Overlooked extruder problem?

A friend recently got a little printer as a kit for a bargain and it was time for some first prints.
He already tried for a few days but always ended with jammed nozzles or really bad prints.
We did the whole calibration thing from scratch it it was there that I noticed a problem.
No matter how I adjusted the stepping it would never really match.
Sometimes there were two or three lenths correct but the next too short.
Other times it was too long.
When we cleaned the extruder, a printed older wades model, I noticed that the hobbed bolt was loose in the big gear!
The printed gear was fo a 14mm head but the bolt in use had a standard 13mm head.
Every once in a while it would get a bit loose and "twist" in the gear a bit.

So if nothing works out with your new printer kit you might want to check it the hobbed bolt actually matches the extruder gear
I printed him one in 13mm and he is now happy to do the final tuning with a working printer.