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Thread: Fine vs. Corse Threads
03-05-2007, 10:51 PM #1
This may have been covered before, but I used the search function, and did not find too much.
I am making some steering trees (triple trees) for my dirt bike project.
Anyhow, I am making the upper and lower trees out of 6061 T6 aluminum, 1.5 X 4 X 19". The holes for the forks are 2.25". I am slitting them and installing clamp bolts to pinch the fork, like any other dirt bike or motorcycle. Being 1.5" thick, I was going to use one 1/2" allen bolt or two 3/8" allen bolts for the pinch bolts. There is more than enough meat to use one 1/2" bolt, and a 1/2" bolt is very strong. Then I was thinking two 3/8" bolts would spread the force a little better. Then I was thinking the 3/8" bolts would be easier to snap off if torqued too tight, where as the 1/2" bolts could take a lot of tightening abuse without breaking. Then I started thinking about using corse thread, as I am threading into aluminum, be it strong aluminum. I know fine threads are stronger, but easier to strip in softer materials. Thread engagment is about 1", so that is a lot. I know optimum would be fine threads with a heli-coil. But I am not fishing a heli-coil down in that deep hole. And for some reason, all my heli-coil inserts have a loose thread. I have heard using a tap eaten smaller by dipping it in nitric acid, but no. No heli-coils.
So what do you think I should use for pinch bolts on my dirt bike project?
One 1/2" corse bolt?
One 1/2" fine bolt?
Two 3/8" corse bolts?
Two 3/8" fine thread bolts?
Thanks for you help.
--Doozer (who wants to finish this thing by spring time), in Buffalo
03-05-2007, 10:55 PM #2
like you said yourself, a pair of bolts spreads the force out better....
as far as using 3/8 coarse (or similar sized metric) every manufacturer I can think of (US, Japan, Euro) uses that size, or SMALLER for pinch bolts, and with their degree of liability for getting sued, I'm thinking 3/8" is sufficient....
03-05-2007, 11:18 PM #3And for some reason, all my heli-coil inserts have a loose thread
03-05-2007, 11:25 PM #4
The heli-coil tap is too large giving me a final loose fit. But I am not using them on this project. I just was rambling off into la la land.
I still am thinking of using one big meaty 1/2" corse thread pinch bolt.
03-05-2007, 11:29 PM #5
I always used 3/8 coarse threads in triple trees. Unless you are going to be removing the bolts often I don't see the need for Heli-coils.
Have you thought of using cam-locks instead of slotting and pinch bolts? I think they hold a lot better.
03-05-2007, 11:31 PM #6
Helicoils require a special tap made by helicoil. A standard thread tap will not work. They also require a special tool to insert. All of this usually comes with the helicoil kit. With the right tools (drill, tap, inserter) they are a breeze to install and very reliable. If you are just buying the bare helicoil inserts and don't have the correct tools then you are doing it wrong.
03-05-2007, 11:32 PM #7
Fine threads in steel with the depth of thread equal to at least 1.0 times bolt diameter.
Course threads in everything else with the depth of thread equal to at least 1.5 times the bolt diameter.
The strongest thread pitch is 12 tpi per an engineer friend.
03-05-2007, 11:34 PM #8
I have never seen a cam-lock used in a triple tree. I think I know what you mean... like some ball bearings have an eccentric locking collar. Is this simmilar?
03-05-2007, 11:53 PM #9
Similar to the quill clamp on a B'port. Cam lock is probably not the correct term but that is what they are called in the custom bike world.
03-05-2007, 11:54 PM #10
Most Motocross bikes have M8 pinch bolts. I like 2 per clamp area. Also be careful not to over tighten the lower set as you can make a tight spot in the tube and this will affect performance.
Judging by your choice of front rubber you probably won't be charging through the whoops at Area51 so it might not matter.
Do you have more pics?
03-05-2007, 11:55 PM #11
I believe a B'port quill clamp is called a "Cotter".
I have tons more pics, mostly of machining the components, just not all in photobucket. I will post some more of the better ones.
Do you think I will die if I try any jumps with this thing? Having the tires it does, any jumps will likley bounce me higher that the origional hill!!!
All I know, this thing will be un-break-able, but HEAVY!
03-06-2007, 12:05 AM #12
I have always heard it called a quill lock and that's the term I use too.
When I opened a shop in Mo. a customer I knew came in one day and asked to borrow a 3/8 course tap so I handed him the tap. He gave me a blank stare and then said, this is not a tap. I said yes it is. He said, I want a tap to cut the threads on a bolt. I then said, oh, you really want a die and got one for him. Apparrently in So. E. Mo. they call a threading die a tap. I never did find out what they call a real tap or if I did I have forgotten. As you know terminology is regional to say the least.
03-06-2007, 12:08 AM #13
Doozer, you mentioned you wanted to be done with this project by spring -- what are you worried about, in Buffalo you have another three months to go!
03-06-2007, 12:54 PM #14
I'd toss the inserts and pass the bolt through the clamp and nut and washer on the backside. Threading into aluminum, while plenty strong, can lead to problems with frequent removal. What kind of motor are you putting on this thing?
03-06-2007, 04:49 PM #15
"...but no. No heli-coils."
Motor is 500cc 4 stroke 4 valve Honda single.
03-06-2007, 05:01 PM #16
The advantage of going with the coarse (NC) thread is that, if the aluminum gets stripped, you can then just tap out to the fine thread for the same nominal diameter bolt, which uses a bigger starting hole.
But if I were doing such a project, I think I would use (NF) thread pitch to start. The fine threads are also called SAE (for "Society of Automotive Engineers?"). I believe that fine threads were adopted as a standard for automotive applications, because they are less prone to coming loose from the vibrations of an automobile. This would suggest that fine threads are preferable for your project.
In general, when I have access to a shop, I think that fine threads are nicer. But if I am using handheld power tools on a project, or if precision is not important, then the coarse threads are faster and easier.
03-06-2007, 07:21 PM #17
03-06-2007, 10:14 PM #18
It doesn't matter if you like fine or course threads at all. Course threads have more holding power in aluminum than fine threads. It's not my opinion, it's a fact. It you are going to tell someone what to do or use be sure to know what is right, not what you think is right or what you prefer.
This is a site for profesional machinists to give correct info.
03-06-2007, 10:39 PM #19
If you strip out the threads on a 3/8-16 bolt there is no material left to tap for a 3/8-24 bolt!
I am always amazed at the number of people who use fine threads in alum. mostly because they look cool.
On the other hand I have made a lot of money repairing stripped threads in alum. parts.
03-06-2007, 10:45 PM #20
I'm a little concerned with your thinking Dooz.
Two 3/8 bolts are going to give you more holding power in a pinch configuration along the length of your triple tree than one 1/2 incher...physics will rule here. However, if the tree is not going to be heavily stressed it may not matter.
The concern is "...take a lot of tightening without breaking..." If you have to use a breaker bar and an ox to tighten; then you probably engineered it wrong. Stick to the max torque for the designed bolt.
Most allen bolts are at least grade eight so twist away. You will probably strip the aluminum before you break the bolt. Use some small diameter washers under the bolt head for best results. If you do go with the 3/8, go to the auto performance shop and pick up some header washers. You could probably use a couple of 5/16" socket head bolts and have plenty of pinch.
Fine threads are not stronger, coarse threads usually are; fine are for fine adjustment or vibration prone areas. They both hold stuff together pretty good though.
So far your project looks good....Should be a Harley motor though...they got little ones.