Replacing the crankshaft seals usually means a complete engine stripdown as its best to push the old ones out from the inside of the crankcase halves - and new bearings should be fitted too. Seal failure allows air to be pulled into the engine from the outside, which gives unreliable running and tickover. The common reason for their failure is either old age or the wrong grade of lubricant - in which case there is usually piston damage too.
John, April 2009
