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Header tank caps


florencethedog

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ASAP Supplies or Calcutt Boats. Someone else will be along to explain which make of cap (Polar or Bowman) tend to fail  regularly and how to stretch the other make to do the same job.

 

I would never run a 1.5 with more than a 6psi pressure cap (radiator cap) because of those end caps and would go lower or non-pressurised cooling if it did not spit loads of coolant out.

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17 hours ago, florencethedog said:

Cheers,

We have just put the boat in the water and the cap went after afew miles and compared too the other cap this one looks new. 

I await an expert to guide me lol

Thanks stav

Mr Tony Brooks in the post above yours,is an expert.

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If very fine chicken wire or similar is molded tightly around the caps and secured with an additional hose clip, it will prevent them expanding too much under pressure.

Edited by bizzard
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1 hour ago, Peter Reed said:

Mr Tony Brooks in the post above yours,is an expert.

Not really - on those rubber caps. We converted all our hire boats to keel cooling, removed the heat exchanger tubes and had the places where those caps fit caped by a welded on aluminium plate. No more split rubber caps.

I THINK its the Polar caps that are poor and the Bowman ones are better but a bit harder to fit.Don't quote me on that - confirmation from others please.

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52 minutes ago, Tony Brooks said:

I THINK its the Polar caps that are poor and the Bowman ones are better but a bit harder to fit.Don't quote me on that - confirmation from others please.

Spot on, Tony.

The end caps still branded Polar are modern copies, using old Polar moulds, (Polar no longer exist, I think).  They are crap, and balloon after a year or two, and split on one of the poorly made seams soon after.  It is not always obvious they have split, as it tends to evaporate fast enough that you don't actually see a dribble, (at least at first).

The Bowman cap can be persuaded on to a Polar tank, (remarkably easily, actually), is much better made, and far less expensive.  We ceased to have problems once we discovered this, but until then were splashing out on a Polar cap every couple of years.

IMO it is also best to run with a pressure cap, "radiator cap" of 3 or 4 psi, and absolutely definitely no more than 7 psi.  At 4 psi it should never come close to boiling, if thermostat works and skin tank adequate, so why exert a greater force on these caps than you actually need to?

Note if swapping pressure caps there are both "long reach" and "short reach" types, and you must get the right type for what you are going to attach it to.

EDIT:

The cap pictured is the blanking one, with no take off I think.  I have never known these to fail.  It is the one that has a take off for a pipe that does - almost invariably on the seam joining the outlet to the rest of the cap.  This is possibly exacerbated if the weight of the attached hose pulls unevenly at the pint where the two "rubber" parts are seamed together, but I was never able to get longer life from a cap, by "jiggling" how the hose entered it or was supported - the "copy" Polar caps are designed to fail, IMO.

Edited by alan_fincher
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2 hours ago, alan_fincher said:

Spot on, Tony.

The end caps still branded Polar are modern copies, using old Polar moulds, (Polar no longer exist, I think).  They are crap, and balloon after a year or two, and split on one of the poorly made seams soon after.  It is not always obvious they have split, as it tends to evaporate fast enough that you don't actually see a dribble, (at least at first).


The cap pictured is the blanking one, with no take off I think.  I have never known these to fail.  It is the one that has a take off for a pipe that does - almost invariably on the seam joining the outlet to the rest of the cap.  This is possibly exacerbated if the weight of the attached hose pulls unevenly at the pint where the two "rubber" parts are seamed together, but I was never able to get longer life from a cap, by "jiggling" how the hose entered it or was supported - the "copy" Polar caps are designed to fail, IMO.

I replaced the endcaps on my oil cooler late last year. I was surprised how much the original ones had distorted:

DSCF3083.JPG

DSCF3055.JPG

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I have never had to replace Theodora's Bowman caps in 5,000 hrs of running since we bought her.  I don't know how old they are.  Soon after we bought her I modified the cooling system and as a consequence of the mod the excess pressure in the whole system is governed by the height of the system expansion tank, about 5 feet above the rectangular header on the engine.

So, yes, use low pressure and Bowman's.

Nick

Edited by Theo
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