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Deathdeelr

Breaking Bolts

12 posts in this topic

I was on a mission to get the motor on my '99 400f back together today and ran into broken bolt after broken bolt.

First was the cam caps. I'm using 88 in/pds as in the Clymer manual as a spec. The exhaust bolts went in and torqued down perfect but the intake cap was a horror. I broke four out of six as they passed around 70 in/pds. All of these bolts are the original 6mm Yami bolts from the factory. After a run to Tractor Supply the broken ones were replaced with 6mm stainless capscrews and torqued to spec. Problem solved or so I thought.

Next was the clutch spring bolts. Two out of 5 snapped. Another trip to TS and more stainless. Now that's together.

I gave up when I snapped two of the front motor bracket bolts in the frame. I'm at the point where I'm just going to make a list of fasteners and buy stock in TS. I checked the Clymer specs after reading some other threads on bolt torque and broken fasteners and not all of the specs correspond to the maximum torque of that particular size fastener therefore I'm inclined to follow the specs they give.

Am I doing something wrong?

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You are not reading your touque wrench corect.

Remember there is 12 inch pounds in one foot pound.

Also on an inch pound touque wrench like my snap on, there is Nm and LB IN.

Example cap bolts on my 2008 YZ450F are 10 Nm and 7.2 ft*lb

10Nm is like 85 in pounds.

Hope you find this helpfull

Greg

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My first thought was that the setting was wrong. I'm using three different SnapOn wrenches (manufactured '03 last calibrated in '08). 1/4" drive 0-50 in/pounds, 3/8" drive 0-75 ft/pounds, and 1/2" drive 50-250 ft/pounds. For the cam caps I converted the 88 in/pounds into 7.3 ft/pounds and set my 3/8" wrench for 7 pounds. This is when they were snapping.

As for the wrenches being inaccurate the more I think about it, the more I think maybe my 3/8" wrench may actually be inaccurate at low settings. My old Craftsman 3/8" wrench started at 5 in/pounds but I used to routinely set it to 4 in/pounds for the motor mounts on my RC truck. I figured SnapOn is much better than Craftsman so why wouldn't it be as accurate at low settings? I have to check this out today.

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Depending on how many times this bike was rebuilt/serviced, this could be totally normal.

My 2000 Wr400F had similar issues, motor mount front frame - snapped at half torque value, clutch cover - same thing, banjo bolt - same.....

I think it all boils down to how many times were these bolts torqued down, over torqued etc. if the machine was serviced regularly, I can see the cams coming off a couple of times a year, then the odd rebuild - engine mount bolts getting over torqued - any which way you look at it this always will weaken the fastener, and you know how many people are - after chugging that beer they just crank 'em down good and tight!

Remember that most commonly, the bolt is designed to fail before the part it bolts into.

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Yeah your torque wrench must be screwed up because I have never broken a clutch bolt? You really must not wrench a lot because you can feel how tight a bolt is getting and judge from it's size what it will take.....

I would get another wrench for sure...88 in.LBS isn't too much for a 6mm bolt...

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My first thought was that the setting was wrong. I'm using three different SnapOn wrenches (manufactured '03 last calibrated in '08). 1/4" drive 0-50 in/pounds, 3/8" drive 0-75 ft/pounds, and 1/2" drive 50-250 ft/pounds. For the cam caps I converted the 88 in/pounds into 7.3 ft/pounds and set my 3/8" wrench for 7 pounds. This is when they were snapping.

As for the wrenches being inaccurate the more I think about it, the more I think maybe my 3/8" wrench may actually be inaccurate at low settings. My old Craftsman 3/8" wrench started at 5 in/pounds but I used to routinely set it to 4 in/pounds for the motor mounts on my RC truck. I figured SnapOn is much better than Craftsman so why wouldn't it be as accurate at low settings? I have to check this out today.

I just read the rest of your post......Typically you never use a Ft. Pound wrench for in Pounds....Get yourself an in.pound wrench for your small bolts

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It's less a matter of which units your using than it is the range of the wrench. Your 1/4" wrench is too light for the task, and your 3/8" ranges so high that it's accuracy is questionable at 7 ft/lb (It's only 10% of the range). Being that it's a Snap-On, it has a better chance of being accurate at that setting (and digitals are even more trustworthy), but it's still a lot easier to differentiate between 80 and 90 inch pounds on a 0-120 in/lb wrench than 6.67 and 7.5 on a 0-75 ft/lb wrench.

Also, to nit pick, the cam caps call for 86 inch pounds, or 7.2 ft/lb ( I personally use 75 in/lb), but the extra 2 inch pounds would not have caused the bolt to fail if it were healthy.

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I just compared the 3/8" wrench set to 8 ft/pnds to a dial type inch pound wrench here at work and she went off at 90 in/pnds the first shot but then didn't click at all the second and third times before the in/pound wrench went out of range at 100 in/pnds. So much for accuracy down low.

I appreciate all the input guys. I'm just going to have to replace bolts as I break them. At least the motor's together and mounted. I've only got three months left before any decent riding weather and I feel like she's fighting me.

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It sounds like a lot of these bolts were going into areas where oil would get on them. Is oil getting on them and causing improper torque readings?

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It sounds like a lot of these bolts were going into areas where oil would get on them. Is oil getting on them and causing improper torque readings?

I would think that would make the torque readings more accurate. Many of the bolts on my bike get a very light coating of grease before I torque them.

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It sounds like a lot of these bolts were going into areas where oil would get on them. Is oil getting on them and causing improper torque readings?
I would think that would make the torque readings more accurate. Many of the bolts on my bike get a very light coating of grease before I torque them.
In the service manual, and in the broad technical industry at large, torque values are assumed to be given for clean, dry threads unless specified otherwise. Oiling the threads does in fact increase the clamping tension load applied to the bolt and the part clamped by it, sometimes by considerably more than 15%. It also tends to reduce the torsional stress applied to the shank of the bolt. Lubing the clamping face of the bolt head has a similar effect on clamping force.

However, the service manual specifically calls for the cam cap bolts to be cleaned and oiled before torquing. That's another reason why 86 in/lb seems higher than necessary to me.

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