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).
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).
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.
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).