Using the less expensive mini motors in standard motor rockets is a real money saver. And these are so easy to use

Building a model rocket with a dedicated 13mm motor mount tube is fun, but you can't use 18mm engines in it. Another option is to build the rocket with an 18mm motor mount, then use these handy little adapters so 13mm Estes engines are a drop in fit. After a few launches, you'll notice that the top of the adapter will get covered in soot and some parts may partially deform a bit. However, as long as the engine fits into the adapter and the adapter fits into your rocket, it should continue working just fine.

For the sale price that these were plus the versatility of flying cheaper 13mm engines they are worth it. You get 4 13mm engines per pack instead of 3 to fly in 18mm rockets. Money saver and a little experimentation makes these great. They can get a little burnt on the aft end but hey they're plastic.