Thorn Cycles Forum

Technical => General Technical => Topic started by: phopwood on November 03, 2012, 08:07:01 AM

Title: Seat post bolt size
Post by: phopwood on November 03, 2012, 08:07:01 AM

Hi,

When I built up my Sherpa I assembled my seat post adjuster with a small nut and bolt, as I could not find a suitable bolt, I tryed an m5 and m6 neither would bite.  So I am going to put a tap though it, do any of you know the correct size.

Peter

Title: Re: Seat post bolt size
Post by: Danneaux on November 03, 2012, 03:29:58 PM

Hi Peter!

I don't have my Sherpa to check, but standard threading for bikes (to choose tap size) would be M5x0.8mm pitch or M6x1.0.

Should be one of those two. Hmm. Thinking...if either of the two bolts you tried didn't "bite", I can think of four possibilities:

1) Powdercoat in the threads, in which case -- yes! -- tapping would fix things right up.
2) Malformed end on the bolt with damaged threads; try another bolt.
3) Bolt not long enough (or effectively so...did you check for swarf or debris in the socketed opening where the bolt head seats? That can sometimes keep the bolt from reaching the threads on the other side).
4) Bosses misaligned, such that the bolt doesn't approach the threads perfectly "square".

Have you tried threading in the bolt from the other side (looks silly, I know) to see if the threads will catch that way? If nothing else, that should confirm bolt size and let you see if they're clogged with powdercoat (the most likely cause).

I hope something here helps.

Best,

Dan.

Title: Re: Seat post bolt size
Post by: phopwood on November 04, 2012, 04:46:48 PM

Dan,

Thanks I tried a couple of M5 but they did not bite at all, so my money is on the powerder coating being my issue, I did not want to start tapping until I was sure of the size.  Tomorrow night I will get my taps out, they have not been used for many years, it will be a nice outing for them.

Peter.

Title: Re: Seat post bolt size
Post by: Dave Whittle Thorn Workshop on November 07, 2012, 12:19:20 AM

Bolt spec M6 x 1.0 thread 20mm long not including head.

Title: Re: Seat post bolt size
Post by: phopwood on November 07, 2012, 05:11:32 PM

Dave,

Many thanks.  I never got around to this on Sunday, but I will have a look this weekend.

Peter

Title: Re: Seat post bolt size
Post by: phopwood on November 09, 2012, 05:30:40 PM

I run my tap though today and now have a decent M6 bolt holding my seat post, rather then a M4 nut and bolt.

Peter

Title: Re: Seat post bolt size
Post by: Danneaux on November 09, 2012, 05:31:59 PM

Yay! Happy outcome, Paul! Much safer, too...

All the best,

Dan.