Welch plugs/ core plugs

I have a little weeping and rusting around one of my core plugs on the exhaust side of my 3.8 mk2.

Is it a job that can be done with the engine in situ? It looks easy enough to me - remove the exhaust manifolds/headers, prise out the old, clean up and push in the new. There appears to be plenty of room. But I don’t want to start and then find out that it is impossible to get the new ones in.

Thanks for you help.


On certain ones, it can be done: if you have the early cup style ones they can be a little more difficult to do, but just be very careful and make sure they go in square and use something like the old fashion Permatex Number Nine on them.

I’m not sure how you could get a big enough hammer swing to set a brass plug inside that engine bay. High likelihood it will just leak again. In place repairs are best done with rubber Dorman plugs that you bolt in. They are supposed to be quite stable and probably last longer than the real ones.

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There is that too: I had three or four of those expandable rubber plugs in Tweety, and they had been in there for over 30 years.

Further to the Dorman plugs: Here on a 3.4 block and access is much easier indeed, although you will have to clean the holes first.

Bob K.

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If the plug on the rear of the engine ever leaked, seems you’d have to pull the engine to replace it. So I’m considering using one of these Dorman plugs in the back. Any reason not to?

Nope! Just make sure it’s well-set, with a good sealant.

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I have always thought that core plugs/welch plugs were to stop the block from cracking in the event of the coolant freezing. Until an engine builder (ex jaguar technician) told me they are purely to allow the sand to be removed after casting… so seal them up as water and pressure tight as you can.
Every day a school day.

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That is correct. I worked in a steel foundry in my early career. There is a sand “core” made in a mold the shape of the water passages, mixed with resin, and baked in an oven. It is set inside the main sand mold. Where you see those Welch plugs is where sand lugs on the core stuck out to support the core inside the empty mold.
Then the molten metal is poured in. When it solidifies, the sand is shaken out, and the sand core is blasted out in a shot blaster. Pouring sprues and molding defects are chipped or ground or blown off with a torch. Then it gets heat treated in a big oven. Then it is ready for the machine shop, all the cutting and drilling.

Sometimes the sand cores have wire stiffeners in them. Here I am pulling out a core wire in a Mark V head.

The Welch plug got it’s name from the Welch brothers. About 1905 they were out in the country around Detroit testing a car they had built. A screw threaded plug fell out, and to get home they hammered in a coin. A great invention was born that day.
The Hubbard Machine Co. is the direct descendant of the Welch Motor Co. and is very proud of their heritage, and still makes the plugs.


I hope you hung that wire on the wall: that is a historical artifact!

I’m afraid a concours judge will say, “Where’s your core wire?” :rofl:

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I thought internal passageways were not judged? :slight_smile:

The cars’ or the owners’ ?

Yes… plus 20 latex gloves.


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