Classic cars forum & vehicle restoration.

exbmc · Posted: Sun Mar 04, 2012 6:35 pm Post subject: Head Torque

I too was taught to slack and retorque headnuts, as Jim describes.
On the BMC A and B series, the head nuts are half exposed, and the rest under the rocker cover. The young lads were told by the foreman,on a 1st service, to slack (1/4 turn only) the nuts that are exposed, then torque back up in one smooth action. We used a breaker bar to slack, then put the torque wrench on the extension to tighten. The oily ones under the rocker cover were just torqued. This practice gradually died out after the foreman retired, i think.
It was routine though, on some laboratory engines such as MWM and Petter to slacken and retighten head nuts, when i worked in the oil research industry.
I don't recall ever actually seeing this method required in any manual, and never applied to bolts, only studs and nuts.

jessejazza · Joined: 18 Mar 2009 Posts: 75

Jim.Walker · Joined: 27 Dec 2008 Posts: 1229 Location: Chesterfield

Quite simply. The torque needed to overcome friction (sticktion) and start a nut turning is usually greater than the torque needed to tighten it down to the correct figure. It gives a false impression that the nut is already tight enough.
The only way to tighten to the correct torque is from a slackened position.
Does it not always in manuals say "RE-tighten" rather than "tighten"? Does that not indicate loosening or backing off first?
Being of the "old school" when studs and nuts were the norm I always RE- tighten head nuts AND bolts after a suitable period to be sure.
Stretch bolts are a different matter, where the elastic period of the bolts is sufficient to maintain tension as gaskets etc. settle.
Jim.
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Penman · Joined: 23 Nov 2007 Posts: 4755 Location: Swindon, Wilts.

Hi
ALSO if doing this on an OHV engine don't forget to recheck/set the tappets
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Julian · Joined: 19 Apr 2010 Posts: 278 Location: Warrington

exbmc · Posted: Mon Mar 05, 2012 8:03 pm Post subject: Head Torque

The 1/4 turn off a previously torqued nut, was usually enough to slacken it before retightening. This way has always worked for me, and i will continue to do so.On the BMC engines, the outer nuts, were painted, could be dirty, or even a bit rusty on a 500 miler, so a false reading could be had.
Now, in the light aircraft world, this method is not used much. The routine is to torque all the cylinder base nuts to spec then go round again in the correct order, but holding the torque on the wrench for 10 or 15 seconds, to ensure their is no more movement in the nut. There is no more tightening, after the engine is in service. No gasket there, only a big O-ring.
There are few aero engines with cylinder heads that come off, most are in assembly with the cylinder, so the base nuts are as critical as a head nut on vehicle engines.

Roger-hatchy · Joined: 07 Dec 2007 Posts: 2135 Location: Tiptree, Essex

Just notice what I posted as a tittle Laughing

Have had a few sticky keys hehe

Have you been cylindered lately.

RUSTON · Joined: 07 Mar 2011 Posts: 144 Location: Matlock.

At Rolls Royce they advised us to mark the position of the head nuts and the cylinder head with Tippex etc, then slacken the nut's off a quarter turn. Then retorque them, making sure that the nuts were at least as tight as they were previously by ensuring that the marks were still at least in line or had prefferably tightened past them.

Pete.
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BigJohn · Joined: 01 Jan 2011 Posts: 954 Location: Wem, Shropshire

My Old Man taught me the torque up, slacken, retorque method, that was 40yrs ago, and he served his time pre war on Rolls Royce, Daimler and Minerva, so that was good enough for me. Laughing

jessejazza · Joined: 18 Mar 2009 Posts: 75

Jim.Walker · Joined: 27 Dec 2008 Posts: 1229 Location: Chesterfield

I don't think "backing off" a 1/4 of a turn is very significant. As long as it is sufficient to remove the risk of "stiction" giving a false re-tightening torque.
Jim
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

Julian · Joined: 19 Apr 2010 Posts: 278 Location: Warrington

This thread got me thinking, so I dug out the destructions for some ARP head bolts that I used a few years ago. (I really wanted studs, but that would make it impossible to remove the cylinder heads with the engine in the car without first winding them all out first - all 34 of them Sad

)

Clearly lubrication of the threads is very important as ARP specify using either oil or thread lubricant on the underside of the bolt heads and washers and the threads. Also there is a table for recommended torques for optimum preload and there are two columns, one for 30-wt oil and one for ARP moly assembly lubricant, the difference is about 10% with the torque when using Moly lubricant being less.

Julian.