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OEM or aftermarket radiator cap?

1998 blazer 4.3L

I think i need a new radiator cap.
The overflow tank is full and overflowing.
a couple of days ago i cleaned the overflow tank and then filled it halfway with antifreeze,everyday since then ive had to add 1-2 cups of water to the radiator because it was low. I havent seen any water on the engine or on the ground.It seemed like the water was just disappearing.
Today i checked the overflow tank and now it is completely full is completely full up to the overflow tube least now i know where all that water went.

anyhow im going to get a new cap because i think this one is kaput,im leaning towards getting an aftermarket cap.The cap that is on there now doesnt look like any cap ive ever seen im not sure how its even suppose to work.
It`s a thin 1/2 inch thick plain plastic cap with no spring on it.
can I get any old aftermarket cap or are aftermarket caps vehicle specific?

If the resivour tank is filling up I do not think the radiator cap is the problem. Easy check when cool remove the radiator cap and see if the radiator is full. A failing radiator cap will in my experience allow fluid to escape via the cap and never make it to the resivour.

When GM built your vehicle, they didn’t make the cooling system pressure cap. It was purchased from a vendor/supplier.

So if you buy it from the dealer/parts store, you’re probably getting the same part.

One major supplier to the auto industry for pressure caps is Stant.


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no it`s not full, i have to add 1-2 cups of water everyday.

i only filled the overflow tank halfway and now it`s full, water is going in to it from the radiator but not going back out to the radiator.

Thats what i was going to replace the plastic cap with, a metal stant cap. maybe the plastic cap is suppose to have a spring or did have a spring but it doesnt have one now.

This cap has a 15psi pressure release but all the stant ones that the local parts stoer has are 16 psi pressure release, will that be ok?

The radiator has just a plastic screw on cap i said it doesnt even a spring on it so i dont know how it releases pressure, that`s why i want to get a new cap.

It’ll be okay.

All 1 PSI will do is just slightly raise the boiling point of the coolant.


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this is what it has now:

I want to get one of these:

If it screws onto the radiator, it should work.


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Replace it with exactly what came off. Don’t try to force that different cap on it.

The cap’s design depends on the dimensions of the place where it screws on. The cap has to match up with those. Especially important is the distance between the top surface (on the Stant photo above, the surface corresonding to the bottom of the red lever to the lower rubber washer.) The spring force acts on that distance, so it has to be correct for it to open & close at the correct psi. And of course the diameter of the sealing surface where that washer seats has to match up so it makes a water-tight seal. Make sure to compare the old cap to the new one, make sure those dimensions match. You don’t show the underside of the black cap, so hard to say if the Stant cap would work or not. Me, I’d only replace it with a cap the parts store had verified was OEM or an an aftermarket OEM equivalent.


Some parts I buy OEM and some I buy aftermarket OEM equivalents. If the price difference is small then I usually buy OEM from the dealer. This is one of those situations. How much more can an OEM radiator cap be than an aftermarket? I found an OEM cap for your vehicle online for less than $12. You have spent a lot of time on this to save maybe $6. Time is money. You’ve already lost.

Its a Chevrolet arent they basically disposable cars? drive em till they stop running then throw them away,no maintanence required. :imp:

Another vote on Stant cap particularly their lever vent radiator cap.

I don’t think those 2 cap types you show in the picture will interchange on any type of vehicle.

As the old saying goes, Turnabout is fair play


Why ? It seems that when replacing parts you should get what came on the vehicle to start with. The people who made the vehicle just might know what works.

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A radiator cap is not as serious as an engine replacement.