Head Stud Torque Check - Hot or Cold?

[IslipMinor]

I have now done ~700 miles since the head gasket was changed, and with a new rocker cover gasket to hand, am about to check the head torque for the final time in the sequence.

I have always checked the head stud torque 'hot' with a cast iron head, but have seen comments recently that made me think if that is right? Equally with an alloy head always cold - but why?

A little research provides some potential logic for whether head studs should be checked hot or cold, and whether the head/block/stud material combinations make a difference. The conclusion is that the block material does not affect the decision, but the combination of head stud/bolt and head material certainly does.

It's all to do with differential expansion, and at what point in the heat cycle is has an effect - if you know all this then stop reading now!

Standard A-Series with steel studs and cast iron head - steel expands slightly more with heat than cast iron (~10%), so when hot the studs will expand more than the surrounding head and this is the point that the torque should be checked. As is cools the clamping force will increase, so no problems. Torque them 'cold' and when hot the studs expand more than the head, and the clamping force is reduced, just when you need it most!

Conversely with steel studs and an aluminium head, with increasing temperature, the head will expand more than the studs (+100%) and for this combination the torque is checked 'cold', so that when hot the head tries to expand more than the studs thus increasing the clamping force. Torque them 'hot' and as the engine cools, the clamping force reduces with potential leaks around the gasket.

Seems to make sense - what are others' opinions?

[bmcecosse]

I never touch it after the first heat cycle - feeling it's better to leave well alone.

[philthehill]

Whilst the theory may be correct I doubt that it has little effect on the holding effort whether the studs are tightened hot or cold provided the faces of the block and cylinder head are true.
Yes - the torque of the cylinder head nuts should be checked after the first heat cycle.
Non of the BMC/BL workshop manuals I have give a recommendation to re-torque when hot.
More importantly to me is whether the studs/nuts are lubricated and what they are lubricated with.
Different lubricants call for different torque settings.
Whilst the above method is my preference - if it is felt that re-torqueing the nuts when the engine is hot is of benefit by all means do so.
Most modern cars with alloy heads are torqued by degrees and then only the once as the majority use stretch studs which have to be replaced when the head is refitted.
Phil

[IslipMinor]

Checked them at short while ago and all 4 main (3/8") rocker pillar studs pulled up a bit, but none of the others. Tappets only needed a tweak here and there.

They are ARP studs with ARP's 'ARP Ultra-Torque Fastener Assembly Lubricant' applied before anything was tightened up after the gasket change. The recommended torque with their lubricant is 45 lbf. ft for the 9 main studs and 25 lbf. ft for the 5/16" rocker pillar studs. Still used 25 lbf. ft for studs 10 and 11.

[philthehill]

Richard
What tensile strength ARP studs did you use - 170,000 PSI - 190,000 PSI or 220,000 PSI.
The reason I ask is that there are different torque settings for the different tensile strength studs.
I use the 190,000 PSI ones and they are set at 40 ft/lbs for the 3/8" x 24 tpi with ARP Moly.
The lower 170,000 PSI are only 35 ft/lb and the higher 220,000 PSI are 45 ft/lb - again with ARP Moly.
The front 11th stud has been opened out to 3/8" x 24 tpi and is set at 33/22 ft/lbs with ARP lub.
The above specifications are taken from the ARP BMC/BL fitting/torque setting instructions.
Phil

[bmcecosse]

The later Mini engines with the flanged headnuts are rated to be tightened to 50 ft lbf. The front ' S ' bolt was limited to 25 ft lbf. Increasing it to 3/8" makes no real difference - it's the metal deck thickness ( thinness!) at that point that is the limiting factor. Easy to pull it out... Note that MG didn't consider the Turbo engine (most powerful of all A series from the factory) to need either the little bolt - or an '11'th stud at the back of the engine.

[IslipMinor]

Phil,

I am using the 200,000 PSI ones, with the ARP lubricant, so 45 lbf. ft for the main 3/8" studs, 25 lbf. ft for the 5/16" studs and I used 25 lbf. ft for the 2 additional studs as well (both 3/8"). As you, taken from the ARP tech sheet (except studs 10 and 11).

[bmcecosse]

Why such low torque with the fancy studs - when MG specified 50 ft lbf with flanged nuts on their standard studs? Are they even stronger than the ARP studs? Can't see the point of paying a lot of money for ARP studs that can't be tightened even as much as standard studs. The rocker pillar nuts will often tighten slightly each time you go back to them - as the pillars crush down. Hence the search for steel pillars. Special Tuning used to sell 'Competition Head Washer Set' - thick hard steel washers to go under the head nuts - ordinary washers are too soft and will crush down releasing some of the tension in the studs. I know some people tighten the flanged nuts to 60 ft lbf - but I have never gone (or seen the need to go) beyond the 50.

[IslipMinor]

Roy,

It's not just how much the nut is tightened, it's achieving a pre-determined level of clamping force from the fastener(s) - the use of a torque setting or angular movement is just a convenient way of achieving it. The conversion from torque to clamping force is affected by thread size, pitch and material, lubricant, temperature, cleanliness etc.

The reason for the apparently low torque figures is the use of the ARP moly-based lubricant, which converts angular movement of the nut into downward movement via the thread pitch more effectively than oil or maybe grease, typically used with standard studs.

Another way of looking at it is if a thread is tight and needs 20 lbf. ft of torque just to move the nut, then using a torque figure of 50 lbf. ft would only result in an effective torque doing any work of 30 lbf. ft. If by using better lubrication it needs only 10 lbf. ft to move the nut, then 40 lbf. ft is available to do the work. Like any mechanical process the output from it is highly dependent on the efficiency of the process itself.

I have seen elsewhere that using oil on 3/8" UNF ARP fixings for the A-Series would need 55 lbf. ft torque to achieve the same clamping force, and be less consistent in the process. ARP have been going a long time, with a very good reputation, and the principle of increased sliding under friction as a result of better lubrication is sound.

[philthehill]

As above with additions.
Regarding the fitting of 3/8" studs to a 940 head:-
When converting a 9 stud head to a 10 stud and 5/16" bolt it is easier to convert to a full 11 stud x 3/8" configuration but still only set the front stud over the water gallery to 25 lb/ft.
There is no reason to set the new 10th stud at the rear of the head to 25 lb/ft but may be a matter of preference.

Regarding the ARP studs:-
Whilst it may be overkill to fit ARP studs to a standard engine where the later higher tensile studs and flanged nuts are sufficient - any self respecting performance engine builder would not use any thing other than ARP studs and bolts.
I have ARP head studs (19,000 PSI), ARP head nuts with the special steel washers, ARP big end nuts and bolts, ARP main bearing cap studs/nuts and ARP flywheel mounting bolts fitted to my engine.
Not cheap but for peace of mind are priceless.
AARP studs are very much stronger than any offering from BMC/BL and tightening with the ARP moly to the ARP recommended torque on Richards engine will give a pre-load of 11,211 lb.

The alloy and sintered rocker pillars (which can and will) crush should be ditched and steel ones fitted which will not crush. That is why when the Cooper S came into production the forged steel ones were fitted.
The same forged steel rocker pillars (Pt No: 12G210 & 12G211) were fitted to early 'A' Series engines but did not have the chamfer either side of the 5/16" hole as the 'S' pillar had.
I would recommend every time that those 12G210 & 12G211 rocker pillars are fitted when upgrading the valve train and you also have the ability to remove metal from the side of the pillar to set the pressed steel rocker centrally over the valve stem.
Phil

[BLOWNMM]

Hi Guys
My con-rods were supplied with ARP 2000 series rod bolts. I use their recommended stretch method of assembly which eliminates any variation of friction between nut and bolt threads, nut face to cap friction etc. , which can ultimately interfere with desired clamping forces. After all the only thing that stops the rod/piston assembly continuing on it’s upward journey at TDC at 7000 + RPM is the rod bolts hence the caution. Head studs and main bearing studs are made from 4140 HT steel and lubricated with ARP thread lube and torqued to their recommended figure.
Cheers Bob

[IslipMinor]

Bob,

The 2000 series are 'waisted' and designed to be 'stretched' by a predetermined amount, as in current engine design, which as you say overcomes all the vagaries of friction, lubrication etc. Which rods are you using?

Do you know if there is an ARP 2000 set available for the standard 11/32" rod bolts and nuts?

[BLOWNMM]

Hi Richard
My rods were made by a firm called 'Argo' here in oz. You can google them to check them out. The rods are CNC machined and shot blasted from billets of 4340 steel. I followed their recomendations as to the proceedure to tighten the bolts they supplied. I have looked on my current ARP catalogue which is only 2013 and there is no mention of Pro Series rod bolts at 11/32 dia. May be best to look at a current cat.
Cheers Bob
PS I should have said shot peened not shot blasted.

[smithskids]

Just going slightly sideways. On the large diesel engines before hydraulic tightening of stressed component studs and bolts we used ground finished steel washers with a ptfe washer on top between the washer and nut that was made from EN19t steel.this was to elimanate as much friction as possible, all covered with PBC copper slip lubricant. First the 4 nuts on the main bearing caps were torqued up to 250 Lbs Ft then the nuts were rotated 120 degrees with a 125:1 reduction gear box. this almost destroyed the ptfe washer so it was easy to see if you missed one. Stud dia. was 1 3/4"BSF. On engine holding down bolts you had to always check the pitch and diameter as this affects the torque figure as IslipMinor says.

[bmcecosse]

I well realise the clamping force is what matters -and obviously don't assemble 'dry'. But I struggle to believe the tiny amount of moly that will get in there is going to make a huge difference. Never had a head gasket blow - or a stud snap or strip. Before I had a torque wrench I would just tighten till I felt the stud 'yield' - seemed to work ok!

[BLOWNMM]

Hi Smithskids
Interesting reading, however although the proceedure takes into account the friction between the nut and it's friction contact surfaces, it does not seem to address the clamping reduction force applied to an assembly due to the friction between the threads of the nut and stud/bolt. Thanks for your input.
Cheers Bob

[BLOWNMM]

G'day all
Richard - I have just looked at the current ARP catalogue and it lists on page 34 rod bolts for BMC A and B Series 11/32 inch dia. They are listed as HI-Perf 8740.
Cheers Bob
http://arpcatalog.com/#34/z

[IslipMinor]

Hi Bob,

Thanks, those are the rod bolts that I fitted a few years ago - ARP 206-6002. Looking at the current ARP instructions, these rod bolts can be either torqued or stretched, but at the time I did not realise that the 'stretch' method was available for these bolts, so used the ARP recommended torque with their moly lube. I would prefer to use 'stretch', but not sure that I could now use it, as I do not have the original starting position, and the limit for permanent stretch is just 0.001".

[BLOWNMM]

Richard
Looking at the pic on page 34 the 206-6002 have a head style C and the dimple in the head is no where near as big as that on the 2000 series with a head style E. I would have thought it not as reliable as the larger dimple on head style E.
Bob

[bmcecosse]

Just how powerful is this motor you are building ? With fancy rods, and stud worry - let's see the full spec - just out of interest, not for any other reason.....i'm guessing it may be 'blown'.......