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An interesting topic I know :runaway:

I've taken off my carbon undertray to apply waxoyl. I've noticed that the bolts are a little on the rusty side and being the ponce that I am I wanted to replace them with stainless steel ones.

They are hex bolts with the numbers 4 and 7 on the heads. Does this mean something to someone wise and full of interesting trivia? :D


Cheers

Jules
 

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4 and 7 on the heads doesn't make much sense! when you say "Hex Bolts" doe you mean standard hex head (i.e. Across Flats bug a socket or spanner on 'em) style or cap or allen head?

if they're the former, they should be marked with radial lines and/or possibly a number (i.e. 8.8 for grade 8). If they're the latter (i.e. cap head bolts) they should be marked A-2 for stainless or not at all for steel.

Either way, just go to your fastener supply shop and ask for Mn stainless bolts x long, where n is the metric size (probably M6 or M8) and x is the length of thread (consult your local ruler!)

For a simpler explaination, see here:

Bolt Depot - Bolt Grade Markings and Strength Chart

I have a bit of a fetish with fasteners :nervous: and have a bespoke collection in my workshop - currently I'm going through a stainless cap screw phase... they're shiny and mechanical - what's not to love! :thumbsup:

enjoy!
 

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vernier caliper and a set of thread pitch gauges is what you need to find out what exactly the size you need.

Mn x p
the n bit is the major diameter of the thread, the p bit is usually the thread pitch. i.e. the distance between each thread, then there are various grip lengths of the bolts which is how long the thread is.

Would have though M6 & M8 would have been A bit on the large side for the undertray, would have thought they are probably M4.
 
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