Improving the 3.8 Sleeve Thermostat

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rfs1957
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#1 Improving the 3.8 Sleeve Thermostat

Post by rfs1957 » Sun Jan 24, 2021 1:55 pm

I'm aware that there are reams of exchanges concerning the 3.8 thermostat, but I couldn't find one that dealt uniquely with the subject, so rather reluctantly am starting this one.

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If a moderator wants to tack it on somewhere else, please intervene.

Any student of 3.8 thermostats has to read this first.

https://www.coolcatcorp.com/thermostats ... ifold.html

My understanding of what matters to us at the present, in view of what is available, is grasping that the famous slot we're trying to uncover / cover dictates whether the bypass hose has coolant flowing through it or not.

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When the engine is cold, the thermostat is "shut", and according to which of the essentially two types of thermostat is fitted, there is a sleeve that is positioned to leave the slot open, uncovered.

Furthermore, no coolant gets past the thermostat mounting flange, because the orifices there are shut too.

Hence, cold engine, coolant circulates uniquely within the block ; no radiator participation at all.

The two types of thermostat are

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1. the early one : here, the sleeve is "deeper" towards the engine, and upon warming up it moves outwards, or towards the mounting flange, to cover up the famous slot.

2. the later one : here, the sleeve is "retracted" hard against the mounting flange when cold, leaving the slot uncovered, and when the coolant warms up it moves away from the mounting flange to cover up the slot, and blank the by-pass hose.

This type is shown in all the pictures that follow here.

From the pictures available here on the Forum, it seems that the depths a) and b) are not constants, and I think it's important to measure these on your car, and then measure up the open/closed heights of the sleeve, to be sure that you're getting the effect you expect.

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I believe that the latter "retracted" type are the only ones generally available now, sold as C3731/1 from SNGB and C°, but that there is some debate as to how appropriate the diameter of their sleeve is.

Indeed, if the sleeve is not a good match to the bore of the housing, its effect as a "porting" device will be compromised, and the full flow of the coolant from the water pump will not all be going through the radiator.

Our friend in the CoolCat link discusses this, giving 49.6mm as the housing bore, and approving the original 48.5mm thermostat sleeve diameter as the figure to aim for, which would suggest a gap of about 0.5mm.

Measurements I made on my car, confirmed by Plastiscene approximation, suggested that there was about a gap of between 1.5mm and 2mm ; bear in mind that the outer diameter of the flange, and the diameter of the guiding rebate in the housing, will influence how much lateral slop there is in the position of the thermostat.

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I post these pictures, then, without truly knowing how critical this gap is, but as it seemed very easy to address on my car, I added a 180° strip of 1mm brass onto the RHS of the periphery, soft-soldered. An initial attempt with 1.5mm strip wouldn't quite fit, so the 1mm feels about right now ; it can only be sealing the port better now.

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Of course, there is a school of thought (Ken Jenkins et al) that blanks the bypass hose off almost completely with a brass bung, bored 1/8" to allow it to bleed, accepting that a slower warm-up is a small price to pay to be sure that the radiator is functioning at full capacity ............
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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Heuer
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#2 Re: Improving the 3.8 Sleeve Thermostat

Post by Heuer » Sun Jan 24, 2021 2:22 pm

There is more information here: http://forum.etypeuk.com/viewtopic.php?p=65692#p65692

Good idea to braze on a piece of brass though.
David Jones
S1 OTS OSB; S1 FHC ODB

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