TR2/3/3A Any metallurgists in the group?

[karls59tr]

I was wondering how extreme cold affects iron in relation to torquing a cylinder head? I know metal can expand or contract depending on temperature so a cylinder head should probably not be torqued in below zero temperatures as the metal would not compress correctly to the right torque value? Do I have this right?

 

[JHaydon]

I did the math on thermal expansion once (for freezing a bearing to slip into an aluminum casting, wanting to know how warm the casting needed to get to relax the fit).

The issues (that I can think of) that you'd potentially face include shrinkage of the cylinder head, shrinkage of the bolts, and to a much lesser extent, shrinkage of the block and head gasket.

The math that I did with the bearings -- from memory -- is that chilling a 3+" OD bearing race to below 32F shrank it by something like 0.0002". Warming the aluminum casting was really the key to happiness, as cooking it to 300F made the 3+" bore grow by 10x that. Again, from memory. I did not do the math on cast iron, but I believe it expands less than aluminum does, closer to the rate of steel.

Since the head would shrink by some small amount, you'd theoretically be overtorquing by some small amount, which would only show up when the engine warms up and the head grows. But on the other hand, if the hardware is also below freezing, it will also have shrunk and will thus grow with temperature. And since you re-torque after the engine heat cycles, I *think* this amount of overtorque may be negligible or may even disappear.

Also, if it is assumed that assembly takes place at room temperature, it is also understood that everything will expand at operating temperature. I *think* they take that into account in the design of the hardware (i.e., length and diameter). The expansion that occurs when going from 72F to 200F is considerably more than the expansion that happens from 32F to 72F, and still quite a bit more than from 0F to 72F.

So... I don't believe the issue is one to lose sleep over.

 

[CJD]

Since...the head, block and sleeves are all iron, and the studs are steel, which has the same thermal expansion coefficient as iron... thermal expansion is not a problem. You may torque the head at any temperature you can tolerate in your shop. If you get into artic temps, you may have trouble with the studs becoming brittle, but that would be -40F or below.

That said, copper is a soft material which is subject to creep. That is a fancy term for the fact it will slowly squeeze thinner under pressure, and this tendency is accelerated as the temperature goes up. That is the reason the shop manual recommends re-torquing your head nuts shortly after a rebuild and then annually after that.

 

[karls59tr]

Since...the head, block and sleeves are all iron, and the studs are steel, which has the same thermal expansion coefficient as iron... thermal expansion is not a problem. You may torque the head at any temperature you can tolerate in your shop. If you get into artic temps, you may have trouble with the studs becoming brittle, but that would be -40F or below.

That said, copper is a soft material which is subject to creep. That is a fancy term for the fact it will slowly squeeze thinner under pressure, and this tendency is accelerated as the temperature goes up. That is the reason the shop manual recommends re-torquing your head nuts shortly after a rebuild and then annually after that.

I did not know that the cylinder head was to be re-torqued annually! Is that what other TR3 owners on here have been doing?

 

[mctriumph]

Having once, in the dim past ,done this in a freezing garage. I can assure you that once the engine is warmed up
the head will need re-torqued in the worst way. Most values were in the 80 ft lbs range if memory serves...
Mad dog

 

[CJD]

I must admit...I torqued the head the first couple years after the rebuild...but it did not need any tightening, so I now let it slide.

 

[Jim_Stevens]

Agree w John. Sealing the valve cover is always the tricky part

 

[DrEntropy]

So... I don't believe the issue is one to lose sleep over.

Even with aluminum heads and cast iron blocks (Lotus) and the coefficient of expansion between the two, when properly assembled at room temps it has been a non-issue for years of running. Even without re-torque. Did re-torque some clients' Alfa heads over the years (aluminum head and block, with sleeves), but my own were built and left alone after that, the only issue being caused by not allowing sufficient warm-up time in western PA winters. A known Mia Culpa.

As John observed:

I must admit...I torqued the head the first couple years after the rebuild...but it did not need any tightening, so I now let it slide.

The cast iron TR-3 lump should be fine.

 

[mctriumph]

Everyone forgets the orphan, Tr7's must have the head re-torqued every 30k or so . Some horrible
designing went into the head studs. Could have been set right with the Dolomite head , but too late now!
Mad dog

 

[JHaydon]

Everyone forgets the orphan, Tr7's must have the head re-torqued every 30k or so . Some horrible
designing went into the head studs. Could have been set right with the Dolomite head , but too late now!
Mad dog

Peugeot as well -- only the factory didn't know it! My family were good friends with the Peugeot dealer, so when they saw head gaskets blowing like clockwork at 30k miles, they re-torqued ours at 20k. It didn't blow until 45k!