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Overflowing anti-freeze in engine header tank

I have a new Vetus M4.45 engine with a coolant header tank that came with the engine.
Since installation the header tank has leaked when the water got hot and expanded. I took it off thinking that the tank had a small split, only to find that there was a small hole in the back at the top, which (it turns out) was supposed to have a return pipe to deliver any overflow back to the skin tank of the engine -the expanded excess water was overflowing out of this hole.
The dealer has given me a replacement header tank without this hole, so now it will be properly pressurised. However, the dealer asks if maybe my skin tank is too small, since he says the coolant shouldn't expand so much in normal use.
My previous engine was a BMC 1.8, which never had any problems with overheating in normal use.

Asked by: Andrew Denny  | 12.14pm, Tuesday 27 November

WW says:

Since water cannot be compressed, the volume change when water is heated is very small; it is the expansion of the gas dissolved in the water that is responsible for the volume change. So if you are getting a lot of water being expelled from the cooling system when the engine is hot, it is probably due to air in the skin tanks.
Careful bleeding of the cooling system should solve the problem.
The expansion bottle should be the highest point on the system and be about half full leaving room for the coolant to expand.

Rupert Smedley  | 6.05PM, Tuesday 27 November

The pipe fitting at the top of the expansion bottle is usually so that a pipe can be fitted to direct any overflow out of the way.

Rupert Smedley  | 11.17AM, Wednesday 28 November

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