Do I need to split the cases to replace the main seal on the right side of the motor on an 82 RM250z. I am getting black tranny oil coming out my silencer. There is a bearing that overlaps a little bit onto the edge of the seal from the outside.

You are most likely going to have do as Ray said and pull engine up with cherry picker or GOOD JACK to back the tranny off the rear main seal. Keep in mind that The oil pan has a front and rear main seal that may need to be replaced when you do this. Not a quick job. Get a Chilton book and read the section regarding oil pan gaskets and seals. Hope this helps

Lots of tools!!! engine crane, engine stand, tranny stand, fluid collections buckets (collant, engin oil, tranny oil, transfer case fluid.
Remove transfer case, remove tranny. drop oil pan, lossen off rear eng bearing and remove slowly, remove seal materiial from upper seal have, install new seal material in both halfs, reinstall bearing and turgue to specs for bearing, install new seal and reinstall oil pan, reinstall trannt, reinstall transfer case, refill all fluids. start it up and check for leaks everyday for next week. it take 2 people to reomove case and tranny. it wiil take 16-18 hours of constant work for an experienced DYI

Hydrostatic Transmission Fluid is actually 10W-50 synthetic motor oil (like mobil). Do not use hydraulic fluid or transmission fluid.

Hydro static transmissions on most lawn tractors are very similar. Many of them are made by the same manufacturer. Tuff Torq makes a lot of them. They definitely make all the John Deere lawn tractor hydro drives. The lawn tractor manufacturers (like JD) often say they are sealed and not serviceable. That is flat out, a lie. All of the JD hydros, and most all others, are serviceable. A complete line of rebuild/replacement parts can be found at Tuff Torq. Info and instructions can be found there. It's quite likely they made your tranny too.

Additionally, I have found that the mower manufacturers fill them (or spec them) with motor oil, 10W-30W. In the service manual, and at the hydro manufacturer, you are told if your tractor is used in extreme or heavy duty situations it's best to drain the oil and replace it with 10W-50 synthetic motor oil (like mobil). Truth is the tranny should have this oil from the get go. It will give you more power and extend the life of the tranny. The mower manufacturers are all just being cheap. They figure why not, you're the one who has to pay for the early failure. Oh yeah, one other minor detail, the tranny manufacturer also recommends regular oil changes, like every 100 hours.

Virtually all of these trannys have a black plug on the top. You have to remove the tranny from the mower (not a difficult things to do, takes about 30 min). Pull that black plug and turn the tranny upside down for about 30 min to completely drain. Most hold about 2 quarts. Flip it back, right side up, and refill. Install plug and reinstall tranny. Note: under the plug there is often a magnet which collects metal shavings. Lift this magnet out while you have the plug out, clean it and replace it. It may fall out while the tranny is upside down. When you refill the fluid just keep filling till it comes up within a quarter inch of the plug. You may have to go through a bleed process after reinstalling the tranny. The tuff torq site explains the bleed process.

If you have the space drop the drive line . You should have a tranny shaft poking out of the housing and the seal should be right at the back. 2 ways you can go. Get a good heavy screwdriver and lever out the seal or drill a small hole and with a slide hammer pull ot the seal. To fit the new seal use a piece of pipe That has the same dia as the outside of the seal tap the new seal in keeping it square to the hole. Tap in till level with edge of case. Check oil level and replace any lost oil as some will run out when you drop the drive line

well the bell housing is for the transmission side, there is a main seal for the crank and oil pan combination, they get soft and deteriorate over time, and the transmission has an inner and an outter seal same with these. you did not specify if it was red, or black, Im assuming its black, and in this case it would be the oil main rear seal needs replaced.

dougnuts are never a good thing. the front seal on transmission could be leaking,if so the fluid will be red.it will drip down between the engine and transmission. if so, the transmission will need to come out fo r a replacement of front transmission seal. if it's clear fluid,it may be a bad steering rack. if it's black oil, it could be a bad rear main engine seal. if bad rear main seal,that transmission will need to come out for replacement of rear main engine seal.a abd rear main engine seal will also leak down between the engine and transmission and be black oil. check for color of the fluid leaking to give you a starting point to diagnose what to look for. thank you for choosing fixya.com

you have several points to check!
the dip stick tube has a "O" ring seal at the bottom where it fits to tranny.
a reverse switch on tranny & the shifters selector shaft has a seal that may be leaking !
there are the cooler pipes from cooler to tranny [in & out] plus pump seal to the torque converter [leaks out between engine & tranny bellhousing] & then there are the drive shaft or shafts seals
so wipe it all clean & park it in a diffrent clean spot!
some will only leak when its runnung due to tranny,s internal pressure!

Here is a website where a manual for you outboard may be purchased.  Outboard Marine went out of buisness in 2000 and thus no parts of origial manufacture are support is available . Therir are secondary markets .    THe Lower case splits in half and their is a seal around the prop shiftm which is pressed in. The seal between the cases are cut o-rings which are placed and glued into place with gasket seal.
The water pump is located where the case splits a little bit higher. You must open the little port on the side and take out one of the screws to the connecting shift linkage to split the case. After splitting the case the drive shaft and water pickup tube(s) wil come out of the powwer head or the tubs may stick into the power head which is ok. Place the lower skag into a vice and remove the 4 or 5 screws to the water pump housing. Remove and replace impellor and impellor plate if available. If part(pieces) of the rubber impellor is missing flush out the tubes and the power head to remove them.   When reinstalling the lower unit use a flash light to make sure that the tubes return  to the proper place in the power head and the shift lever is resecured. If you need more assistance, just write back. I was an Outboard motor mechanic for five years about the time of your motor.  

Thats not likely. What you are seeing is excess oil from you gas and oil mixture. Not sure if your still running the oil injection pump but it sounds like you have been running a very rich oil/fuel mixture.
The gearbox oil is not accessible through the crankcase. They a totally separate sealed sections of the motor. If by chance you are losing gearbox oil your bike will billow a very white disgusting smelly cloud of smoke. If that is what is happening you may have a right side crank seal blown which will allow the oil to be sucked from the gearbox into the crankcase. Hopefully this is not the problem because with your model I'm 85% certain you will have to split the cases to replace the main seal on the right side engine center case. If thats the case I'd highly recommend you replace both right and left main seals as well as main bearings.
You will need some special tools to do this job. I can tell you what you need to do the job and if you like to DIY I can tell you how to do it and what to look out for.
Best of luck.