Yes they definitely do fail closed. The bellows is filled with
volatile liquid not unlike the bulb in a temperature gauge and any
tiny crack or pinhole renders it inoperative leading to an overheated
engine if the driver doesn't catch it in time.
Could someone with one of the old style bellows/sleeve thermostats
please measure the diameter of the sleeve. Some time ago I did a
comparison between the diameter of the sleeve and the distance to the
bypass port, and on a Longbridge head it was about 1/4" radially. This
gap means the sleeve does very little to block the port when hot, I'd
like to check it against a later head but don't have a thermostat on
hand.
Andy.
On 9/14/18, Bob Spidell <bspidell@comcast.net> wrote:
> One other consideration: I have heard--note: hearsay--that the OEM
> sleeved stats are of the 'fail closed' variety. Have seen modern ones
> called 'fail open' or, even 'failsafe.'
>
> FWIW.
>
> Bob
>
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