With a cast iron head the difference between hot and cold isn't that great.
Yes, the bolts heat up and expand, but so do the cylinder head and block, by a
similar amount (similar temperatures, similar thermal expansion
co-efficients). But the answer to the first question is yes, the resultant
torque from re-tightening the nuts when hot is what the design engineer
intended.
As for a summer and winter spec for wheel nuts, for a start the temperatures
are not different enough to be very significant. In the UK at least, summer
is rarely hotter than 25C, and winter rarely colder than 3C so we are talking
a 22C swing. Engines go from around 20C (or whatever it was when assembled)
to 130C around the bolt bosses when hot - 5* the swing.
Ignoring this fact, it is the change in temperatures that causes the problem.
So, if the temperature outside stays around 5C most of the winter, your wheel
nuts will stay at the prescribed load all winter. When summer arrived, the
bolts would expand, and then your nuts would be (a tiny bit) loose, so you
could re-tighten them, but again just to the defined load, where they would
stay all summer. None of this is remotely necessary, I am just following the
theoretical arguement here about what to do if you wanted to make sure your
nuts were at exactly the right load all year.
Richard
eyp_jl@netzero.net on 22Sep2000 10:24 PM
To: Richard B Gosling/1M/Caterpillar@Caterpillar
owner-spitfires@autox.team.net@INTERNET
cc:
Subject: Re: cylinder head
Retain Until: 22/10/2000 Retention Category: G90 - Information and
Reports
Perkins Confidential: Green
My question about this is the retorqueing after engine warm-up. When
the
engine warms up the studs warm up and since it is allot longer than it is
around it lengthens with the expansion due to heat and that means the
applied torque is lower. So, you re-torque it and then when it cools and
shrinks the applied torque will be much higher than what you originally had.
Now, is this resultant torque the actual specification that the design
engineer wanted?
When you deal with high temperature pressure vessels and you want to torque
the 2 or 3'' bolts to 1.5mega foot pounds force the temperature elongation
is calculated and the bolt is heated then torqued to a lower value.
This also makes me wonder if there should be a Summer and Winter spec for
wheel nuts
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