Friday, February 3, 2017

Estes Sprint XL #7224 Build, Part 3, Tail Cone Fit

First up you'll have to widen the low end hole.
200 grit was wrapped around an 18mm casing. Constantly check the fit of the BT-50 engine mount tube.

The engine hook slot was widened with a small diamond coated file.

Even though the slot was wide enough at the back, the hook was still getting caught up inside. The interior walls had to be hit with a knife.

Here's the final fit with the mount slid in place. The BT-50 tube is flush with the end of the tail cone.
I did remove the finger tab and flipped the hook over.
The fins don't sweep back on this one so the engine hook seems longer than it should be.


  1. I'm considering doing away with the engine hook altogether on my second Sprint XL - trim the hook tunnel off, fill the gap, then retain the engine with tape like the original Sprint.

    1. Hi Openroad,
      Some forum post builders didn't like the engine hook and tail cone tunnel on the Sprint XL. I didn't mind it too much, I prefer an engine hook over friction fitting.
      That Sprint XL nose cone and tail cone are used again in the upcoming Estes Protostar kit.

  2. I happened to run into Dave Bucher at NARAM this past Summer. He was carrying around his XL and I was carrying my scratch made. We compared models and agreed that his looked better with no hook. His method of retention was to leave about a quarter inch of motor tube sticking out of the cone, and then to just tape the motor to that 1/4-inch of tube. A simple and elegant solution (and so very typical of California Dave.)

    1. Hi Tim,
      First time I heard about a simple wrap of masking tape around the exposed motor tube and engine, I didn't think it would work! It does and very well!
      To protect the 1/4" of exposed engine mount tube, I'll give it a light coat of CA glue. Otherwise, pulling the tape could delaminate the tube or leave a gummy residue.

  3. I've just started building the Sprint XL. Since I have a kit and the tail cone has a grove for the engine hook I think I'll use one. A scratch built one probably would look nice with out one - and be easier to build. Adding an engine hook to a paper boat tail may be tricky and prone to cause issues. Thinking of Chris's Starlight Jayhawk which had a paper boat tail and used friction fit. Too bad you had to let that one go.

  4. Wow! I'm building this model and that is really a lot of plastic you need to remove. If someone just tried to assemble this model, they would never be able to insert an engine.

    I'm surprised that the end product is not closer to a usable part.

    Thanks to your blog, mine is in fine working order.

    1. Hi Sparky,
      I'm glad the blog is helping your build. There was quite a bit of plastic to remove. The plastic part could have been molded better, but the real problem is the vague instructions. They don't tell you how much work goes into getting the best fit.