front rack bolt size?

Brock_71

I bought a pair of Blackburn low rider front racks for the woman's bike, which came with this fitting kit. I fitted the first one ok, but while tightening the long bolt that goes through the fork bosses on the second rack the head of the bolt sheered off. It was getting a little tight as I was watching for it to protrude through the inside of the fork but I didn't expect it to just twist off leaving the shaft in the fork. I managed to twist it out again with a pair of pliers. Since I couldn't now fit the pair I went to remove the first rack and the same bolt on the other side sheered off in the same way!
What the heck am I doing wrong? I've fitted and removed similar racks on my own bike a number of times.
I'm about to place an order with SJSC and was going to add some stainless steel bolts to replace these putty ones they gave me, but I'm confused about the sizes.
which should I order (m4 50mm?)?
And what the heck went wrong the first time?

Thanks in advance learned wise ones.

david 142

I'd have expected the thread to strip before the head sheared off, so I would say that you did indeed have some cheap and nasty bolts.
All the same, was it hard work to wind them into the bosses? If so the threads may be dirty or corroded a bit. The 100% solution is to carefully run a tap through the boss, but if you dont know how to do it you could do a lot of damage, so treat this as the last resort and think about getting a bike shop to do it for you. The DIY approach is to use lots of oil and run the bolt in and out repeatedly until it runs through freely, then grease and assemble with care. Tighten but dont overtighten...
In answer to your title, the normal size for rack bolts is m5, length as per sample!

Brock_71

M5, thanks for the help David.
I wouldn't expect the threads in the bosses to be dirty or corroded, it's the first time they've been used on a newish bike, and all the little plastic plugs were in place. I didn't use any lubricant on the bolt though, and it did start to get stiffer than I would expect in the final turns before it failed.
I'll put it down mainly to horribly cheap cheese metal bolts rather than my ham fisted mechanical ignorance then. Makes a change

david 142

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Brock_71</i>

I didn't use any lubricant on the bolt though, and it did start to get stiffer than I would expect in the final turns before it failed.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Well IMHO not using a lubricant is asking for trouble when the threads are as delicate as m5. Think of it as taking care of the threads inside the boss, which could cost you a fortune to renew! I'd actually use copaslip or similiar so that they can be removed easily at a later date. If you're worrying about them loosening off, well it might be necessary to degrease to use a thread locking agent, but dont worry about that unless it happens - and you'd still be well advised to check 'em regularly!

On a new bike, if you encounter increasing resistance on the last few turns before the bolt comes right through, then its likely to be "paint" in the end of the thread. It might be possible to work it out as described, but the easy solution would be to take it back to the (hopefully local!) shop and ask them to run a tap through.