Examples of jobs I would take in the garage days: replacing a steerer in a fork. Back in the day, all steerer tubes were threaded and every once in awhile they would break or get damaged. I would get the request to replace a steerer occasionally. I no longer do this as I now know that re-heating a fork is potentially creating a very unsafe- yet solid appearing fork . The chance of someone hopping a curb on a repaired fork and getting really badly hurt when the fork broke made the job not worth it.
Some jobs like the fork repair are obviously risky and have the possible result of serious injury but other jobs that I no longer do are not nearly as risky. What makes them something I avoid is less a question of liability and more a question of what I feel is the right thing to do when given the choice.
An example of what I get asked to do lately is to take an older MTB frame that will only take a 1" steerer fork and replace the head tube with one that will take either a 1 1/8" steerer or even a tapered steerer. The problem with this request is that someone wants to take a functional older bike-one that was probably really high end about 20 years ago-and try to make it modern. Trouble is, not only are the steerers different these days, the forks are a lot longer ( if you are talking shock forks ) so simply making the frame have the ability to run the newer fork does not mean that the bike will ride correctly as either a cool old bike or a cool new bike. What one will have is an old bike pretty much turned into a piece of shit-neither classic nor current- almost like what the Soviets were doing in the '50's with vivisection-a really inhuman experiment of grafting parts of living animals together-two-headed dogs and the like........disgusting .In effect , people are asking me to practice vivisection on their bikes. " Yeah, he had a Bontrager OR from 1994 but now he has a two- headed dog".
Another thing folks will ask me to do is to graft on a disc brake setup on a really old bike that was never made for that kind of structural stress. A lot of these requests are for doing this modification to aluminum frames. I usually have to explain that the welding process will weaken an already really old and tired frame. To re-temper the frame will require heat treatment. The heat treatment will destroy the paint so now the frame will have to be repainted.Now, the cost of the job exceeds the value of the bike by a large sum. At this point folks thinking that they could take the old Cannondale and put disc brakes on it for $ 20 and a sixpack get a significant reality check .
The sad thing is that for the most part these requests are coming from well meaning people who honestly don't have enough know how to realize that what they are asking for-even if it can be done-is a mistake. Back maybe 30 years ago a kid brought in a beautiful old English frame from the '50's and wanted me to 'legthen' the whole front end of the frame. Ignorantly I took in the job and attempted to heat and remove the tubes from the seat lug and BB shell. The lug and shell literally disintegrated and the frame was destroyed. I felt terrible and wished that I had never touched it.The frame was a Hopper 'Vampire' and I have not seen one since. The kid was understanding and did not rake me over the coals as he knew that he shared some of the blame for the demise of this irreplaceable old relic.
So now, many years later when someone calls me with a request for a job that I know will turn a functional bike into a two-headed dog I'll think of the Hopper Vampire and also those insane Soviet scientists in the '50's that just couldn't let a dog be a dog.