I am combining parts from two different ‘91 V12 engines to make one good engine. However, on removing the three sensors on the “A” bank water manifold, I noticed that all of them were installed with a sealer material on the threads (made them fanged hard to get out, too!)

is this factory/correct? If so, what do I use to ensure these don’t leak?

I've attached a pic with the sensors/locations circled.

 

Yes to the factory.

It was a type of sealant on the threads of the sensors as packaged from whoever made them. When installed, and then heat applied, running engine, it set off and made sure no leaks, so no warranty complaints/cost.

I use a Permatex #3 Non Hardening Gasket Goo.

Being in the USA, the GM head bolt sealant of the old days is the same stuff.

Nothing rocket science, as long as any OLD crusty stuff is cleaned off the sensor and the hole. No need to go all out, just clean them up, smear the Goo, fit and forget.

 

Thanks Grant!!

 

I use Loctite 567 thread sealer. You're not looking for a thread locker, but a sealer.