[Healeys] radiator thermostat

Fri Sep 19 13:26:25 MDT 2025

Alright so far. 25 years or so…………..

From: David Nock <healeydoc at sbcglobal.net> 
Sent: 19 September 2025 17:27
To: Simon Lachlan <simon.lachlan at alexarevel.plus.com>; Harold Manifold <manifold at telus.net>
Cc: Richard Antal <rantal243 at yahoo.com>; Healeys <healeys at autox.team.net>
Subject: Re: [Healeys] radiator thermostat

If you are using the original style Bellow thermostat be sure that you DO NOT use a higher pressure radiator cap. If you run a bellow thermostat and a higher pressure cap the excess pressure will close the thermostat once the cooling system gets up to temperature. Causing the engine to overheat.

On 9/18/2025 1:15 PM, Simon Lachlan wrote:

Hi,  

The original sleeved ones are - I think - easier to find in the UK. Certainly, I managed to find a few. I was advised that the Morgan people were a good source. I bought mine on eBay but that was a while ago. Simon

Sent from my iPhone

On 18 Sep 2025, at 18:54, Harold Manifold  <mailto:manifold at telus.net> <manifold at telus.net> wrote:

Hi All,

Thermostats and Healey cooling always generate interesting discussions, and I'd like to clarify my previous comments on what controls (or often doesn't control) the coolant temperature.

To summarize some of the points made: Coolant flows down the radiator, out the bottom hose, and into the water pump. It's then pumped into the engine block's cooling jacket. The goal is to direct the coolest fluid to the hottest part of the engine for optimal heat transfer (i.e., the largest delta T). The coolant, having absorbed heat, then moves up to the cylinder head, through the thermostat, and back to the radiator. This process is known as thermosiphoning, where heated coolant rises through the engine and falls as it cools in the radiator.

The heat generated by the engine is a function of the combustion process (timing, carburation, etc.) and horsepower efficiency loss. More horsepower output means more heat transferred to the coolant. The radiator removes this heat, but its effectiveness is proportional to the difference between the air and coolant temperatures.

Assuming the radiator has sufficient capacity to remove heat, if the coolant temperature the thermostat "sees" is less than its opening temperature plus 20 degrees, the thermostat will slightly close to restrict flow to the radiator. In this scenario, the thermostat is actively controlling the coolant temperature. However, if the coolant temperature exceeds 20 degrees above the thermostat's opening temperature, then the coolant temperature is primarily a function of the incoming air temperature, and the thermostat is no longer a factor. In such cases, the coolant temperature is effectively uncontrolled by the thermostat.

To add to the discussion, the engine fan has minimal effect at speeds greater than 15 miles per hour.

Harold

On Thu, Sep 18, 2025 at 8:46 AM David Nock via Healeys <healeys at autox.team.net <mailto:healeys at autox.team.net> > wrote:

The thermostat does not control the high end temperature of the engine. Its job is to get the engine up to an operating temperature as soon as possible. Then in maintains the low end temperature of the engine. Once the thermostat is open the engine temperature is controlled by the radiator, radiator fan and air flow.

On the 6 cylinder engine there is a sleeve attached to the thermostat, that blocks off a bypass in the cylinder head. The bypass is there to allow the coolant to flow when the thermostat is closed. Once the thermostat is open the sleeve is there to block the bypass allowing ALL the coolant to flow thru the radiator. 

David Nock

British Car Specialists

On 9/17/2025 12:54 PM, Richard Mayor wrote:

165 is too cold for good running.  Our engines run better in the 180 to 190 range.   

Sent from my iPad

On Sep 17, 2025, at 12:13 PM, Robert Markovich via Healeys  <mailto:healeys at autox.team.net> <healeys at autox.team.net> wrote:

 165 is the one I use

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On Wednesday, September 17, 2025, 2:50 PM, Richard Antal via Healeys  <mailto:healeys at autox.team.net> <healeys at autox.team.net> wrote:

Gents,

     What is the desired coolant temperature for optimal engine performance? I've searched the tech articles and can find nothing that specifically answers that question. This summer my BJ8 seemed to run fine with a 160 thermostat which, according to the gauge which I've checked and found to be accurate, kept the coolant at around 165. Thanks to all who respond.

rich antal

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