I plan on pulling my engine to replace my oil separator plate with a new metal one and also doing the clutch. My question is....do I replace the main seal while the engine is out? My car has 255,000 miles on it.

General consensus is don't touch the rear main unless it's obviously leaking.  Too many chances for a poor installation!

they rarely leak and people sometimes don't install this seal well - it's large and awkward. 

 

if you can carefully and methodically install a seal and buy a Subaru OEM seal, you shouldn't have a problem.  plenty of us have never had a leak.   after a quarter million miles and coming up on a quarter century there's certainly good reason to consider it if you want. 

 

10 years and 100,000 i'd say pass.  20 years and 250,000...i'd personally do it, but recommend caution as stated already. 

I guess I don't quite understand why it is so diffficult to replace the main seal and get a good seal with the new one? Everyone seems to be afraid to touch it but it just seems like another seal to me?

oh yeah - if you want to replace - by all means, like i said, replace it.  you're right, it's just a seal, there's nothing different about it mechnically speaking.  

 

they have two compelling data points, regardless of guessing, technically pontificating, ruminating, arm chair quarterbacking why: 

1. Subaru rear mains have a higher percentage of leaking after replacement.  (even from folks with Subaru experience).  this has been true for like 3 decades....it's nothing new.

2.  they very rarely leak - fairly typical for a Subaru engine to never need one replaced - that's not the case with front crank and cam seals.  also true for 3 decades (probably more)

 

for that reason - it doesn't make sense to always recommend replacing them any time an engine is removed and some caution is very beneficial to average DIY folks.

 

but if you want to replace - by all means, replace it. 

I would replace it but that just me. I've replaced many and never had one leak that I know of but I do understand everyone's concern. The inner lip of the seal can leave a very slight groove on the crankshaft especially when you have a lot of miles. If you don't install the seal at the exact same depth as the original it can cause it to leak if its rubbing against the groove wrong. If the seal is flush with the seal housing lip then it's pretty easy to get it done properly if you have a flat piece of wood like a 2x4. Just keep going around it until your 2x4 is bottoming out on the engine. If it's recessed a bit you can use a PVC coupler that is the same size but if it's recessed then I  just install it flush as long as the mating surface is smooth. If there is crust built up on shaft surface I use very light pressure with a green scotchbrite pad and some brake cleaner to get it smooth. Just make sure to clean out the seal area really good to get any grit out of there.