@Hollywin,
No 3 in your picture has the largest inner diameter, and has to be the 8mm collet. Make sure (by measuring with a vernier) that your bits are indeed 1/4". Bits that are 1/4" will not slide into the 6mm collet, whereas 6mm bits will go into both, but should NOT be used with the 1/4" collet. Colour-code with a dot of paint.
No 3 in your picture has the largest inner diameter, and has to be the 8mm collet. Make sure (by measuring with a vernier) that your bits are indeed 1/4". Bits that are 1/4" will not slide into the 6mm collet, whereas 6mm bits will go into both, but should NOT be used with the 1/4" collet. Colour-code with a dot of paint.
