How can you tell a bolt material?

[evan326]

First, if this is the wrong place sorry. I wasn't sure which forum this should fit into.
I just got some black Ti bolts from ebay, and they're astonishingly light. One is 1.7g, the set is 10.8g. The only relatively simple way I know to is just to see if it's ferrous or not by putting a magnet to it. The bolts that came with a kalloy uno stem I have fail the magnet test, but weigh 20.6g as a set and 3.5g each. The bolts are the same size and neither have markings.
I'm trying to get a better idea so I can decide which other ones are worth tuning.

[gilles75]

You can only tell by density. Alu is three times lighter than steel, and Ti is twice as light as steel. Compare weight to a similar steel bolt. Stainless weight about the same as steel and doesn't attract a magnet.
If you don't mind damaging one bolt for the sake of science, use a file to see how it bites to different materials. Alu and steel will obviously feel and look very differently, and you you will be able to also tell Ti appart.
Hope this helps.

[evan326]

More than I knew before you wrote it, thanks!
I didn't know stainless wasn't ferrous, that explains a lot now. I had a few non magnetic bolts that were surprisingly heavy.
Now to get a shopping list for toronto bikes....

[jlok]

One question about the stem bolts. With repeated installation cycles, will the bolts become fatigue and break?

[gilles75]

The weak points are the aluminum threads in the stem. If you are carefull to engage threads straight, not cross threads, and never over torque them, threads are not at risk.

[Marin]

If the bolts are the same size as ti but lighter they're alu and should not be used for the stem.

The magnet test is problematic because some steel grades aren't magnetic either...

[youngs_modulus]

Stainless steel is absolutely a ferrous metal. Some grades are strongly magnetic, some weakly magnetic and some are essentially non magnetic.

Another way to tell the difference between materials is to touch them to an abrasive wheel. A bench grinder or a Dremel tool will work just fine. Steel bolts will shed yellow sparks; titanium bolts shed sparks that are nearly white and aluminum bolts shed no sparks at all.

[Rick]

Actually, most stainless WILL be attracted by a magnet. SOME of it will be weakly attracted, but still noticeable. There is some virtually non-magnetic stainless, but you have to anneal it after forming the bolt, so it is highly unlikely you would have that.
Ti will be completely non-magnetic.
The most obvious difference is, of course, the density.

[jfranci3]

Stainless is ferrous. Ferrous meaning that it contains iron. Stainless Steel is primarily Fe (Iron) has a minimum of 10.5% chromium in it. Not all ferrous alloys are magnetic.

As for your black mystery bolt.
-Is it paint? Probably not I'm guessing by your post.
-Probably not high-carbon steel based on the weight.
-Could be anodized aluminium. Is it shiny and somewhat see through? https://encrypted.google.com/search?biw ... JfSgFPszBM:
-Could be magnesium if it's grey, but not likely.
-if you shave of the bottom tip of the bolt with sandpaper past the finish, what color is it? Is it a darker grey than aluminium foil?
-Do the TI bolts look like this: http://www.torontocycles.com/Selling/Bl ... Bolts.html

You could take a piece to the grinder and see what color the sparks are.
One way to test that's not recommended. Try to cross thread it into steel. If the bolt wins and the steel loses, it's a TI bolt. If the bolt loses, it's an aluminium.

[evan326]

I may not have been clear enough initially. The black stem bolts I have are titanium, I just bought them on ebay as a flier. The bolts that I now believe to be stainless steel are the ones already on various parts of my bike. I am trying to get my list together of all of the bolts that are worth changing to aluminium/titanium so I can just do one order from toronto cycles.
The tip with the sparks color is pretty darn good. I didn't think of that at all.

[jfranci3]

Oh... TI common alloys nature are that it's pretty damn strong in tension and equal to SS in shear. One bit of caution, just like steel bolts can easily cross thread in AL, TI takes it to the next level. TI can cross thread in AL and steel.

[Kurets]

Another way to figure out the material of a bolt is to measure its resistance. Aluminium is a very good conductor, steel less so and titanium quite a poor conductor. This requires a benchtop power supply and a multimeter, but pushing an Ampere or two through the bolt and comparing voltage drop to a known bolt will show what material your mystery bolt is made of.

[djwalker]

An easy test for all the steels except the super non-magnetic stainless steels is to get a small rare-earth magnet. Hang it on a thread a few feet long. Bring the bolt near it. If the bolt is steel the magnet on the thread will deflect toward the bolt as the bolt gets close. This works better than the parts sitting on a table because the friction of the parts on the table is stronger than the magnetic force on weakly magnetic steels.

[Marin]

Cool idea!

[UpFromOne]

If you can order an extra eBay ti bolt or two, then you have a spare with which to do the destructive testing.

Chances are they are 3/2 rather than 6/4 ti. But if you grind it and the sparks are anything but bright white,
it's not true ti, might be an alloy with some crap filler in it.

Nevertheless, I've used eBay ti bolts and never had an issue.
But Toronto (Tom M.) has some great black ti pieces.