Sine wave output from crystal oscillator

I built a dual-inverter crystal oscillator circuit, but the output is a bit less than sinusoidal. Adjusting the capacitor between the inverters cuts down the distortion a bit, but it's still obviously not a sine wave. What's the best way to go about this? A bandpass filter? I tried a ladder filter and that only seemed to make it worse, and I'm not familiar with any other filter designs that give the astronomical Q values required (oscillator is around 13Mhz).

Any suggestions?

You could use a narrow bandpass filter centered at the crystal frequency.

Alternately you could use an oscillator configuration that generates a waveform closer to a sinewave such as a Colpitts or Pierce.

You just need a sharpish low pass filter. Filter out everything above the fundamental.