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Coolant Reservoir Cap spec?

dyeager535

dyeager535

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Anyone by chance able to measure the diameter of one of the vent holes on the underside of the coolant cap? Drill bit, decimal, standard, metric, not picky.

Done a couple cheap/easy things to try and stop the ejection of antifreeze out the cap as the truck bounces and jostles around, but not there yet. Getting close, but I feel I should never lose coolant.

Realized there is small diameter threaded tubing available, and if I know the hole size, I can see if screwing a piece of tubing into the hole(s) to move the vent away from the top of the reservoir will solve my issue.
 
Yeah I have ejected some coolant on bumpy/steep terrain. My 84 has a large elbow/ neck on the top of the reservoir, just for that purpose, I suppose. And it's a weird oval shape so I can't even put a hose on it to route it out of the way.
 
Plastic bulkhead connector in cap and long vent hose with filter in end ?
 
Basically same idea as an axle, trans, etc. vent? I'm sure that would work if my tank was low enough, but mine is up on the fender. Not sure I have much room for something like that.

I was hoping to come up with something inside the tank. I've used screws to block off a couple of the vent holes already, so I know that threading something in there will last and hold, with a short length of tubing protruding inside the reservoir I think it would be the end of leakage.

At least on my setup, the reservoir is nowhere near full when hot, so my idea, I think, will work. Heck even a small hose barb would work, if I knew what diameter hole I was working with.
 
Now if the truck will survive a week of driving to find out if this works.

What I tried first. It helped, but still lost a bit of coolant:
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10-32 threads, just pushed it through the existing hole:
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I *think* the added slots will help make sure coolant can't flow easily if it does splash just right:
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Interesting, let us know how it works. I cant imagine there needs to be a lot of air venting from the cap unless you are in full boil-over.
Im wondering if a small tube (1/4" copper waterline) in the shape of a upside down fish-hook "J" with the opening close to the top of the cap would work?
 
Eventually splashing fluid will build up at the bend, and as the only vent, will probably just get pushed out via pressure.

A sintered bronze filter would work fairly well I bet, but I didn't find one that was threaded 10-32
 
Yep! I bet those would help. The longer one, could probably shrink wrap all but the end to reduce the effect of slosh.

Could also leave the shrink wrap long, like a short straw, to further limit the ability of coolant to be expelled.
 
So you're losing coolant out of the overflow tank? Doesn't that imply the radiator cap is weak?
 
No, the coolant is at the proper level in the tank.

The coolant isnt so high it's overflowing the tank if that is what you are picturing. As the truck bounces through hundreds or thousands of potholes over a few miles, the minimal coolant that is in the tank repeatedly sloshes and splashes. Eventually some of that finds it's way out the cap.

The new "nozzle" vent seemed to work good. Good enough that on a random check I noticed the remainder of fluid that leaks is getting past the gasket on the cap. I'm using what GM did, a thin paper(?) gasket. Even though I block sanded the mating surface flat, there is some sort of sealing that isn't happening. Probably just need a thicker, slightly softer gasket that will allow for a snug cap, but able to conform better to whatever isn't sealing right. Not sure what material that would be, but I'll have to figure it out to try.
 
Here's what it looks like after six days of bouncing around:

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If you look close you can see the fill level is at the cold mark, so it's not like it loses a ton of coolant, but I hate leaks, period.
 
are you sure your jug is not cracked? Had that happen before...
 
Guess the question is did your modification substantially reduce the ejected fluid or was it a small change?
Was that a new cap and gasket?
Seem to remember you saying you used a paper gasket. Have you tried a rubber gasket or non-hardening sealant around the cap gasket?

As mentioned, cracks, especially in the seams are common. I replaced two tanks over time. I couldn't find one crack on upper seam until I removed the tank, removed the cap and placed a cover/stop in its place, then turned the tank upside down and added coolant thru the overflow hole and let it sit upside down. I found the crack/fluid leaking out half way up the seam. Might be worth a check but it sure looks like the cap is not properly sealing around the threads.
 
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I'll just answer all here.

I don't think the tank is cracked. I cleaned it all up initially, and happened to check it after doing a bit of off-pavement driving. The only spot that was wet was around the fill cap. The way the inner fender is wet after far more driving, and what I saw on the tank, indicates it was on top of the tank and ran down the sides. I've never seen evidence of a leak after long periods of it sitting either. But it's going back to storage so maybe I'll clean it up again and verify.

If it isn't a leak, I assume others either don't hit that many potholes that often for that long, and/or the cap just isn't sealing right. I am using a paper type gasket on it. I have no problem trying a different material, but I think it would need to be something soft so the cap doesn't need to be cranked down very hard. I'm not really sure what material is up to that task.

The modification did seem to make a difference, I lost very little coolant this season, normally the overflow would be empty.
 
The picture makes it look like the cap is dry, implying a leak around the threads. Maybe a flat rubber seal like from a gas can would work.

I suppose it's possible I don't notice a leak because things get wet from wheeling anyway, but I don't think I've added coolant this year and I just checked it a couple days ago.
 
I agree, a thin rubber gasket you can make from an inner-tube or ?? or you can try a small amount of non-hardening gasket sealant (permatex, Loctite) on the top lip of the tank so when the cap is cinched down, its sealed. This will at least verify if the cap/threads is leaking which is what it appears.
 
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