Advice on replacing 135 thermostat please.

Thanks Glyn.
Not knowing what it looked like under the manifold and fishing in the dark I decided it was too risky to put back together "unseen'.
Now I know "how it works" it is much easier to see how it all goes back and those two bolts holding the water rail are vital in keeping it all together.
The small water pipe on the inlet was blocked. When I tried to poke it through with a long thin screwdriver I couldnt poke it out. I had to use a gimlet to screw its way through. The aperture on the head was also solid.
I can only think it was not noticed when the HG was done because in 7k miles I can't see how it got so bad.
So the good thing is the waterways will all be clear and I have learned a lot about my car!
I think the HG will be ok but fingers crossed.

It will, Brian 😊

Well done for finding the blockage. I had many weird and wonderful effects with the cooling system on the Mk1 F including the radiator hoses going completely flat under load. All due to a blocked pipe.

Cheers Pedro..I am on your side 😁

Thanks Chris.
I am really glad I found it. It lets the head breathe like a spout on a kettle.
I know water flows thru the head through the water jackets but any air is released thru this pipe.

Still waiting for the thermostat. Want to get that done whilst I'm waiting for an inlet gasket!!

@Delbourt...that part number you provided is the one I used to order the plug from MGObsession and it is the right part. Thank you.

Just a thought that you probably won’t like.
If it was bunged up where you have described what is it like in the radiator?
Very difficult to flush that clean through all the channels/tubes but a fibre optic inspection might be worth considering.

It's a very good point Delbourt.
I know from experience in central heating that the system can be clear but have a localised blockage that can effect the whole system.
The radiator and heater rad flush clean and so do the pipes. The rad gets warm top and bottom and the fan comes on to cool it so I am pretty happy with the pipes and rads.
I was very surprised to find that pipe so solid. No amount of flushing would have made any difference to that pipe.
I think it will be ok now and function as it should...hopefully

I will be running it on just water for a week or so then another flush before I add antifreeze.

Two points there. The first is to try and identify what the sludge is and what contributed to its deposition. If temperature change creates the conditions that enhance sludge formation you may find blocked rad cores.
If still at work the mass spectrometer would indicate the elements and the chemical engineers or chemists would tell us what and how the sludge formed.
As things stand I suspect we are going to have to guess! 
My guess would be interaction between coolants and the possibility of someone using one that is incompatible with the one used most recently. I can’t remember if it is Evan’s coolant or 4-Life that doesn’t necessarily like OAT or similar etc.
So how are you going to clear it? Perhaps a “rad flush”. And I think I’d use one of those to try and ensure the system and the galleries in the block are cleaned.

Someone on here might know where the temperature senders are for turning the radiator fan on/off but I think that is at the outlet from the top of the block/cylinder head. So I can’t see that as a problem in terms of proximity to the radiator. I think the engine bay fan in yours will be initiated by the sensor positioned near the back of the engine bay. Presumably that is working (easy to test by just disconnecting it).

Secondly.
Not sure I’d use clean water to circulate for days. Nice hot water with plenty of O2 in it will like the steel water rail unless you have a stainless one.

Lets see what others have to say.

Good morning
Two good points here.

It's widespread knowledge that the two main kinds of coolant can't be mixed. But why and what are the effects of that, I don't know. Maybe some weird hard sludge like the one you found?

And I thought the same Delbourt mentions when I read Brian´s lines. Plain water is not what you want to have in your cooling system, and one week seems quite a long time to me. 

It could even be the result of some miracle head gasket gunk that had been used to "fix" a problem in the ancient, dark and distant past that's come to haunt you.

Many moons ago a mechanic mate had a loverly injected BMW 2002 tii, this long suffering car had a nice mix of oil snot in the coolant, he made this great revelation the day before he was going to drive up to Bathurst for the big annual 1000km race, he asked me if I could think of a quick bodge that he could do to get him there and back a round trip of @ 1700 km, I told him that he needed to fix it properly, but instead he took a punt and tipped a bottle of a miracle head gasket cure called Chemiweld into the cooling system and off on his mission our intrepid adventurer went. Well the miracle muck actually worked, he got there and back with no problems. 
Now being a typical mechanic did he do anything about the head gasket upon he's return?.........No of course not, he left well enough alone for some many years, the resultant soup ended up solidifying in the heated core matrix blocking it completely, thus he no longer had a functioning heater, did he do anything about that?........No of course not! He continued to belt the living crap outta that poor car.

I recall he'd worn the front brake pads down, and still persisted to drive the car with the backing plates of the pads making a hideous noise as the ground into the discs, he still didn't do anything about that until both discs had snapped off the hubs, from the lathe cutting action of the backing plates against the discs!

He would drive like an absolute maniac and crashed that car (and a great many others) several times and in often in spectacular fashion! By the time he sold the car, there were only the boot lid and one rear guard (wing or fender to you johnny foreigners who talk funny) of the original panels left, all the rest had been replaced due to damage in various incidents........the bonnet had been replaced about 3 times that I knew of.........even the bloody roof panel had been replaced!

He had many monumental shunts and prangs over the years.
Mind you we were all absolutely fearless, 10 foot tall and bulletproof back then, but sadly my mate had no driving talent with which to back it up, how he has survived to become and old retired bastard to this day is truly astounding!
 

A lot of good points above and I thank you all for your contributions..genuinely so.

I only know that the HG was done about 2k before I got the car and I had no idea what antifreeze had been put in because it wasn't on the bill. It was however done by a previous Rover garage.
I had a tiny leak on a hose from the central SS pipes to the radiator hose and decided to change it asap.

 

Knowing the hose was coming I drained and flushed the system. It didn't appear sludgy just red antifreeze. I had no overheating problems at all. Heater worked all thru my first winter and the rad fan worked.
I refilled with water, cut the bad bit of hose off and ran it a few days. When the hose arrived I drained it again. It still had red in it so I fitted the new hose and ran it again with water.
I used correct MG OAT antifreeze and I had flushed it as clean as it was possible to do.
That blockage had been their for a very long time. It wasn't just blocked since the HG was done. It was missed at the time and was solid. You could have tapped a thread in it!! It had occured over years of running.
I know from heating engineer experience it was magnatite sludge..my magnet stuck to the aluminium head. This happens in central heating systems and you can find a localised blockage with a magnet. Magnets won't stick to copper but it will if there is sludge at a tee etc.
Easy to demonstrate to a customer they have a blockage! The cars heating and cooling system is the same as a sealed central heating system.

The car. I am not worried about running it on water for a final clean out. My rally cars hardly ever had antifreeze in as they were worked on that often. Not saying it is right.
Some of that remaining sludge will break down now it has been broken up and most of it removed.
I will use the same antifreeze as before so it is mixing with the same tiny amount left.
The car had behaved perfectly for the last almost two years and I actually use the car. 
I can't prove it but I am certain the thermostat is the fault. It just stuck closed. Hence no heat to the rad and no need for the rad fan to come on. Just localised heat in the engine.
I don't think its even worth trying the stat in boiling water because its likely to open!
Although it is a pain to change I am glad I was led to the stat as otherwise I would never have found that pipe blocked.
The other thing is that cylinder 3 had a tiny break in the inlet gasket that I don't think I did when it was removed. Cylinders 1, 2, and 4 had signs of normal stains in the inlet manifold tubes but cylinder 3 was almost clear showing it was running lean. That could account for the feint stumble on tickover. I might have accidently cured that.
Really pleased for your help guys.

Still waiting for the stat posted Monday on a 48hr courier delivery..🙄


 

As you have found this blockage I would recommend you also check the fixed pipes around the outlet side. Take the half inch hose off the spigot near the temperature sensors and look inside as crud can really build up in there.