Since these bolts go all the way through into the crank cavity and water pump cavity, what do you guys use to keep the oil/water from leaking past the threads?

I just use some nickel anti-seize for the threads on the water pump bolts.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

For the water jacket: Loctite 1158514

For oil exposure: Loctite PST 592

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I replaced the water pump on my '81 300SD about six months ago (thought it had gone bad and was squealing but it turned out to be ok after 260,000 miles). I did not use anything. Just replaced the bolts. No problem with leakage so far.

Smart that you checked. Enough of that and you can realize a leak-free engine. I coat threads w/ Permatex "gasket sealant". It is a purple liquid, thin & gummy. Cleans up easy w/ ethanol. Its main purpose is to coat cork gaskets, but must be careful that you don't use too much so the gasket gets slippery and extrudes.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans