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Old 07-02-2008, 14:01   #1
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What is the purpose of a petcock in the cooling system?

I've got a petcock mounted in the thermostat housing of my Universal M18. It appears to be a device for bleeding air out of the cooling system. Unfortunately, it is not mounted properly and does not function. Is bleeding the air out of the cooling system critical?

I've got a new petcock but the existing one is incredibly stuck in there and made of brass. It deforms easily and I'm afraid I'll wind up breaking it off...
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Old 07-02-2008, 14:09   #2
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I've got a petcock mounted in the thermostat housing of my Universal M18. It appears to be a device for bleeding air out of the cooling system. Unfortunately, it is not mounted properly and does not function. Is bleeding the air out of the cooling system critical?

I've got a new petcock but the existing one is incredibly stuck in there and made of brass. It deforms easily and I'm afraid I'll wind up breaking it off...
It is good to get all of the air out of the cooling system (I'm assuming that you have fresh water cooling). Air can get much hotter than water. If you have 300* airbubbles circulating in your system, that is far worse than having 212* water or 225* water/coolant mixture. It also takes air for water to boil. If you can keep all of the air out, you lessen the chance of boiling.

Now, having said all that, if your engine gets hot enough to expell expanded water out of the radiator cap, air will be ingested, after the cooling system cools. That's why it is importnt to have a recirculating overflow tank. The water that expells goes into the tank, then as the engine cools and the expanded water contracts and causes a vacuum, it will suck the expelled water back into the engine instead of air.
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Old 07-02-2008, 17:40   #3
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I'm willing to bet the the petcock is only being used as a plug for the hole that use to support a sending unit for a thermometer or temp alarm.

Never ever had to bleed the air from a cooling system but it could be possible, I guess!
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Old 07-02-2008, 17:52   #4
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I'm willing to bet the the petcock is only being used as a plug for the hole that use to support a sending unit for a thermometer or temp alarm.

Never ever had to bleed the air from a cooling system but it could be possible, I guess!
they were factory installed.

Yes it's to bleed the air out of the cooling system. Don't worry if it breaks off. It's NPT thread. Just retap it.
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Old 07-02-2008, 18:01   #5
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Does your boat have a heat exchanger? If so, on the freshwater side, any air in the coolant will "burp" out the coolant reservoir. And all you need to do is keep the reservoir topped up to the correct level. If the petcock is on the seawater side of the coolant system then you do indeed need to prime the cooling system since the rubber raw water impeller can burn up relatively quickly if it is run without raw water. If the petcock is higher than your raw water impeller and near it, then most likely that is its purpose. I prime my raw water cooling system by forcing water using a garden hose, in to the sea strainer until I see a steady stream of water coming out the wet exhaust.

Also, a small petcock on the raw water side is required for dripless shaft seals...but this probably has no relation.
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Old 07-02-2008, 18:03   #6
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I'm willing to bet the the petcock is only being used as a plug for the hole that use to support a sending unit for a thermometer or temp alarm.

Never ever had to bleed the air from a cooling system but it could be possible, I guess!
When you fill the coolant tank with coolant, it will fill up as soon as the water seals off any escape for the air contained in the upper part of the engine on some engines. By opening the bleeder valve, you allow that air to escape and water to fill the voids in the cylinder head before filling the tank. The other way is to start the engine while filling but you will have to start and stop it several times as air bubbles reach the reservoir. You always stand the chance of air bubbles being trapped in the engine.

Some engines are designed with the upper hose at the top of the cylinder head (above the thermo housing) and that eleviates that problem.
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Old 07-02-2008, 18:18   #7
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You don't want to do your initial filling of the cooling system through the upper coolant reservoir. Pull the radiator cap and do it there. As Kanani says, open that bleed valve (if it exists) on your engine. There will always be some "burping" of your cooling system after changing the coolant...watch for that in your coolant reservoir.
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Old 07-02-2008, 18:32   #8
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Thanks guys. The petcock is a factory part. I bought a replacement but I can't get the old one out. I'm certain it will just shear off if I keep torquing it.

The petcock is a $4 part. The thermostat housing is $100. I'm chicken

The only problem with the petcock in there is that it wasn't screwed all the way in and opening it doesn't do anything now.

Here is a link to a picture of it... part #1 is the petcock.

Torresen Marine.
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Old 07-02-2008, 18:37   #9
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Yeah, if it looks like it wont work its way on its own then leave it in. I know how expensive Westerbeke parts are!
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Old 07-02-2008, 18:43   #10
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The petcock is a $4 part. The thermostat housing is $100. I'm chicken
Darrel,
take the housing off and run down to your local machinist. They'll get it out. The petcock is brass and the housing should be Aluminum. If the threads get messed up. Drill it out and tap it to IIRC 1/4" NPT or what ever the next size up is. Go to ACE Hdw and get a bushing. It'll be fine.
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Old 07-02-2008, 21:30   #11
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Darrel,
take the housing off and run down to your local machinist. They'll get it out. The petcock is brass and the housing should be Aluminum. If the threads get messed up. Drill it out and tap it to IIRC 1/4" NPT or what ever the next size up is. Go to ACE Hdw and get a bushing. It'll be fine.
Dotto, but.........

Get a bronze reducer (bushing) at the marine hardware. Less difference in the metals, less erosion.

And when you put the next one in, use Rectoseal® #5 pipe sealant. It leaves a barrier and seals between the two surfaces if screwed in just snug. The exchanger's internal pressure shouldn't be over 10 psi.
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