Engine assembly labor and machine shop labor are two different animals.One relys on precision machine work that sometimes take a lot of setup and check/recheck time,assembly and engine build time should be pretty much basic,checking the clearences and fit and putting everything together correctly.Based on the amont of machine work needed a estimate of cost can be established and quoted within reasonable+- dollars and the same with the assembly or build.The customer shouldn't have to pay for dead time or problem solving issues that arise.I personally would never have work done based on an hourly shop time,if someone is charging by time and materials in any industry someone is getting screwed.I can understand that if someone brings a job in that will only take a few hours then it would be appropiate to charge a flat hourly rate since it is taking you away from shop productive time.Both partys should have a clear understanding of the the cost before entering into an agreement and if something unexpected comes up discuss it and come to terms as how to proceed.Too many times when the job is done and the cost is over the agreed amont it's too late salvage a relationship and both partys feel cheated.With this economy any good customer and business relationship is important to maintain.