Regarding bolts we advise to use hardened steel hex bolts as the best option. Regular hardwear store bolts are a definite no-no. Not sure of the Yen rate but GR12.9 bolts are under £2 for a set of six in the UK and will last for ever. Make sure you have 7-8mm minimum in the hub securing the cog (use 10mm bolts for 3/32", 12mm for 1/8") grease the bolts and tighten them up as per the FAQ.