Cylinder Head Gasket Replacement? ?

[Robert Leigh]

I have read this string with interest as I was always taught to retighten cast iron heads when hot. Not cold only alloy heads to be retighten cold. What is the opinion of other forum readers as to what is correct hot or cold. I have never had problems doing it hot

John Mason

I agree with the tightening hot brigade. I always grease the gasket. All studs should be sealed into the block with Loctite or similar. With modern non asbestos filled gaskets I always find that they will tighten further up to four or five times after a run. Following this procedure I have had no problems with leaks.

Robert Leigh

[Chris KC]

The coefficient of linear expansion of cast iron is about 10% lower than steel, so if we disregard temperature-related friction effects and the rather thin layer of copper in the gasket, the axial force in the studs will slightly reduce when the engine warms up. As the amount of this reduction is relatively small it's not a big deal whether the 'coup de grace' is applied hot or cold. The nuts will probably nudge up a fraction and when it cools down the clamping force will be slightly higher than it was cold. You could achieve the same effect by applying a marginally higher torque in cold condition. Where cold wins out is that it is a known and stable condition, whereas a hot engine is how hot? and gradually cooling as you work your way around the nuts, unless you tighten with the engine running. The main benefit of using a torque wrench is consistency but this is undermined somewhat if conditions are changing as you use it. Either way, for a cast iron head with decent studs you will get away with it. An alloy head is an entirely different kettle of fish as aluminium expands twice as much per degree of temperature rise as steel. This means not just that the effect is reversed but that the differential expansion is greatly increased. Torqueing up an alloy head while it is hot will lead to leaking gaskets and potentially a warped head.

I would not advocate skimming block faces unless you absolutely have to. There is a surprisingly small margin against the stroke of the piston, take too much off and you are forever after stuck with having to skim piston crowns or space under the block to keep the pistons from hitting the head.

[John Mason]

Chris KC, I read your input with interest (I do not doubt your figures on expansion etc.)but you make no reference to the expansion on the steel head studs. Surely these increase in length when hot thus require re tightening to bring everything to the correct tightness. When cold they maybe slightly overtight with contraction but that is a small price to pay to have them correct when running.

John Mason

[Chris KC]

Chris KC, I read your input with interest (I do not doubt your figures on expansion etc.)but you make no reference to the expansion on the steel head studs. Surely these increase in length when hot thus require re tightening to bring everything to the correct tightness. When cold they maybe slightly overtight with contraction but that is a small price to pay to have them correct when running.

John Mason

They do expand John, and it wouldn't be a problem if everything expanded the same amount. But a cast iron head expands less, and an alloy head more, than the steel studs do.

If the torque applied at cold condition is correct the change in clamping force at hot condition will be accommodated.

[John Mason]

I will continue to retighten when hot as Chris says the head studs have expanded with heat and therefore a little longer. In my view the best time to get the correct tightness of the head and head nuts. I have only ever had a problem with head gaskets once when that was an uneven block face not stud tightness. I never use a torques wrench either just the feel of my hand. My advice just do what you prefer and what has served you best in the past. We all differ slightly in how we tackle a job.

I have just consulted my car bible Oldham Practicle Automobile Engerneering Illistrated which states always use a torque wrench when tightening a cylinder head, ( 1. To Chris) it goes on to say cast iron heads re tighten hot. aluminium heads cold. ( 1. To John )



John Mason

[Tony Press]

Chris KC, I read your input with interest (I do not doubt your figures on expansion etc.)but you make no reference to the expansion on the steel head studs. Surely these increase in length when hot thus require re tightening to bring everything to the correct tightness. When cold they maybe slightly overtight with contraction but that is a small price to pay to have them correct when running.

John Mason

John,
Yes he does-

"The coefficient of linear expansion of cast iron is about 10% lower than steel, so if we disregard temperature-related friction effects and the rather thin layer of copper in the gasket, the axial force in the studs will slightly reduce when the engine warms up."

[Jackson]

Its best to tighten Al cylinder heads when cold (very cold) I always put mine in Freezer for a few hours before putting on engine. Tighten studs to 20,ft.lbs. expension of head increase stud tensiion, but not torsional stress, tension and torsional stress together makes studs more likely to fail.

[JonE]

a Qu. on modern/supplier copper-inner layer-copper head gaskets...

Does the serial number face go upwards or downwards please?

[John Mason]

I was always taught crimped side upwards on head gaskets

John Mason

[Chris KC]

I'm not sure that it matters - both sides have to seal.