Main Crank Seal Replacement
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First of all you'll need the Lycoming SI1324A as a reference download here

Next you have to make a decision as to use a split seal or a solid (non-split) seal. Be sure to glue the ends back together again if you use this type of seal.  Personally,  I prefer a non-split seal as there is one less chance for the seal to leak.  The leaking seal I had to replace was a split seal.

One can use a solid seal but it is a bit of work to stretch it over the crank flange. I have done this in the past but you'll definitely skin up your knuckles!  If you decide to use this method be sure to protect the seal by wrapping lots of duct tape around the flange and oiling it prior to stretching it over the flange. 

After installing seals both ways, I decided I had to have a installation tool.  I decided if I couldn't borrow one from my A&P, I'd make my own.  If you want to make your own seal installation tool click here: Lycoming ST-383 Tool

After talking to the Lycoming rep, he told me to

A.  Carefully clean the seal area with a solvent (acetone, or MEK).  Cleanliness is vitally important!
B.  Stretch the seal over the flange (make sure you read the SI1324A).  I was surprised to find Lycoming does not heat the seal to soften it, or let the seal rest after stretching.  The just take it out of the box, stretch it and immediately glue it into place.
C.  Using Pliobond Nbr 20, put a light coat on the seal and sealing surface.  Be sure to protect the crank shaft sealing area from the adhesive.  LET DRY.
D.   Reactivate the adhesive by putting a second coat on the seal.
E.   Press seal in place and let set for 24 hrs.

OK, here we go!

The most critical step in replacing the seal is cleanliness of the seal and the case seal area I tried various types of cleaner, MEK, Acetone, Xylene, GooGone, etc and finally found that fresh MEK worked the best.  A tooth brush, acid brush, and wood stir stick seemed to work the best (heard a scotch bright pad will also be helpful).  The cleaning took about 2hrs.

Next is to stretch the seal.  Remove the spring and put a little grease on the flange and the internal part of the seal.  Next, put the seal over the flange making sure you protect the sealing area.  You DO NOT need to heat up the seal (according to Lycoming).  Just install it at hanger temp.  It was only about 65 f when I installed mine with no issues.

 Make sure you are careful of the prop lugs…

 Just run the tool around the flange and you are done.  It only took about a minute.  With that the seal now on the crank, reinstall the internal seal spring.   There is no need to wait hours for the seal to contract.  It will quickly do so while you are preparing it for the glue step.

 I then just took some stiff paper about 1” wide and wrapped it around the crank to protect the shaft from the glue.  Carefully clean the seal with MEK.    Place a thin coat of Pliobond on the seal and on the case sealing area.  Let dry, then reactivate the adhesive by recoating the seal and quickly push the seal into place.   

 Reinstall your seal retainer plate and you are done!