What is this part called?

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Echelon

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I have an aftermarket radiator from a mk3 vw, it does not have a radiator cap. We have an inlet and outlet, then there’s this bolt with a rubber seal/gasket that blew out. I can’t find the replacement plug for this anywhere so I’m thinking I’m just the wrong term?

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You can see it from the top down view of the radiator it has the square in the middle
 

CascadeTraveler

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Dilldog

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Are the threads in the radiator still good? If so just get some liquid pipe sealant and reinstall it. If the threads in the radiator are not still good, you can always try heli coiling it, or honestly for the easy not really right repair JB weld it in place, youll just have to pull a radiator hose to drain it in the future.
 

smritte

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Being as its not in the bottom of the tank, I would hope its not a drain. Unfortunately it there doesn't look like anything else is lower. It also appears to be a plastic tank. If that's the case, do not put Teflon tape and you cant heli coil it. The threaded area is not designed to be the seal. The rubber seal is what you get. What bothers me is you said it popped out and it looks like someone already tried to seal it with tape. In a plastic tank if you make that too tight its going to stress crack. Ive seen plugs like that before. most likely its specific to that radiator.

The threads on that plug are not going to strip even if its a metal tank. The torque on something like that will be measured in inch pounds. You really only have two options, clean the threads and epoxy it and hope the radiator isn't cracked in the thread area or buy a new tank.

I have seen quite a number of "backyard" fixs on things like this in the shop and out on the trail. The question is, do you want to possibly have it fail on you, leaving you stranded and possibly costing you a motor.
 

MarkD

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it looks like a plug in place of a fan switch to me, perhaps if not fitted to your car other cars use the same radiator with an electrical fan. as smritte says i would be concerned about it popping out, especially as its metal. do you have a local hydraulic shop, this kind of plug is likely to be used in their field so they might be able to help you.
 

CascadeTraveler

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The pic of the radiator didn't load when I first looked at it. I'm inclined to agree with MarkD looks like a plug for adding a switch or temp probe. Threads look similar to the switch sold below, you could go to the parts store and compare threads on switches they have in store. Just but a switch and use it as a plug or use it to wire a fan later.

 

smritte

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Look at the picture of the plug. Its metal. The radiator tank is plastic. Someone has put what looks like Teflon tape on the plug threads. If it was done for a leak, the threads in the radiator are probably gone. If it "popped out" like he said, there are no threads left. The plug isn't going to be the issue. The threads are not the seal area on that, the gasket is. If the tape was cleaned off the plug, you would have threads. A plastic tank is not going to strip threads off a metal plug.
I do agree, the plug is most likely there because a sensor would be installed for another option. If replacing the plug fix's this, it will only be until enough pressure builds to blow it out again.
This is something I've seen before. The sensor or plug leak's, the customer either over tightens the plug, ruining the thread on the plastic tank or tries to seal the threads causing the plastic threads to deform.
 

Echelon

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The plug was still seated in the hole just fine and I had to use a wrench to get it out, it was just the rubber seal split apart. The radiator was brand new and the teflon tape was on it originally either from the shop or the vendor? Either way I went to a hydraulic shop and they had a really high quality seal that should hold just fine. I will give it a go and see if it holds pressure.
 
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smritte

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The plug was still seated in the hole just fine and I had to use a wrench to get it out, it was just the rubber seal split apart.
Good deal. Your post gave the impression it came out on its own. I've seen many of those plugs factory and aftermarket. I'm still bothered by the tape on it. Don't over tighten it and as long as it bites in you will be good.