Correct position of thermostat within sandwich plate assembly
Turbo Mini Forums

This is a good one for you all.............

Normally I would reassemble components back to the way they were before I disassemble, BUT in an older car like a Mini, perhaps having had several owners, maybe subject to not the best standard of maintenance how can we be sure that the the assembly we find is correct! I know you could argue if it was working like that before........... but perhaps it could work better!

So thermostats and sandwich plates....... On our SPi the thermostat is straight into the head, then the additional outlet goes to the heater valve, then the top complete with top hose.

On the ERA the thermostat is again striaight into the head, the additional outlet goes to the additional water radiator which then goes back to the bottom of the radiator.

I read an old Mini Magaziine last night which suggested that on some cars the thermostat sits not in the head but in the top of the sandwich plate and below the top hose section..

The only Haynes reference I can find to the order of these parts was in the Haynes Metro one where the thermostat is definately shown above the sandwich plate. I have thought about this and you can present an arguement for either installation - your collective comments would be most welcome

It depends what the sandwich plate is being used for.

On the Metro the thermostat is above it because of two reasons,
1 - The Metro A series engine didn't have the small bypass hose from head to block
2 - It also didn't have the heater tapping at the far end of the head.

The sandwich plate provided the bypass flow through the heater matrix while the engine was warming up - unlike the Mini, there is no valve in the water line to the Metro heater, the heat is contolled by flaps on the air side of the matrix, so the line provided a good bypass and heated the car (if required) before the thermostat was open.


Presumably you have the small bypass hose under the head and your heater is connected to the far end of the head ???

So if your sandwich plate is just to feed your Aux Rad which only needs to be fed once the thermostat is open (otherwise it would slow down the engine warm-up) then logically the thermostat would be beneath it.

Schrödinger's cat - so which one am I ???

The thermostat in the SPi needs to sit in the top of the sandwich plate and not in the head. This provides coolant flow round the inlet manifold where the coolant temp sensor is located, alowing the sensor to measure the temperature of the coolant. Poor cold running will result and warm running also suffers at cruse if the thermostat is in the wrong place.

The SPi should not have the head bypass, as this function is via the sandwich plate and the inlet manifold. There should be un interupted coolant flow from the sandwich plate, through the inlet manifold back to the bottom hose.

I have seen all sorts of incorrect plumbing situations on the SPi's and only one works properly.

There should be a short tee hose from the sandwich plate that connects to the heater valve. The tee leg connects to the upper inlet manifold stub and the lower inlet manifold stub should connect direct to the bottom hose. The heater return should then connect to the remaining bottom hose connection. The hoses are shaped such that they pretty much fall into the right sort of place anyway.

Where there is a sandwich plate fitted, metro or mini, the thermostat should ALWAYS sit in the top of te sandwich plate and not the head. The reason for this is that it allows a low flow of coolant to circulate around the thermostat and the engine, maintaining a far more even coolant temoerature through the engine, and the thermostat sees the actual temperature, rather than just a hot spot at the top of the engine. If the thermostat does not sit in this location, the cooling system will not function as it should during warm up conditions, the pump will cavitate like a bitch and in the case of the SPI, will run like shit

Edited by Sprocket on 28th Apr, 2011.

Thank you Rod S for your concise answer. As sprocket correctly states there is no bypass hose fitted to SPi or the ERA (is presumeably the same for Metro Turbo) On the ERA feed for the heater valve is fed from the outlet at the back of the block (left hand front of engine - this is not open on a metro turbo head)

Thank you sprocket for a detailed and expert answer! I remember the garage who last had the thermostat housing off of the SPi - they were so "expert" I am surprised they managed to assemble it at all!

The car has been in this configuration for about 10 years - it warms up ok, runs ok - the electric heater for cold start emisions is of course blown as always!

The only problem I have had with the car is the one you are discussing with someone else here today who has failed emissions during mot - It has always been a struggle! Replacing the air temperature sensor seemed to help last year along with the vacuum pipes.

I was wondering how sufficient bypass was achieved without a byepass hose and now i know!

So the recommendation is to move the thermostat to the centre of the sandwich plate - i wonder how many other cars have it incorrectly fitted!

Just waded through the parts micro fiche of the ERA register and as Sprocket indicates it is shown fitted ABOVE the sandwich plate!

So there we go - I have 2 cars both with the thermostats fitted incrrectly and the legacy would have continued were it not for turbominis!!