Tap size for bolt hole on triple crank

[jgspin]

Does anyone know the size for a tap for the small bolt holes on a triple crank spider? Also where can I buy it? I just got an older Takagi crank and the the granny ring looked like it hit a rock that bent the bolts and the previous owner tried to put it back but didn't do it right. The threads of a couple of the the holes are stripped. I used one of straight bolts to fix the threads but would like to acquire a tap to chase the threads and just so I can say I have one.

[RPS]

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The threads of a couple of the the holes are stripped. I used one of straight bolts to fix the threads but would like to acquire a tap to chase the threads and just so I can say I have one.
I’d take one of the bolts to my hardware store and try to thread it on various nuts until I found the right one. My Ace Hardware has a great selection of odd-size bolts and nuts, including metric.

I know it’s not what you asked, but if you can already start the bolts on the damaged crank spider I doubt tapping with same size will do much to make it stronger. If the bolt holes are stripped running a tap won’t replace the metal. If there is enough material you could drill and tap one size larger but that could introduce misalignment, plus the holes in the ring would also have to be drilled out to the larger diameter.

[DfCas]

Bike Tools Etc suggests it is a M8x .75mm here

http://www.biketoolsetc.com/index.cgi?id=669025618617&d=single&c=Tools&sc=Taps-and-Dies&tc=Taps&item_id=MS-TST

But I would take a bolt to a hardware store to make sure.

[Peter P.]

The bolts typically used for the granny ring for a triple crank, those with the blind holes (they have a bottom), are 8x0.75mm. This is from my Sutherland's Handbook for Bicycle Mechanics.

Since they are a blind hole, you will need 2 taps, one to start recutting the threads properly, and a tap to follow up and get all the way to the last thread in the hole.

The first tap is called an "intermediate", "second", or "plug" tap. The last tap is called a "bottoming" or "plug" tap, which can be confused with the plug tap above. I don't think you'll be able to use a tapered tap as they are better for through-holes.

http://upload.wikimedia.org/wikipedia/commons/f/fb/TapTypes.gif

[bicycletricycle]

http://www.mcmaster.com/#taps/=ch9qzg

[RPS]

jgspin, whatever you do I recommend you proceed with caution because your safety may be on the line. If multiple threads are truly “stripped” which normally means you can’t get them to tighten enough before there is plastic deformation (i.e. – the bolt keeps turning without higher tightening torque), and assuming the crank spider is aluminum and the bolts steel, it’s very likely that the “stripping” process essentially imparted all thread damage on the aluminum crank threads and little on the steel bolt threads. If that’s the case, replacing bolts may be a good idea and certainly won’t hurt anything, but it doesn’t address the soft aluminum metal that was already sheared off inside the spider. Running a tap through the damaged holes will likely not restore enough of the integrity of the crank system to make it safe to ride in my opinion.

You may want to read this short article about tapping damaged threads to see why it “may” not be a good idea, and also to see other options.

http://www.virginiawind.com/tips/stripped_threads.asp

Assuming aluminum cranks and steel bolts, in my opinion the thread damage probably runs the entire depth of the bolt. And since you probably can’t use longer bolts, and you mentioned at least two stripped bolts, I think you are taking a big risk, particularly on the granny ring which carries a lot of chain tension.

Personally, I wouldn’t ride cranks with stripped granny bolts any more than I would ride with a stem with a stripped faceplate bolt. When that happened to me a while back I immediately took the stem off and cut it in half before tossing in trash.



P.S. – Sorry for the engineering lecture but I can’t resist giving unsolicited advice when it may affect someone’s safety. :)

[Ken Robb]

How about heli-coils in the holes?

You are in luck. Marshall's Hardware at about 8423 Production Rd. (just north of Miramar Rd.) has the biggest selection of ALL kinds of hardware and fasteners that I can imagine.

[jgspin]

Thanks for all the advice and responses. I'm going to try and install this crank on a beater bike and see if it holds, keeping in mind that it might be unsafe. I did clean out the threads as much as I could. If it feels secure I will probably locktite them for good measure. Re: helicoils if I have to go that route I'll probably just throw the cranks away; I only got them for $15 anyway, so no big loss.