Jorn, I've changed freeze plugs but not specifically on a 103 and I don't recall changing them on a Benz at all for that matter. Yes sealer is a good idea to prevent leaks since new plugs often leak/weep. They may stop leaking after numerous heat cycles.

Clean the bore sealing edge as much as possible and keep the new plugs in the freezer at least overnight to contract them some and don't take them out of the freezer until seconds before the install.

Try to block the water jacket with a rag to prevent the old plug from falling in and getting lodged.

I'd try to use OE plugs to increase the chance of getting the correct size and material.

Did you get a new hose to go from the water pump/t-stat to the head? Remember to put that on the water pump and slide it as far on the pump as possible BEFORE putting installing the head. Use the highest quality hose you can get.

Good luck!!!

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