Re: [CR] More on Stronglight extractor size regardless of whether anyone cares or not.


Example: Component Manufacturers:Avocet

Date: Wed, 30 Jun 2010 17:34:11 -0700
From: verktyg <verktyg@aol.com>
To: Classicrendezvous@bikelist.org
References: <185909036.32082.1277932682929.JavaMail.root@sz0035a.emeryville.ca.mail.comcast.net>
In-Reply-To: <185909036.32082.1277932682929.JavaMail.root@sz0035a.emeryville.ca.mail.comcast.net>
Subject: Re: [CR] More on Stronglight extractor size regardless of whether anyone cares or not.


Billy, (long boring explanation)

23.35mm would be the outside or Major Diameter of a perfectly manufactured tap, crank arm extractor or dust cap.

As you pointed out, your extractors for old style Stronglight crank arms measure 23.1mm and 23.3mm diameter.

The thread accuracy on the original Stronglight extractors left a lot to be desired - same with the 23mm T.A. extractors.

There needs to be a little "slop" for most fasteners to easily fit into an internal threaded hole.

That's usually accomplished by slightly increasing the inside diameter of the internal threads by a small standardized amount or decreasing the outside diameter of the mating part(it's referred to as the ID or OD "Pitch Diameter").

There is also the Minor Diameter of the ID hole to consider.

The theoretical Thread Height of an individual 60° V thread is approximately 60% of the Thread Pitch. So, the Thread Height of a 1mm Pitch thread is about 0.6mm (0.0236").

Times 2 (for both sides of the thread), it's about 1.2mm which means the Minor Diameter of a 23.35mm ID thread needs to theoretically be at least 22.15mm in a perfect 100% thread. In reality it needs to be a little larger.

The Minor Diameter of an internal thread is usually calculated to give about 60% to 70% Thread Height - 0.36mm to 0.42mm (0.014" to 0.0165") of thread height per side.

In that case the Minor Diameter of 22.15 becomes closer to 22.8mm

If the threads on a 23.1mm extractor are already 0.25mm (0.0098") undersize and the thread height is only 0.36mm to 0.42mm it doesn't leave much thread height for the extractor to grip.

When there is too little amount of thread engagement it's very easy to strip the ID threads out of the aluminum crank arm and you never now what kind of ham fisted hammer mechanic had a go at your prized possession before you got to it!

Some people claim that they've never had a problem using a 23mm T.A crank arm extractor on an old style 23.35mm Stronglight crank. I guess that they are just lucky.

Chas. Colerich Oakland, CA USA


> Hi all. I thought i'd double check as my memory is a bit faulty of late. Sutherland's 4th and 5th eds. list Stronglight extractor threads as 23.35mmx1.0mm. The 7th ed. couldn't be bothered w/ such arcane outdated nonsense. Barnett's Manual, 5th ed. gives the same specs. I measured my original Stronglight tool - it's about 23.1mm but my Stein/Stronglight extractor that i got from Bike Tools, Etc. is a healthy 23.3+. Both thread into a Stronglight 93 arm. For comparison my original TA extractor tool which is nominally 23.0mm measured out at about 22.7mm. I'm just concerned that if people start having 23.15mm tools made, they're gonna be a bit small. Granted we're talking about 1/5mm, but why not be accurate? Cheers. Billy Ketchum; Chicago, IL; USA.