Any Japanese Bolt Experts Here?

[Toestr]

So I snapped a head bolt on my sled (Yamaha has a stupid torque pattern) and I'm looking to purchase a bolt with equivalent specs without paying the OEM price of $5 each plus waiting for it to come into the dealer.

The bolt itself is an M6 x 40mm but I'm having trouble determining the class/grade. The head is marked only with an '8', (I'm sure many of you have seen similar bolts on older Japanese vehicles but I'll get a picture later). From what I understand this relates to older JIS specs and was later phased out in favor of the ISO classes (ie 8.8, 10.9, 12.9, etc). I found a video where someone else found an equivalency table but it only goes up to the '7'.

I haven't been able to find out ANYTHING about these markings beyond this video. Admittedly I didn't look beyond the first couple of Google pages of the most obvious search terms. Anyone here have some forgotten knowledge?

[Ecapnation]

According to my chart it's a class 8.8 medium carbon steel metric bolt

[Toestr]

Thanks. On the table I saw a '7' bolt was equivalent to an 8.8. Any chance I'd be able to see the chart you're referencing?

[Ecapnation]

Its confusing....

But because it lacks the lines of inch bolts its read as metric class 8.8

[Chuck E Ducky]

The single “8” is almost certainly the maker’s mark rather than a grade designation.

Aftermarket bolt kits often supply 10.9 zinc-plated hardware as a direct upgrade/replacement for OEM-style fasteners.

8.8 is a safe, direct strength upgrade over an unmarked/commercial original.

10.9 common upgrade choice for sleds; good balance of strength and toughness.

I would not go harder as they can become brittle in the cold and are more likely to snap over stretch. Make sure you follow torque spec so you don’t strip it out.

[Carp]

Good thread you got going on here. 😏

Just recently snapped a 10.9 M6 x 20mm bolt before, I remembered that I'm supposed to use a torque wrench there. I still need to grab a replacement for it.

[Toestr]

I emailed a British importer of Japanese hardware asking what ISO class each 'mark' corresponds with and this is what I got back:

Mark 4 is 4.8
Mark 6 is 6.8
Mark 7 is 8.8
Mark 8 is 8.8
Mark 9 is 9.8
Mark 10 is 10.9
Mark 11 is 12.9

Hopefully this is actually based on some knowledge/documentation they have and not just the first AI slop/Google result the salesperson found. I'm going to leave it at that for now.