 2:36 am

PeterCrespin wrote: On the other hand, as one of my teachers once told me, "If you can't do anything useful with the answer, it may be better not to bother asking the question..."

We'd have never got to the Moon thinking like that :-)

Soo... 14 studs at 7500lb apiece is about 100,000lb. The head is about 24" by 8", say 200in2, minus the area of the six combustion chambers (about 15in2 each) so say 100in2 to make the maths easy. So 100,000lb over 100in2.

The answer to my question is about 1000psi or only about 70bar which is less that I expected. I was trying to get a feel for what the pressure in the cylinder would be on detonation and assumed it must be substantially less that that holding the head on. I could work out that you get 10bar on the compression stroke alone but had no idea how to calculate it from there.

norman m. macleod · Post by **norman m. macleod** » Sat Dec 13, 2014 12:17 pm

Hi again Andrew,
How did I calculate this figure, you ask.....I didn't. Being an engineer of some threescore and ten years, I have a sizeable collection of Great Bookes and Tomes that will give this sort of stuff at the flick of a few pages............
Having said that, if you'd wished for some reason (?) to calculate a value, the following simple formula would suffice:-

Axial Preload=Torque(lbf-ins)/ K x thread diameter. In this case:-
= 672/0.2 x 0.4375 = 7680 lbf.
The "K" value (or "Nut Factor") is determined by a somewhat complex formula which is supposed to take account of a number of other factors affecting the case. As with so many of these formulae, it is partly theoretical, but with a substantial empirical content. The above K is for steel on steel, a K value of 0.15 - 0.16 would be applicable for lightly oiled or cadmium plated threads. Incidentally, K is not the coefficient of friction.
The value thus calculated correlates quite well with the figure of 7500lbf I gave you earlier.
I must confess, however, that I am no further forward in understanding what anyone would do with the figure so gained :?
In passing, I'd be fascinated if Peter could explain how one measures the stretch on a stud with one end buried deep in the bowels of a long-stud XK engine. Big-end cap bolts and the like, yes, but...... :( Incidentally, Andrew, whilst on the subject of those studs, and clamping forces, etc., it is well to remember that a large percentage of these are 45 years old, partly corroded, and have been subjected to tens of thousands of heating/cooling cycles. What I am saying is that they do not possess the tensile strength and the modulus of elasticity (and consequently the consistent torque retention) they were born with. As a matter of course, I chuck them in the bin and replace them with ARP items. Each to his own...

Hope this clarifies....And before I forget, bear in mind that the clamping
pressure over the entire block/head face is not in any way indicative of the pressures extant in the narrow insert rings in the gasket, around the bore and combustion chamber apertures. These are much higher.....
By the way, it is relatively simple to calculate the bmep (Brake Mean Effective Pressure) in the cylinder during combustion, but I'd rather not go there, if you don't mind.....

:) :) All the best from the West
Norman.

abowie · Post by **abowie** » Sat Dec 13, 2014 9:12 pm

norman m. macleod wrote:As a matter of course, I chuck them in the bin and replace them with ARP items.

Which is exactly what I do as well. $200 well spent.

norman m. macleod wrote:By the way, it is relatively simple to calculate the bmep (Brake Mean Effective Pressure) in the cylinder during combustion.

Now I know what it's called I can look it up. Many thanks for your help.

PeterCrespin · Post by **PeterCrespin** » Sun Dec 14, 2014 12:13 am

Norman, I wasn't saying one should calculate stud stretch, merely that one can in some cases. For a fastener accessible at both ends it's easy enough as you know, but not very common. For a head stud you could mount a dial gauge to the head and measure the tip extension through an open nut, for short or long stud blocks. I would never bother. Theoretically, I suppose a tiny bit of the extension would be due to gauge and head movement/gasket compression but the whole topic is already somewhat 'angels on the head of a pin'...

Andrew, I'm not sure why you want to bother with BMEP as it is a low number since it is the mean pressure over a range of conditions in the cylinder. It's nothing like the detonation pressure / clamping load you were trying to figure out. BMEP is effectively a 'performance' statistic predictive of torque, not a 'structural' figure.

David Oslo · Post by **David Oslo** » Sun Dec 14, 2014 11:13 am

If wishing to make this rather humourous (I didn't say nonsensical) calculation a tad more accurate, one would use the surface area of the head gasket, as opposed to head area minus piston openings. :D

norman m. macleod · Post by **norman m. macleod** » Sun Dec 14, 2014 11:13 am

Andrew,
I think I'll draw a line on this one, before we get too many professors on the head of a pin........

The typical combustion chamber/piston crown pressures (just after TDC on the firing stroke) for an engine of the XK type are around 300 psi on light load, and up to about 900 psi on full load with throttles wide open. The latter will occur at the maximum torque revs of the engine. this assumes engine in A1 condition, accurate ignition and valve timing, and a CR of around 9:1.
I've just had a look at your website.....Wow.....impressive.....! :o

Best regards,
Norman.

David Oslo · Post by **David Oslo** » Sun Dec 14, 2014 11:42 am

Good call, draw the line. Trust you did see tongue in cheek aspect of last post.

Final 5-cents. One should look at local forces and components involved. It is more correct to compare piston combustion force (in one cylinder) with the 'local' part of head reacting to those local forces. Hence I would use only the bolts relating to one cylinder (given only one fires at a time) and the local portion of the head (and gasket). So the big psi figure at the start of the thread can be divided by 6.

Point is the head (and all it's bolts and gasket areas) holds on 6 cylinders, firing one at a time. So to use the total head values in comparison to only one combustion pressure is off by a factor of 6.

Don't blame me, I only an engineer :D

mike837go · Post by **mike837go** » Mon Dec 15, 2014 9:12 pm

ARP head bolts!?

Gotta add them to the shopping list for reassembly!

abowie · Post by **abowie** » Mon Dec 15, 2014 9:47 pm

mike837go wrote:ARP head bolts!?

Gotta add them to the shopping list for reassembly!

They really are quite nice and only about twice the price of standard ones if you get them from XK's in the US. The guys I work with have been using them routinely in rebuilds for the last couple of years.
Only difficulty I have run into is getting a correct length dowel stud for my 3.8. These are designed to have the engine lifting brackets (not fitted on the 3.8 ) attached and so are a tad long. Shorter ones do not seem to be available. I ended up compromising by putting 2 extra washers under mine as I wasn't confident that cutting the stud down would be a good idea.

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