I collect motors (they are a nice, dirt cheap rocketr item to collect), and Quest motors are a real collectors dream (or nightmare!)
There are some that are made 17.5 mm x 18mm. They are too slender and allow EJ pressure to excape. These are often the Blue A6-4s, but some Blue B6s I have are 17.5mm as well.
Th Quest 'Black n Blue' label C6s are 18mm in dimeter, but are too long, by about 3mm. These are made in Germany for Quest, and have German printing under the label.
The older Quest 'White/Orange' label motors are usually 'spot on', but are no longer manufactured. (Most of these were made on the old MPC motor making machine that FSI had)
I am looking to buy some of the 1991 'Green/White' label Quest motors if anyone has any.