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Una

Changing the Thermostat

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During my RYA course, I heard about someone's thermostat breaking/failing - don't know what the term is - and this costing him a fortune in engine repairs. He was kicking himself for not having replaced the thermostat earlier and recommended that, on our new 'old' boat, we replace ours as a precautionary measure. I'd like to. How easy is it to change?

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21 minutes ago, Una said:

During my RYA course, I heard about someone's thermostat breaking/failing - don't know what the term is - and this costing him a fortune in engine repairs. He was kicking himself for not having replaced the thermostat earlier and recommended that, on our new 'old' boat, we replace ours as a precautionary measure. I'd like to. How easy is it to change?

What boat have you got, what engine is it and what is access like

 

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It's a 38ft Cole Craft semi trad BMC 1.5 engine. Access to the engine is easy enough in the engine room.

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I'm not sure I'd bother - they don't fail very often, and I'm not certain that one that has worked OK for years is necessarily more likely to fail than a new one you have just fitted.

 

If you do replace a thermostat on a BMC...

 

1) It can be quite fiddly, assuming your BMC has only a very short length of hose connecting it to an engine mounted Polar or Bowman header tank.  It can be hard to get the thermostat housing over the studs, and put the pipe in place at the same time.

2) You need to know what temperature stat you are replacing.  74 degrees and 82 degress are common values.

3) If you have a special type of stat known as a "bypass stat" that has an extra extension with a disk on, you must replace with a bypass stat, and if you have standard non bypass type, then you must stick to that.  Putting the wrong type in, (assuming you can actually manage it),  will likely cause more problems than it solves.
4) One standard supplier, Calcutt, are not very good at understanding what you currently have, and actually sending a correct replacement - I know this from experience!

 

Unless you know of anything wrong at the moment, personally I'd leave well alone.

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43 minutes ago, Una said:

During my RYA course, I heard about someone's thermostat breaking/failing - don't know what the term is - and this costing him a fortune in engine repairs. He was kicking himself for not having replaced the thermostat earlier and recommended that, on our new 'old' boat, we replace ours as a precautionary measure. I'd like to. How easy is it to change?

That depends if the thermostat housing is cast iron or aluminium. If aluminium then you may have a fight to get it off without damage, otherwise its take off a hose and three nuts.

 

However He was misinformed. What he should have been kinking himself over is ignoring an overheating engine. If the engine boils no damage will be done to the engine itself until the coolant level goes below the top of the cylinder head. A well boiling engine can cause modern wax type thermostats to vent wax so they open at a far higher temperature than normal. I would not trust a number of so called engineers to take that into account.

 

I would never consider changing a thermostat as a precaution although I  may well fit a temperature gauge or overheat warning lamp if the engine did not have one. One can never totally rule out a thermostat failing but it is, in my view, far more likely that overheating would be caused by an undersized skin tank, low coolant level, gas/air in the skin tank, loose or broken alternator belt,  leaking head gasket or a badly fouled propeller that has been ignored.

 

It is far more important to keep on top of your hoses and drive belt because they can and do deteriorate with age.

 

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Lots of useful advice there. But isn't there a recommended shelf life for thermostats? Won't they all eventually fail - I thought that was the danger: that it suddenly fails and causes expensive damage to the engine when the thermostat itself is a very cheap piece of equipment.

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Thank you. You're convincing me to leave well alone. As it's a new 'old' boat, I think the best thing would be to have the engine serviced so that we know where we stand. Is a service expensive in London? Anyone have any great contacts?

Finding it tricky to upload a photo for my profile - all are deemed to large.

 

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6 minutes ago, Una said:

Lots of useful advice there. But isn't there a recommended shelf life for thermostats? Won't they all eventually fail - I thought that was the danger: that it suddenly fails and causes expensive damage to the engine when the thermostat itself is a very cheap piece of equipment.

A thermostat that fails open or opens at a lower temperature than it should can cause no expensive damage to the engine, but it may end up overcooled and running at a lower temperature than optimal.

 

One that fails closed or opens at a higher temperature than it should may cause the engine to overheat, but this will be full evident long before you do expensive damage.

 

I can't see one failing and wrecking anything in an expensive way , unless you choose to ignore clouds of steam or hot burning smells.

Edited by alan_fincher
"Optimal" not "Optional"!
  • Greenie 1

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There, now I've learnt lots more about thermostats. Thanks very much. I do need a mechanic to check out the engine though.

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3 minutes ago, Tony Brooks said:

BMC 1.5s use a normal thermostat and a small bypass port in the pump casing/cylinder head. I suspect the 1.8 is the same.

My 1.8 had to have a bypass stat, and if a normal one was fitted, the bypass would never have been blocked, meaning there was always a short circuit route for the water, that didn't involve passing through the skin tank.

I neither know if all 1.8s are like that, (I've both been told that they are, and that they are not!), and I don't know about 1.5s.

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And we will not ignore clouds of steam or hot burning smells... 

 

Any recommended mechanics in Central London would be very welcome.

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Thank you for that. It does sound a fiddly operation. I may use a mechanic. I really don't understand all the mechanical terms you're using - I'd prefer to watch an expert in action - however, I'm no longer convinced that the thermostat needs changing. Think I might wait for signs of steam and hot burning smells... I'd still like an engine service though.

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25 minutes ago, alan_fincher said:

My 1.8 had to have a bypass stat, and if a normal one was fitted, the bypass would never have been blocked, meaning there was always a short circuit route for the water, that didn't involve passing through the skin tank.

I neither know if all 1.8s are like that, (I've both been told that they are, and that they are not!), and I don't know about 1.5s.

Mine’s like that. 

Not saying they’re all like that. 

Just saying mine is.

 

I bought a replacement thermostat awhile ago when I was having over heating problems. But after boiling the old one and finding it worked ok I put it back in. 

 

It was easy enough to take out and put back. 

Had to make a new gasket though. A bit of card from a cereal packet did the trick. All still ok after a couple of years. 

 

 

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Just now, Una said:

That sounds lovely and simple - cereal box sounds good too. 

 

Thanks for sharing.

If you do boil it, use a thermometer to find out at what temperature it opens. 

There’s a number etched on the thermostat to say at what temperature it should open. 

 

But I do prefer the ‘if it ain’t broke don’t fix it approach’. 

 

 

 

 

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9 minutes ago, Una said:

Think I’ll just leave well alone. 

 

Thanks

Doing small jobs are a good way to understand a bit more about your engine and build up the confidence to work on it. While you’re doing the little jobs you may spot other things which really do need attention.

 

 

I’m no mechanic, I only start fiddling with the engine when I know there’s someone around to help me out when I stuff up. 

 

 

 

 

 

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As in a lot of cases, if you have a spare, then it never fails. A thermostat is a cheap item, and takes up little space. A set of coolant hoses and jubilee clips also falls into this category. As time progresses, you may include a spare alternator, battery connectors and a control cable. (keep a spare of the longest one you have, you can always make it fit for the shorter one.

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