I have a couple of M8 Hi Tensile cap heads 65mm long and I would like to extend the threaded section of them. I have an 8mm x 1.25 die and went at this but it's proving harder than I expected. It's been regularly soaked in lube and I am using the correct High School Metal Work thread cutting technic, 1/2 a turn forward and 2 turns back to clear the swaff. It's is almost like the unthreaded portion of these bolts' shank is too big, it measures 7.9 to 8.0mm I would have guessed it would be made the correct diameter to cut the original thread so why is this so hard? I have heard the term rolled thread, are commercially made cap heads rolled thread or cut thread and are they different in shape?
Thread Engineering Question
- Thread starter Vince
- Start date
Max
Full member
A couple of things occur to me. Commercially made fasteners normally have rolled threads because they are much stronger and quicker to form than cut threads. The portion of the fastener to be threaded is forged with the correct pre-rolling diameter and the shank is the nominal fastener diameter, so in theory is the correct size to cut more thread. However, a forged high tensile cap screw is made of very strong stuff and depending on class is likely heat treated after forming. I know my cheapo dies would probably die if I tried!
BlackJota
Senior member
- Location
- Sunshine Coast Australia
The shank diameter is correct. Maybe your die is a bit worn, making it difficult to continue cutting the thread.
GregT
Hero member
- Location
- Christchurch NZ
As has been stated above a HT capscrew is made of very tough material and will be difficult to rethread. It does thake a very good quality die to do it. Also the benefit of the rolled thread is the start of the thread is gradual and does not form a stress raiser. Using a die will almost certainly leave a sharp edge where the thread begins - which is a stress raiser and can lead to failure if the capscrew is used in a highly loaded position.
That said, I have done it myself with no problems. Usually going into aluminium where the loads are low.
That said, I have done it myself with no problems. Usually going into aluminium where the loads are low.
Great tips but it's now a bit academic. I was attempting to use one long cap head to hold my Atlas seat and subframe on AND hold the mudguard bracket on as well as its the same hole and the same captured nut. BUT for some odd reason, I couldn't get the bolt to pass right through it all. So it's now all held on with 2 shorter Cap Heads one entering from outside holding the seat and subframe and another entering from inside the rear guard. There is some bloody odd mounting stuff happening on this bike.
All the above is correct Vince. A good quality, keen die should cut the thread with relative ease, despite the marginally larger shank.
A rolled thread has a quite tough outer layer due to the thread being formed by rollers instead of being cut. A die that is only a little dull will have an extremely hard time getting into the shank of the bolt, despite your efforts with cutting oil, etc. A 10.9 or 12.9 bolt is considerably tougher to cut than a 8.8.
Easier for you to source some longer 8.8 class bolts with threads to the head, no need to be high tensile in that application. Alternatively, you could insert a length of threaded rod and use a nut at both ends.
piet
A rolled thread has a quite tough outer layer due to the thread being formed by rollers instead of being cut. A die that is only a little dull will have an extremely hard time getting into the shank of the bolt, despite your efforts with cutting oil, etc. A 10.9 or 12.9 bolt is considerably tougher to cut than a 8.8.
Easier for you to source some longer 8.8 class bolts with threads to the head, no need to be high tensile in that application. Alternatively, you could insert a length of threaded rod and use a nut at both ends.
piet
I managed to add approximately 25mm more thread before it got insanely hard, I had enough to attempt inserting it from both sides but it just got to a point where I was flogging a dead horse. It's all ok now with 2 shorter bolts like I said, it good to learn some new info so thanks for teaching this old dog.
Basketcase
Senior member
Major thread diameter should not exceed 7.97 mm.
Hi tensile is very hard wear on your die, normally it is either rolled (pre- heat treating) or else cut on a lathe with a single point tool.
The force would be greatly reduced if you used a shank diameter of 7.76 mm (the minimum standard).
Don't use cheap dies, I would used a hi speed steel split die even for a one-off.
Hi tensile is very hard wear on your die, normally it is either rolled (pre- heat treating) or else cut on a lathe with a single point tool.
The force would be greatly reduced if you used a shank diameter of 7.76 mm (the minimum standard).
Don't use cheap dies, I would used a hi speed steel split die even for a one-off.
RGS1000RGS
Senior member
- Location
- Dorset, GB
The split die with grub screw adjustment allows you to open the die cutter up slightly to allow you to adjust the thread size, if you have worn female thread. It will also allow you to make more than one pass with the die cutter, making a small reduction of the cutter each time ( exceptionally useful for harder materials where you need to remove smaller amounts of material.
... correct tensile strength fastener is a factor for axial load and sheer load, very different load rating.
We had a near fatal mishap with a replacement grade 12.9 M12 ( or M10 ) sheering on and RGS fairing centre bolt, failing and jamming the forks, lesson learned higher grade tensile needs to be engineered correctly. 8.8 was factory correct and reliable in that location on the RGS. BTW it is sleeved on the RGS hence the larger Metric size bolt could be applied, before i get shouted down... j
We had a near fatal mishap with a replacement grade 12.9 M12 ( or M10 ) sheering on and RGS fairing centre bolt, failing and jamming the forks, lesson learned higher grade tensile needs to be engineered correctly. 8.8 was factory correct and reliable in that location on the RGS. BTW it is sleeved on the RGS hence the larger Metric size bolt could be applied, before i get shouted down... j
It's a M12 bolt, John. The silentbloc has a 14mm bore, this is sleeved to 12mm to suit the bolt. It's the same silentbloc previously used for the swingarm pivot of the 750s.
Unusual for the bolt to fail, normally it's the bent bracket that's bolted to the frame.
piet
Unusual for the bolt to fail, normally it's the bent bracket that's bolted to the frame.
piet
Tah Piet, recollection here is flawed, no question. Know we adjusted the bolt size to the detriment of the safety factor, not often i get caught out, on this occasion was not making best replacement choice, with severe consequence for the owner, spooked me.
As you say rare, the bolt failing, original failed and the incorrect replacement failed. Could say these 120's are viby.. then i better not..a smile here. j
As you say rare, the bolt failing, original failed and the incorrect replacement failed. Could say these 120's are viby.. then i better not..a smile here. j
Basketcase
Senior member
Higher tensile strength = more brittle
8.8 would be best for shear impact. Just use a larger diameter if higher loads are required.
Accessories are often bolted to the frame with the smallest fastener that will hold up under normal loads and yet shear off during crashes to save the frame (i.e. shear pin).
Don't think so...
The digits denominate the tensile strength and the tensile elastic limit.
12.9 = 1200N/mm² tensile strength, stretch limit is 90% of tensile strength. 8.8 = 800N/mm², stretch limit 80%. Thus, a 12.9 bolt is 50% stronger than a 8.8 and will be able to stretch quite a bit more without permanant deformation, ie, being able to return to its original length. It's the elastic properties of the bolts that determine how tight things can be held together.
A 12.9 bolt will be stronger and more capable than its 8.8 equivalent under all circumstances.
piet
andre g
Hero member
Screws mustn‘t be used to take shear loads direcly, just axial loads. If there is a shear load or radial oad the tension of the screw must be so big that the friction between the fixed materials is so big that it can hold the radial load. So for M10 8.8 the tension is about 26 KN at a torque of approx. 49 Nm, the max. radial load is with mü 0,1 is 2600 N.
Basketcase
Senior member
Not for impact resistance or "toughness".
e.g. tough case hardened axle (with soft ductile core for shock resistance) vs glass hard 12.9 throughout which gives brittle fracture at shock loads way below elastic limit ... let alone tensile limit.
Add to that the problems of hydrogen embrittlement for the higher strength steels above 1000 Mpa (12.9 is 1200 MPa).
Don't forget that even endurance limit testing of steel uses nice sinewave stress cycling.
Impact testing involves sharp striking of a notched speciment. A bit like shear impact on a threaded bolt section.
Notch sensitivity for endurance testing increases with hardness, or tensile strength for steel as they correlate.
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Empirically, by observation, is that a HT fastener has physical properties not obvious, should the maxim "better the higher tensile strength, the stronger" be applied. This 12.9 failed in short order, obvious it was mis-engineered by me.
The replacement 8.8 did not fail, has to be 20 years ago now, RGS is still ridden to my knowledge. Appreciate the highly technical contributions in the above replies, cannot pretend to have a working knowledge all the same, does help me, to use basic factors in assessing parts applied suitability, thank you to notables for this extra knowledge base.
What i was able to do, rebuilding a 24Valve V6 Mazda motor and Hiace diesel motor, actually measure the stretch on the torque to yield ( TTY ) cylinder head bolts, rejecting the one size fits all "replace with new" all these TTY fasteners. Factory Mazda manual gave stretch limit values for these TTY fasteners.
Did purchase new TTY for the diesel, quick inspection showed these replacement bolts were inferior to the genuine Toyota TTY items. Returned the new bolts, money back, measured the oem TTY and reused them. That was 10 years ago, both motors still operating as normal. Make of this section what you will. j
The replacement 8.8 did not fail, has to be 20 years ago now, RGS is still ridden to my knowledge. Appreciate the highly technical contributions in the above replies, cannot pretend to have a working knowledge all the same, does help me, to use basic factors in assessing parts applied suitability, thank you to notables for this extra knowledge base.
What i was able to do, rebuilding a 24Valve V6 Mazda motor and Hiace diesel motor, actually measure the stretch on the torque to yield ( TTY ) cylinder head bolts, rejecting the one size fits all "replace with new" all these TTY fasteners. Factory Mazda manual gave stretch limit values for these TTY fasteners.
Did purchase new TTY for the diesel, quick inspection showed these replacement bolts were inferior to the genuine Toyota TTY items. Returned the new bolts, money back, measured the oem TTY and reused them. That was 10 years ago, both motors still operating as normal. Make of this section what you will. j
martymoose
Hero member
- Location
- Sydney, Australia
Hi John. Same happened to my 120 jota at around 80,000km, main bolt holding the fairing snapped, resulting in the fairing falling and jamming the steering. Has also happened to mark Austin on his 120 jota, now sporting a dent on the top of the guard. Dave H very kindly supplied me a new 12.9 when he replaced the one in his RGS Corsa.