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Cap PSI pressure ratings for Big Block

What PSI caps should be ran on a big block with an expansion tank? Psi for all three caps - the expansion tank, thermostat housing and radiator ?

I have about 1.5 quart expansion tank that hooks to the thermostat housing/full neck and no catch can on the other end (pic below)
 

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Radiator cap 20#, the cap on the "deaeration tank" should be 15#, expansion tank shall be vented, being sure the hose from the Deaeration tank is always submerged in coolant.
 
Radiator cap 20#, the cap on the "deaeration tank" should be 15#, expansion tank shall be vented, being sure the hose from the Deaeration tank is always submerged in coolant.
Thanks - what about the cap at the thermostat housing/fill neck? Or is that what you are referring to as the Deaeration tank?

If that’s the case then what PSI cap at the expansion tank? Pic below:
1657570460635.jpeg
 
There should be a vented cap on expansion bottle from the hose leading from the thermostat/fill neck housing
 
.502,
To be clear, what you show in the picture is not the expansion tank, it is just your fill point. John is talking about wherever that vent line just under the cap goes as the expansion tank.
 
.502,
To be clear, what you show in the picture is not the expansion tank, it is just your fill point. John is talking about wherever that vent line just under the cap goes as the expansion tank.
Thanks- I understand and expansion tank is in the first picture.

What cap setup do you personally use at both the thermostat housing and expansion tank?

I’m trying to figure out is whether all three caps be the same PSI rating (at radiator, thermostat housing/fill neck, and expansion tank) or different? E.g. The radiator cap having the greatest PSI rating and the expansion tank having the lowest?

If a “vented” cap for the expansion tank is needed (suggested a few posts above) - would a low PSI cap (7 or 8) suffice?
 
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Thanks- I understand and expansion tank is in the first picture.
Yah, 10-4, I saw that later.

I’m trying to figure out is whether all three caps be the same PSI rating (at radiator, thermostat housing/fill neck, and expansion tank) or different? E.g. The radiator cap having the greatest PSI rating and the expansion tank having the lowest?
Assuming the top of your radiator is below your thermostat housing and fill point, the goal is for your radiator cap to never open. Note this advice to someone else in another thread.

You want a radiator that does not have a cap.

The radiator should be isolated to guard against vibration.

Given you are not going to swap the radiator out, you want the radiator cap set as high as possible, you do not want it to open. Hence the reason John is recommending a 20# cap on the radiator vs. a lower pressure cap on the thermostat housing.
Radiator cap 20#, the cap on the "deaeration tank" should be 15#, expansion tank shall be vented, being sure the hose from the Deaeration tank is always submerged in coolant.


If a “vented” cap for the expansion tank is needed (suggested a few posts above) - would a low PSI cap (7 or 8) suffice?
No. For starters, drill a hole in the cap you presently have. At some point figure out a better setup that vents down do it doesn't get rainwater or insect nests in it.

What cap setup do you personally use at both the thermostat housing and expansion tank?

I'm still building my LS boat, I have not gotten that far. On my swamp buggy I run my hose to the expansion tank through a hole I wholered out in the lid of an empty anti-freeze jug, made sure the hose runs to the bottom of the jug and tie wrapped it off. That's a vented expansion tank.

The point here is that you want one single place in the system that vents off excess pressure and you want that to be at the highest point of your cooling system. When anti-freeze gets hot, it expands, some of it needs to go into the expansion tank. This also vents off any air in the system. When the system gets cool, it sucks that coolant back into the system from the expansion tank. The expansion tank should only be 1/4 full when cold and the level when cold should remain stable, otherwise you have a leak.
 
The cap on the overflow tank (expansion) is the gasket removed ?

Sometimes guys pull the rubber gasket seals out to allow venting
 
The cap on the overflow tank (expansion) is the gasket removed ?

Sometimes guys pull the rubber gasket seals out to allow venting
Yah, 10-4, I saw that later.


Assuming the top of your radiator is below your thermostat housing and fill point, the goal is for your radiator cap to never open. Note this advice to someone else in another thread.



Given you are not going to swap the radiator out, you want the radiator cap set as high as possible, you do not want it to open. Hence the reason John is recommending a 20# cap on the radiator vs. a lower pressure cap on the thermostat housing.




No. For starters, drill a hole in the cap you presently have. At some point figure out a better setup that vents down do it doesn't get rainwater or insect nests in it.



I'm still building my LS boat, I have not gotten that far. On my swamp buggy I run my hose to the expansion tank through a hole I wholered out in the lid of an empty anti-freeze jug, made sure the hose runs to the bottom of the jug and tie wrapped it off. That's a vented expansion tank.

The point here is that you want one single place in the system that vents off excess pressure and you want that to be at the highest point of your cooling system. When anti-freeze gets hot, it expands, some of it needs to go into the expansion tank. This also vents off any air in the system. When the system gets cool, it sucks that coolant back into the system from the expansion tank. The expansion tank should only be 1/4 full when cold and the level when cold should remain stable, otherwise you have a leak.
I appreciate the information and will give it a try.
 
Here is where I’m at. The systems I have received responses for are for overflow tanks as opposed to expansion tanks (vid below).

I have an expansion tank an I’m still wondering what caps to use at the thermostat housing/fill neck and expansion tank but feedback and reasoning has led me to the following, which I will test:

1) The radiator shouldn’t have a cap since it’s mounted low (a radiator without a cap provision is best). The expansion tank cap on an airboat (with a low mount radiator) will serve the purpose of the radiator’s cap in a traditional vehicle since it is the highest point. Since my existing radiator does have a cap provision - I’ll use a high PSI cap there (20+) with that being the highest PSI cap in the coolant system.

2) I surmise that the expansion tank fill neck should be around the typical size of 16 PSI found in most vehicles, being less PSI than the cap at the radiator radiator. I reached out to some airboat manufacturers about the cap rating at the expansion tank.

3) I’m not sure about the thermostat/fill neck cap rating. The vid below doesn’t mention one, but I would assume it to be an integral part in allowing coolant back and fourth between the coolant system and the expansion tank so I’ll probably go with 16 PSI there as well.

 
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I have an expansion tank an I’m still wondering what caps to use at the thermostat housing/fill neck and expansion tank but feedback and reasoning has led me to the following, which I will test:

If an overflow and expansion tank had a baby it would look like what you got, because it's neither.

3) I’m not sure about the thermostat/fill neck cap rating. The vid below doesn’t mention one,

Most people call that a hint. The video does not mention it because it would be wrong.

At present you have 2 pressure caps in series, the one on the thermostat fill neck and the one on the overflow/expansion tank. If both of those are 15 PSI caps, your system would hold to 30 PSI (2 times 15), which would normally blow up your radiator and/or hoses (nothing worse than a steam explosion), except that 20 PSI cap on the radiator will vent first. Any way you slice it that BB 502 is spewing anti-freeze all over the marsh, so please don't run it as is!

If you delete the cap on the thermostat housing then you would have an expansion tank system as explained in the video. Rather than venting that fluid into the marsh, please add another tank (an overflow tank) on the expansion tank vent line. I have no idea why someone would build something like this as it just adds complexity.

If you want to make it work simply, leave the system just as you have it and follow the previous instructions.
 
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