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Discussion Starter · #1 ·
Not exactly a "parts" problem, but this is the best place I could think to put it.

Has anyone attempted rethreading their fastener holes? I was cleaning out the holes for my coil brackets with my tap set and I noticed they made the holes significantly sloppier for the fastener. Upon measuring several size fasteners laying around me, I found that, on average, the honda fasteners were ~.15mm under the "rated" size. So a thread diameter measurement on a 5mm screw yielded roughly 4.85mm. Held true for the 4-6mm fasteners I had handy.

On my tap set, they were on average ~.05mm over the rated size. So you can see the problem, I'm looking at roughly .2mm of size differential. On these soft aluminum parts, I am worried about stripping threads.

Do higher priced tap sets yield taps closer to honda's sizes? Or did they just make life harder for me? I would think if you were buying a 6mm tap, you'd get damn close to 6mm the more money you paid and honda was just being weird.
 

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Sensei
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I believe the "difference" you are measuring is due to an inherent problem with your assumptions of the threading process vs size, and how the fasteners actually work.... Unless the threads are "rolled on" (as they are on spokes for instance) they are "sized" based on the rod size before the threads are cut away..... So, obviously, the resulting fastener will be less than the rod size due to the cutting process of the die.....
Similarly, the tap has to make a clearance sized spiral hole, so it is larger than the fastener... (if the sizes were precisely identical, it would be an interference fit, and you could not easily insert or turn the screw into the nut )
Threaded fasteners work because the threads are essentially spirally wound wedges, and "tightening" them ultimately bears the wedges together, pressure binding them. As long as the threads are large enough that the "bolt" won't "fall through" the "nut" and the threads are cut at essentially the same taper (wedge) angle (matching shape and number of threads per unit of measurement) the fasteners will ultimately "bind" and hold.....
Until the parts being held together actually resist compression enough to cause the "wedges" (threads) to bind the fasteners should spin against/through each other easily (ignoring friction of course)......
You probably already knew this, but sometimes another explaination can help clarify our understanding of how things work....

Yes, more expensive taps and dies will probably be toleranced more accurately .....
 

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Discussion Starter · #3 ·
I should have prefaced that with that I am an engineer and have a fair amount of design experience, especially with mechanical joints and fastener standards. The strength of the grip is all in how much tooth contact you get and I don't want to be clipping the tops of the teeth off with a tap that's too big just to try to clean up the holes. If they rolled the threads (likely) then to get a diameter less than specified size then they must have started with a piece of material smaller than that size. The material has to go somewhere when you roll threads and it ends up pushing the teeth higher (larger diameter).

The question I'm looking to get answered is whether or not I can actually buy a set of taps that are close to the tolerances Honda used. It's pretty clear that they used smaller taps than I have, but I didn't spend a large amount of money on my taps and perhaps the .05mm oversize on mine is all the difference there is. Does anyone have a good set of taps they can check the diameter of?
 

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Sensei
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Yes, I thought you already knew what was going on.....Never hurts to post it for the newbies though...

Taps and dies, like any other precision tools are available with closer tolerances for more money.... I use a "Craftsman" set and have had decent results....Never actually mic'd them....I also tend to use "bottoming" taps in the aluminum blocks.....
 

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Discussion Starter · #5 ·
I was thinking of trading in my harbor freight set for a set from Sears. They have rethreading kits but the pitch range is limited and I don't want to pay for SAE tools I don't use anyway. I guess it all comes down to how much I want to spend, which is probably right about where the craftsman set is going to run. I'll give it a go and see how it does, worst case is they'll take it back if I don't find it better than the harbor freight set.
 

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Discussion Starter · #7 ·
Well I got my craftsman set and the tap size tolerance isn't a great deal better. The flutes are ground instead of being raw from the casting, so they should provide a better cutting edge, and the dies are made from hex stock so they won't spin in the die handle. Case is nice too.

But the diametric tolerance isn't massively better: on average they're .03mm over the rated size (+/- .01mm for my caliper). Even the cheap home depot screws I bought to replace some are roughly .15mm under the rated size. I think this is just standard and I'm not sure why the holes are designed so sloppy.

Which means, in order to clean threads, the taps aren't going to do me any good. Looks like I'm on to plan B: cut a couple flutes in a matching size screw with my dremel and see how that goes.
 

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Do you have access to any inspection tools at work? You could borrow some thread gauges and you may find that the bikes threads were cut to a different class than what your tap set produces.
 

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Discussion Starter · #9 ·
Unfortunately everything at my work is in SAE so they don't have the tools in the right (and I mean that) size. It doesn't surprise me to find that Honda had their own tooling built to a tighter tolerance than is "standard" for the hobbyist. Looks like something I'll have to live with.
 

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Sharkmonkey said:
Unfortunately everything at my work is in SAE so they don't have the tools in the right (and I mean that) size. It doesn't surprise me to find that Honda had their own tooling built to a tighter tolerance than is "standard" for the hobbyist. Looks like something I'll have to live with.
When I replaced the crank case & rebuilt my 350 engine, I chased all the crank case threads with my Snap-On tap & die set 6mm tap and did not take any aluminum with me on almost every single hole I came across but, there were a couple of holes that the threads were messed up when I got the case, they were the reason I chased the threads to begin with.
 
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