swapping bolts, for well, bolts

After realizing my bike has a blown head gasket, I started removing the head of my 04 rm-z 250. On the engine's left side I saw something that would make any one of us get that" heart sinking" feeling. The previous owner (one of em) had stripped out the two 6mm bolts that help mate the head and cylinder (not the 10mm head bolts). Not being able to get the OEM bolts in, he came up with an alternative, get a longer bolt of the same size/pitch and put a nut on the other end of the stripped hole. Torqued it to spec, and rode it.

Now comes the "engineering" part. The general idea is that you don't want to stray from the manufactuer's design. Well, we all have gotten into situations that disagree with that statement:smirk:. The question is; "Can an alternate fastening system be made to create the same downward force required on the two surfaces to be mated?"

After pulling out my statics book and crunching the numbers, you in fact can relocate the thread location, i.e. using a nut in place of tapped threads, without causing significant issues, and still meet the required OEM specs. As long as you use the original thread pitch and diameter, you may still torque the bolts to the OEM specs. Switching bolt diameter and pitch can be done, but you WILL have a different required torque to create the same required force acting on the two surfaces to be mated.

Just thought I'd share my findings to hopefully help those out in a similar situation and possibly poke some engineers brains to also consider other factors such as required strengths, coefficients of static friction, alternative materials, etc.

IMHO if the threads are clean as they should be, the torque to clamping force of a 6mm bolt and a 8mm bolt should be the same. If you have room for a nut, lucky you! You may want to drill out any remaining threads in the original hole to reduce any added friction.

I am not educated near enough to discuss "required strengths, coefficients of static friction, alternative materials, etc" but I can discuss and have questions about heli-coil versus re-taping for an over-size bolt.

I am not a fan of heli-coil in the first place if an over-size can be done and in my current situation especially. I have a striped hole in a 83 Honda XL600r head. The hole is 6mm with a surrounding alloy material of only 12mm. A heli-coil would require removing much of the material. No room for nuts. I would be happy to tap the hole out to 7mm (8mm would weaken the narrow material same as heli-coil) if I could find a stinking bolt! I have tapped the hole to a 6.3mm and re-threaded a 1/4" bolt. The bolt is loose when installed is the reason for wanting to go up to a 7mm.

Edited by LWright
IMHO if the threads are clean as they should be, the the torque to clamping force of a 6mm bolt and a 8mm bolt should be the same.

It strikes me that that would not be true at all, for a variety of reasons.

It strikes me that that would not be true at all, for a variety of reasons.

I could easily be wrong, but threads are not supposed to induce torque. If they do then your clamping force is going to be lessened by the torque needed to overcome said thread friction.

I am probably completely missing any "surface area of any given thread/diameter calculation of torque/clamping force" and need to take my search for a simple bolt to another forum.

Carry on.

LWright, are you able to simply get a longer 6mm bolt and have it poke out the other side? that's what i did with mine, and with the end of the bolt sticking out, just put a nut on that.

To answer the 6mm vs 8mm having the same clamping force, they would not, assuming they are at the same torque. By increasing the mean radius of the bolt, the lead angle changes, meaning a different downward force applied to the head.

If the said lead angle is lesser than the angle of friction, the bolt is self locking, and stays locked under load, which is why we have specific torques required. Increasing the friction coefficient means it will self lock sooner, and friction can be changed by using different materials, and general friction info can be found online.

To answer your initial question, you can move to an oversized bolt, however then new smaller holes might not be able to withstand the normal stresses involved. Most likely you'll just start to cause deformation, if anything happens at all, but too much stress still can cause ultimate failure (i.e. you crack the bolt hole, the metal snaps off, etc). Knowing the exact limits of the bolt hole is nearly impossible to tell, unless you have test data and the engineer's design specs handy, and the math to check everything. If you can get a bolt to poke out the other side you'd be fine putting as nut on it, otherwise i'd be a little iffy on moving to the 8mm, although they are relatively low torques on the 6mm bolts and you could probably get away with, but again, without testing it, i'd have no idea how "safe" it really is.

Edited by kawazuki250
spelling, makes a difference in the theorem's meaning
LWright, are you able to simply... i'd have no idea how "safe" it really is.

Wow, that all actually made sense. Thank you.

No room for a nut on the other side of this particular location. I am trying to keep the "new" bolt as close to the old one as possible while still getting a good bite. Even if I had used an 8m I would have reduced it down to a 7m while keeping the original 1mm pitch.

After all this speculation, I am going to try using form-a-thread and return to 6m.

I have my doubts about any epoxy living up to their claims, but it is worth a shot. And I even get to test the epoxy! Last night I managed to pull the threads on the oil drain hole for my 82 Suzuki 1100GK. Having no larger plug available, I used the form-a-thread. It did tighten up, though I only tightened to a single foot pound at best, is not leaking oil.

Back to the original clamping load vs bolt/thread size discussion, at least it is not "Torque to Yield" !!

Back to the original clamping load vs bolt/thread size discussion, at least it is not "Torque to Yield" !!

"Yield" literally implies a permanent change in the length of the bolt through plastic tensile stretching. But bolts will stretch elastically under normal loads, and the parts that the bolt is used to fasten will also stretch or compress elastically to some extent. The overall length of the bolt plays a major role in this, and all of this elastic distortion affects clamping force. The thread pitch, diameter, and number of threads in contact are also factors as is anything that affects the friction generated by the threads as they are torqued.

IMO, the best fix in your case, based on how you've described the situation, is some type of thread insert repair, such as a Heli-Coil. You already said you don't like those, and they have their disadvantages, but they work very well when properly installed, and are often actually stronger that the original threads were. If not Heli-Coils in particular, then there are other more advanced and expensive thread inserts you can use instead.

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