For those of you who don't like to do things twice, take notice. I recently replaced the 6-speed in my NC with a new to me previously enjoyed 6-speed transmission. When I pulled the transmission, I also replaced the rear main seal. Here was my first mistake; when you remove the rear main seal on an NC, it is required to drain the oil prior to removing the seal. On the NA and NB Miata's, the oil pan sits parallel to the ground and the oil level is below the rear main and even the top of the oil pan. On the NC, however, the rear main seal includes a portion that covers up the top of the oil pan. The engine in the NC is also tilted to one side when used in a RWD application like the Miata, so the top of the pan is not parallel to the ground and one corner of the pan sits below the oil level. Some of you may know where I am going with this... when I removed the rear main seal, oil came pouring out over the lip. I ended up scrambling to get the rear main back on to slow the leak while I got my drain pan to drain the oil.
This was mistake number 1, and other than an early oil change and a bit of a mess, there was no harm, no foul. Insert mistake number 2... when installing a new rear main seal, the factory service manual says to apply RTV sealant to the bottom flange on the rear main seal that mates up to the top of the oil pan. I had not read the factory manual, and my prior experience with NA and NB Miata's had not prepared me for the use of RTV on the rear main seal. Now when you are working on your back, under the car, you cannot get a great view of the flange on the top of the oil pan to see that there was in fact RTV there to begin with. So, I installed the rear main seal without applying the required RTV. Fast forward two weeks and a nasty leak had developed between the transmission and the block.
I had to remove the transmission again and replace the brand new rear main seal with another new rear main seal, this time using RTV on the top of the oil pan. This time I drained the oil, avoided the mess and got the job done correctly, using RTV.
TLDR Version: When replacing the rear main seal on an NC, be sure to drain the oil first and use RTV as directed in the service manual.
Now back to your regularly scheduled programing.
