Tapping holes
- Thread starter N5GTi6
- Start date
According to that chart I should be ok using a 10.5mm drill on an M10 hole to then insert an M12 X 1.5 bolt.
The thread depth on the casting is still unknown at this stage - from the pic below it looks like I can go for a longer thread though as the current M10 bolt is 50mm.
The (potential) failure is between 2 components of a Megane gearbox mount. Not my pic but you get the idea of what I'm intending to avoid happening. The failure can potentially be fairly catastrophic.
The mount is two parts - that are clamped together by that one bolt. One 'half' of the mount attaches to the gearbox - the 'bottom 'half', and the other half to the chassis. The single bolt clamps them together.
There are all sorts of forces involved. Rocking motion from the engine twisting under load. The weight of the gearbox and engine. etcetc
Cheers
Justin
Ouch. Nasty.
The problem with that sort of setup is that the stronger you make the bolt the more likely it is you're just shifting the weak spot elsewhere. I would give it a go again with the same size bolt as per original but try to minimise relative movement of the two brackets. Ideally you would pin them together but with so little material around the bolt it is going to be an exercise in lateral thinking. How about a split dowel collar around the bolt that locates the two brackets against each other so the bolt only has to deal with clamping? Involves machining, I know, but that is what I would try. Enlarge a bit the hole around the bolt, say to a depth of 5mm, enlarge the hole in the other bracket likewise and use a tight fit collar say about 1mm wall thickness. These collar dowels are used in yesteryear Renaults of all sorts, shouldn't be too hard to find one. There's quite a few for instance in a R12 or R17 and the likes (engine to g'box, starter to clutch cover, clutch cover to g'box, another one locates head to block, etc).
The problem with that sort of setup is that the stronger you make the bolt the more likely it is you're just shifting the weak spot elsewhere. I would give it a go again with the same size bolt as per original but try to minimise relative movement of the two brackets. Ideally you would pin them together but with so little material around the bolt it is going to be an exercise in lateral thinking. How about a split dowel collar around the bolt that locates the two brackets against each other so the bolt only has to deal with clamping? Involves machining, I know, but that is what I would try. Enlarge a bit the hole around the bolt, say to a depth of 5mm, enlarge the hole in the other bracket likewise and use a tight fit collar say about 1mm wall thickness. These collar dowels are used in yesteryear Renaults of all sorts, shouldn't be too hard to find one. There's quite a few for instance in a R12 or R17 and the likes (engine to g'box, starter to clutch cover, clutch cover to g'box, another one locates head to block, etc).
I was going to reply earlier but had to go to Drs.
Clearly having the join spanned by the thread is far from ideal as a 10mm bolt is closer to 8mm at the base of the thread.
The hollow dowel is a good idea.
I was going to say countersink the thread of the casting enough that you can span the join with the shank of a 10mm bolt not the thread. That would pretty give you the strength of the 12mm bolt anyway (if it was relying on the threaded part of the bolt like the one in the photo). I don't think a 12.9 bolt would be the best choice given the various forces involved.
There's a fair bit of metal in that casting, if there's room for a nut in the cavity at the base of the thread you could just drill it out to 10, 12 or even 14mm and use a long shank bolt with just enough thread on the end of it to engage the depth of the nut. Keep the tolerances snug and lube the bolt's shank so it doesn't corrode into the alloy. This would remove the limiting factor in tensioning the bolt (the alloy threads).
Something else to keep in mind is a that finer pitch bolt requires less torque to provide the same clamping force as a coarser pitched bolt.
I would not mess with that casting. They are a pain in the butt at the best of times. In fact if I had to drill anything cast that I would then want to put on my car, I would take it to my milling machine, clamp it solid and drill it there. Come to think of it, I would bore it or drill it with a milling cutter and ream it. Holes run away from you so easily in unknown cast alloys you can ruin the bloody thing in an instant with a garden variety drill bit.
I second the idea of spanning the gap with the shank of the bolt rather than the thread though. Even if you use the locating collar I suggested above, I would still do it even if to meant cutting some of the threaded part of a longer bolt. Come to think of it, my cars have lots of customised fasteners like that for this very reason. I didn't like where/how the factory put the bolts spanning mating faces like that.
You can also use a machined shouldered bolt that would locate both castings on the shoulder section but that would mean enlarging the non threaded casting to a precise diameter, and counterboring the threaded casting. You can not however achieve that with a hand drill or even a garden variety drill press. You need a milling machine or similar to do that properly. And shoulder bolts usually have a very short threaded section.
Locating collar seems the optimal solution to me.
And by the way, now that we have seen the casting, I don't think a taper tap would d the job especially in the diameters we were talking about. The full size thread is reached only at about the last quarter/third of a taper tap and you don't have enough room at the bottom there to drive the tap that deep. You'll need at least a taper and intermediate taps to do the job if you still want to go ahead that way. I mention both simply because that alloy is very likely to crumble and the thread is going to be chewed up if you go with the intermediate tap directly.
I second the idea of spanning the gap with the shank of the bolt rather than the thread though. Even if you use the locating collar I suggested above, I would still do it even if to meant cutting some of the threaded part of a longer bolt. Come to think of it, my cars have lots of customised fasteners like that for this very reason. I didn't like where/how the factory put the bolts spanning mating faces like that.
You can also use a machined shouldered bolt that would locate both castings on the shoulder section but that would mean enlarging the non threaded casting to a precise diameter, and counterboring the threaded casting. You can not however achieve that with a hand drill or even a garden variety drill press. You need a milling machine or similar to do that properly. And shoulder bolts usually have a very short threaded section.
Locating collar seems the optimal solution to me.
And by the way, now that we have seen the casting, I don't think a taper tap would d the job especially in the diameters we were talking about. The full size thread is reached only at about the last quarter/third of a taper tap and you don't have enough room at the bottom there to drive the tap that deep. You'll need at least a taper and intermediate taps to do the job if you still want to go ahead that way. I mention both simply because that alloy is very likely to crumble and the thread is going to be chewed up if you go with the intermediate tap directly.
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The more I look at the one in the photo it looks like the casting isn't threaded to the face anyway.
It looks recessed. Is it possible this one had the wrong bolt in it anyway and that's why it failed?
It looks recessed. Is it possible this one had the wrong bolt in it anyway and that's why it failed?
One more thought, a stud and nut are generally more reliable than a bolt into a thread.
Is it possible to locktite a stud into the casting, drop it through the cross member and secure it with a nut?
Not sure how much room you have to work with.
Is it possible to locktite a stud into the casting, drop it through the cross member and secure it with a nut?
Not sure how much room you have to work with.