T-Type Overheating: Part 2

I rather hi-jacked Part 1 on this subject in the October issue. Sorry about this, but the message, (which was best given in the introduction), was that you need to start with a good radiator, which preferably has been checked for leaks and has been flow tested.

Part 2 deals with the bellows type thermostat, originally fitted to the XPAG. An illustration, courtesy of the YB workshop manual, is reproduced below.

Firstly, a general word about the importance of running your engine with a thermostat and then Eric Worpe will explain how the set-up works in conjunction with the bypass.

The thermostat was provided as part of the engine design for a very good reason i.e. to greatly assist the engine to reach its operating temperature as quickly as possible and once it has done so, to maintain it at that temperature.

Some owners run their car without a thermostat in an attempt to cure an overheating problem. However, this results in too cool running especially in winter and as the engine takes longer to reach its optimal operating temperature (it may in fact never achieve this, especially in winter) fuel economy will suffer as, arguably, will performance and engine wear.

As I am now getting out of my depth, I’ll hand over to Eric…..

The XPAG’s Thermostat

The skirted thermostat used in the thermostat housing should be given some credit for being a rather clever control valve. Hot coolant pumped through the cylinder head is diverted by the thermostat between two separate routes in such a way that the output water temperature is held at a nominal 74 deg.C.

When the engine is working hard and generating considerable heat, the thermostat directs most, if not all, of the coolant water through the radiator, which conducts heat to the air flowing through its core. The cooled water is then pumped to the back of the engine block and upwards to the back of the cylinder head. However, if the engine is having an easy time, or has not fully warmed up, only some of the coolant is directed through the radiator, the rest is allowed to flow through the by-pass port which remains open until the floating skirt progressively closes it off as the thermostat reaches its operating temperature.

Coolant flowing through the by-pass port is mixed with the cooler water from the radiator in proportions controlled by the thermostat such that the engine temperature remains stable and thus minimises any stress caused by thermal variations. This is accomplished whilst maintaining the best possible flow of coolant through the engine block, which helps scavenge any “dead-areas” thus minimising hot spots from developing.

Just replacing the original thermostat with a modern, non- skirted version and partially blocking off the by-pass port reduces the coolant flow rate through the engine when the thermostat is not fully open. This would create a significant thermal gradient throughout the engine, something the engine designers were at pains to avoid.

Finding original thermostats is rather unlikely, so using an alternative skirted thermostat that can still be found at auto jumbles for around £3 to £4 seems worth considering. The down side is that extensive machining is involved. The alternative thermostat needs to be housed in two stainless steel tubes. The smaller diameter tube surrounds the thermostat’s skirt and also has a horizontal slot machined to coincide with the elongated section of the by-pass port. The greater diameter tube houses the main body of the thermostat and is extended to enable the attachment of the top radiator hose (Photo 1).

Photo 1 – The “ingredients”

The smaller diameter tube needs to be inserted by about 5mm into the larger tube and then welded together. The original cast iron thermostat housing has to be machined to accept a push fit of the stainless steel tubes. The two are secured with an application of some Loctite retaining adhesive such as 648 or 638 (Photo 2).

Photo 2 – Here’s one Eric made earlier!

The above does represent a rather complicated solution to the replacement thermostat issue and is probably only suited to the have-a-go home machinist now that replacement thermostats with by-pass skirts have become available.

Eric Worpe

Ed’s Note: Eric mentions that replacement thermostats with by-pass skirts are now available. The MG Octagon Car Club offers part number SBE052 “Thermostat and housing” for £62.36 plus VAT; this is the members’ price – non-members pay £74.83 plus VAT.

MOSS Europe offers part number 434-168 “Thermostat and housing” for £74.95 (inclusive of VAT).

Whilst on the subject of thermostat housings and by-pass skirts I have an original by-pass elbow for sale. It is in good condition and can be sent anywhere in the world. Asking £10 plus £2 UK postage (the £10 will be donated to the TTT 2 ‘hard’ copy fund).

3 thoughts on “T-Type Overheating: Part 2

  1. Richard Evans says:

    Hello Eric
    Interesting stuff. How do you check for rad flow at home? When ever I take my rad to a local repairer he just disappears out the back and then 10 mins later reappears, “Yes mate you need a new core” What exactly are we looking for please? Is it just a matter of sticking a home mains water pressure hose in the top and looking out the bottom to see if the flow is pretty much the same?
    Thanks and Regards

  2. admin says:

    Hello Richard,

    I’ll try and find out a bit more about it from the outfit who checked my radiator and report back.

    Kind Regards,


  3. Duncan MacKellar says:

    Difficult to believe, but while recently doing a mechanical restoration on my Dads ’49 TC that has been stored since 1955, I found the original thermostat crudded up with the typical growth. He had been drained it of coolant in 1955. After careful cleaning along with some help from a bit of muriatic acid to free things up, I tested the thermostat and found it to be fully functional, and opening and closing just as designed. Then again, then as now, the water supply here comes from the High Sierra Mtns snow runoff water, and is quite high quality. None the less, a very nice quality part. If you still have an original thermostat, be sure and give it everything you have got before giving up on it.

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