[Healeys] Bellows Thermostats

A H List austinhealeyslist at gmail.com
Sun Sep 3 21:41:48 MDT 2017

Avoid the bellows thermostat, they are all old now and as you noted,
fail closed. I put one in my engine and it failed within three months,
fortunately without engine damage as I saw the temp rising.

If you look at the design, they don't really block the bypass flow,
more they limit it somewhat. The bypass port is radial and it doesn't
actually seal in any way, the ring just comes down in front to reduce
the flow. Modern engines have the same principle but have a
spring-loaded disc instead of a ring and the disc seals over the
bypass port axially.

The vast majority of engines run just fine with the later wax stat as
long as the system is in good shape and the car doesn't spend a long
time idling in traffic. There are replacement wax thermostats
available with the bypass port block-off ring but they are not
particularly cheap. Considerably cheaper than an engine reco due to
overheating though.


On 9/4/17, Harold Manifold <manifold at telus.net> wrote:
> Hello,
> I am in the process of putting my AH 3000 engine back together and went
> looking for a recommendation for the engine thermostat. I was expecting to
> find a recommendation for the opening temperature but instead found
> information that recommends a bellows type thermostat should be used and
> the
> bellows is important or some of the coolant can bypass the radiator when
> the
> thermostat opens. If a bellows type thermostat is recommended is the
> correct
> type either the Smiths 85025 series or the AC Delco TF series? Is there a
> modern replacement bellows type thermostat that fails in the open position?
> Your comments and insight is welcomed.
> Harold
> 1960 BT7

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