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There was some talk recently about skirted thermostats for the TR2-4 engines,
and I remembered having saved something that was sent to me by (former?) list
member John Pike (Australia), quite some time ago. Upon re-reading it I thought
it would be useful to re-post for newer list members who may not have seen it
before. The bottom part is where it gets more interesting; be patient.
This is long enough but if you read it through carefully it makes a lot of
sense. Here goes:
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Now, the thermostat and the bypass. For some reason this seems to be a
highly emotive issue, and has been the subject of much discussion in our TR
magazine. I found the article from the engineer I mentioned last night,
which seems to me to paint a rational picture. It's a bit long, but I
think it's worthwhile. He says:
"Last newsletter carried an article that recommended the blocking of the
radiator bypass system when a skirted thermostat is not available and a
non-skirted type has been fitted.
I most definitely do NOT recommend that this action be taken, for the
following reasons.
While the radiator bypass serves no useful function when the engine is at
normal operating temperature and the thermostat is open; it serves a vital
role when the thermostat is closed.
When a cold engine is started the thermostat is closed and coolant is
prevented from circulating through the radiator. However, under the
influence of the water pump coolant does circulate through the block and
the head via the bypass hose, ensuring an EVEN distribution of the
generated heat throughout the engine.
As the coolant must pass the thermostat on its way to the bypass outlet the
thermostat heats up at the same speed as the coolant so when the
temperature reaches the specified temperature for the thermostat, the
thermostat begins to open, admitting a small amount of cold coolant from
the radiator at first, which has the initial effect of lowering the
temperature of the coolant in the engine and slowing the rate at which the
thermostat opens; this is important to prevent a sudden surge of cold
coolant into a hot block and head.
When the coolant temperature in the whole system has stabilised the
thermostat will continue to vary as necessary to maintain the correct
operating temperature for the engine.
If the thermostat is closed and the bypass blocked, coolant cannot
circulate and coolant heating is localised to areas adjacent to the top of
the cylinder bores; coolant does not pass by the thermostat and heat only
reaches it by conduction.
In this situation several things can happen and none of them are good for
the engine.
Since heat cannot be quickly conducted away from the cylinders, hot spots
develop. These can be hot enough to cause local boiling of the coolant.
The steam generated forces coolant back through the water pump to the
bottom radiator tank and then up and out through the radiator cap and
overflow pipe.
The steam forces the water out of the head and then reaches the still
closed thermostat which opens immediately, allowing the steam to pass into
the cold coolant of the upper hose where it condenses immediately.
The heated coolant which was forced into the lower radiator tank returns to
the block and head and passes straight through the now wide open thermostat
to the top radiator tank. The very hot coolant is immediately followed by
the remaining cold coolant from the radiator with consequent rapid cooling
of the head and block - an excellent recipe for cracking both of these.
Another scenario is that the coolant does not boil, but coolant that is hot
enough eventually reaches the thermostat and it begins to open. Coolant
begins to flow under the influence of the water pump and, because the rest
of the coolant in the head is much hotter than that which first reached the
thermostat, the unevenly heated block and head are quickly cooled by the
cold water from the radiator, with a similar recipe for disaster as before.
If you are one of the growing number of TR owners unable to find an
original type thermostat, I can recommend the following procedure.
Use an off-the-shelf unskirted thermostat with a heat rating as near as
possible to the recommended rating of 70 degrees C. Do not use a
thermostat with a rating over 80 C.
It is permissible to reduce the bypass access to a minimum of about 3/8 of
an inch. The important thing is to maintain at least some flow during
warmup. It should be pointed out that the skirt on the original type
thermostat did not seal off the bypass, but rather just directed coolant
flow more towards the top hose outlet. "
Hope that helps a bit. You mentioned that your old Volvo thermostat fitted
the TR. However, the inside diameter of the opening is as critical as the
outside. The original TR thermostat had an outside diameter of 54mm (sorry
about the change in units), and an internal opening diameter of 33mm. Many
after market thermostats have an ID of only around 25 mm, and can therefore
restrict flow through the system, leading to the inevitable overheating.
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So, do the thermostats currently sold by the big three have as big an inside
diameter as original?
Regards,
Jim Wallace
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