If you want to gather honey, don't kick over the beehive.
Most people are really bad at seeing the other side of the coin and it's human nature to get defensive when confronted or criticized, even if you know you did something wrong. We all do it. Maybe try a different tact with this guy. He probably knows he did something wrong and doesn't want to admit it, plus you are questioning his knowledge and skill with transmissions (which may not be that good, but to him, he's a great transmission guy and you're someone he doesn't know trying to say he sucks). Try saying something positive and appreciative to him (stick with me here guys, this sounds strange but you'll understand in the end). Something like you appreciate him trying to help you out and that he knows way more about these things then you do (something along these lines returns his feeling good about himself - also...it has to be genuine appreciation, not flattery. Flattery is empty and most people see through it. Think about something you can honestly appreciate and use that). Offer to pay for more parts if he'll supply the labor and that you understand a gamble was taken to save money, and that you again, appreciate him trying to help you out. 9 times out of 10, once you restore how they feel about themselves, they can then admit to making a mistake and will then "fix" the situation. <img src="/forums/images/graemlins/cheers.gif" alt="" />

Also, getting the guy to say the word "Yes" a few times before the offer or compromise helps with him being in a more positive mindset. Even if it's saying yes to completely unrelated questions (Do you open at 7am? "Yes", Is that a Ford? "Yes", Is the sky blue because of the way light refracts off of water vapor in the atmosphere? "Huh?" <img src="/forums/images/graemlins/notooth.gif" alt="" /> ).

Of course if the guy is a total jerk <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> and is self-absorbed, he'll be the 10th and this tact won't work. It's at least worth a try! Good luck. <img src="/forums/images/graemlins/patriot.gif" alt="" />