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If a bolt is calling for a rotational torque value it usually is a TTY (torque to yield) bolt and can not be reused.
Does the service manual call for replacement bolts?
A digital torque wrench with angle is nice, there are also mechanical type angle gauges, but you could just mark the socket and keep track of your rotation.
The Shop
OP, the reason they specify in degrees is because they want a specific tension, or pre-stress, in that head bolt at all times. I’m assuming there is probably a necked down portion of the bolt that begins to taper up toward the threads and bolt head, this section is a specific diameter that is held to a tight tolerance. Because the thread pitch is known, and any slack in the bolt has been taken up by the first two torque sequences, it’s easy to induce a specific amount of stretch (or elongation if we want to be fancy) in the bolt, which translates to a specific tension. This tension is likely close to the yield strength of the bolt, but as long as it’s not over it can likely be safely used again which is why they don’t call for a replacement.
As a simple example, let’s say that the thread pitch of that fastener is 1mm per revolution. If we take all the slack out of the joint and assume the assembly is now rigid, by turning it 50 degrees ( or approx. .14 revolutions) we induced an elongation of roughly .14mm into the fastener. I won’t get into the rest of the math because it would put everyone to sleep, but from that elongation, the known cross section if the bolt body, and the known elastic modulus of the material, we can determine the installed stress on the fastener much more accurately than with only a torque spec. In reality, torque specs are not all that reliable due to variances in friction during install, but elongation doesn’t lie.
The reason this is important is because of metal fatigue. These four strokes are pushing pretty high compression ratios, and that bolt is undergoing a lot of force every time your piston hits TDC on the compression stroke. After a while, that repeated loading would fatigue the bolt material and a failed fastener will be the result. But, by inducing an installed stress that is higher than the load the bolt will see in operation, we can essentially fake it out into not feeling each cyclical load, and prevent fatigue failure. Hopefully that makes sense, I got going and couldn’t stop myself
Kind of sucks to have to take it back apart because the gaskets were so expensive. I'm going to try and call KTM Tech today hopefully there somebody there..
If the studs are reusable I guess I could back it off 50% and then check to see where I'm at in the torque. I did notice to head bolts weren't as tight as they were on older style engine when I was taking it apart.
What you can do to get close is mark a 90 degree angle, then a mark between your 90 marks to get a 45, then just turn your bolt a little past your 45 mark. Id be comfortable with that torque.
Pit Row
If this was my bike I would just run it with it torque at 35 lb but it's not.
I would think somewhere in that range is good for any bike. With the bigger motor it requires more clamping force compared to a 250.
Wouldnt overthink it too much. Get it close.
If you use a torque wrench, different lubricants will yield different results. WAY different.
Using moly paste vs motor oil vs ATF, torquing a bolt to say 50 ft/lbs, and you will be shocked how much more rotation (stretch) you will get with moly. The absolute pinnacle is measuring stretch .006-.008” on the bolt to maintain tension as you said.
Built lots of test engines, I’ve watch so many stock small block Chevy rod bolts destroyed using moly, you just can’t get 50ft lbs of toque without over stretching the rod bolt. ARP has their own moly formula with high end bolts and also say to measure stretch or go to 35ft lbs, then quarter turn approximation. If you use motor oil and a torque wrench, 50ft lbs won’t stretch the bolt enough. Moly is always close
Drives me nuts when machine shops bake your block with the main caps on too.. the bolts are all heat treated.
Simply I don't like lubing bolts. BIG bolts are a different story. I'm talking about engine bolts above.
Post a reply to: Torque sequence question( somebody with a 2018 KTM 350 manual please read)