Our short answer to this questions is NO we do not recommend it – we HIGHLY do not recommend it !
Now with that being said, we have had a few people contact us and state that they have performed the install with the engine in the car and they were also successful – i.e. NO LEAKS. We are by no means stating that it is not possible; however, we are engine builders and have installed many of these seals. Knowing the ins and outs of the install procedure, we would not think of trying to attempt to install one of our rear main seals in an engine with the engine still being in the car. Cleaning the seal groove properly would be nearly impossible with the crankshaft still in place. Another tricky part of the install is the oil pan gasket. Something else we would probably not consider performing with the engine in the car either.
We find that most gasket and seal failures are the result of improper installation procedures and or directions are not being followed during installation. Surface preparation and cleaning are very important when installing any gasket or seal. This is the main reason we do not recommend an in car install. Another issue with the in car install is that it is very difficult to place Permatex Ultra Black sealer in the anti rotation grooves in the block and properly back fill the seal cavity. Trying to do all of this properly while the engine is in the car greatly increases the difficulty of the install process and in our opinion greatly reduces your odds of have a successful / leak free installation.
Or if you would like to purchase a seal with the same sealer we use Shop here….