Getting some helicoils done

[xwhatsit]

Here's a picture, too.

Is the idea of putting shorter bolts in and a thread at the top drastically dangerous and very silly? It seems the smartest way to do it, to me. I'm sure I've got enough bolts lying around to find some the right length.

[Ixion]

Hm. Those are the bolts that clam down the camshaft bearing surfaces. Quite important. I'm looking at those holes and wondering if there are supposed to be dowels in there, would explain why the threads start so far down ?

[xwhatsit]

By dowels do you mean like threaded rod stuff and then I wind a nut on top the of the rocker cover? Because in the Haynes manual they're just normal bolts, except with copper washers.

[Ixion]

No , I mean like little short tubes of hardened steel. They fit into the wider top bit above the trheads, and stick up above the surface. Then when you fit the cover the dowels fit into similar slightly oversize holes in the cover, so that it is precisely located. The bolts go through the dowels.

Usually if a cover incorporates any sort of bearing surface it will be dowelled so that it is precisely located.

[xwhatsit]

As far as I can work out there are none, although there are at least three other dowels which have larger holes in the head and rocker cover to accomodate them. There's also like a little pin that aligns with the little indentation next to the right exhaust valve. That picture shows no dowels because it's from an old pic, but they are all fitted.

Haynes shows no dowels in any of the exploded diagrams.

Because of the importance of these bolts, it is therefore unwise to faff around by putting in shorter bolts? In that case, how is it possible to put in coils so deep? Can you get extender-thingies for drills?

[Ixion]

You can get extra long drill bits. But what worries me is why it's like that.

Engineers don't (usually) do things without a reason. So those threads were buried deep for some good reason. It would have been easier machining to put the threads at the top of the holes like the rest. So the design engineers thought that there was a reason that justified the extra machining work. I don't like changing things until I've worked out WHY it is the way it is.

Um. Another thought. Is that how the oil gets to the cam bearing surfaces ? Look VERY closely at the holes - are they cross drilled at all. How DOES oil get fed to that plain bearing? I've seen that before, an oversize hole with a cross drilled oil way. In which case you definately would NOT want to screw a close fit bolt into the top bit of thread, thus blocking of the oil supply? Very quick seizure.

EDIT: Incidentally that would explain the difficulty of getting an oil tight seal, because that part of the joint would be exposed to pressure fed oil, not just mist and splash like the rest of the joint.

[xwhatsit]

I think you might be right there, Mr Ixion. Would explain why those bolts (and one other) are supposed to have copper washers when no other ones do.

That said, there is the oil reservoir in the middle, where the cam lobes go. The lubrication diagram in the Haynes manual only shows oil going to the oil reservoir in the middle, up through the hole underneath. And by looking on the old head, I can't see any passages in those threads, and when I tried to blow on them with my lips they sure seemed to be airtight.

Obviously if I can get extra long drill bits, I can get extra long taps. You're right, of course, it's better to do it the way God (aka Mr Honda wot designed my engine) intended.

EDIT: Just went out to garage with a bit of wire, and checked the depth. Looks like the supplied drill and tap will reach, but there's just less than 1cm left of the cutting part of the drill and the shaft of the tap left. Maybe I should try drilling and tapping the spare head on my own, to see if it can be done, except not put the coils in of course.
EDITY EDIT: Actually no, the tap won't reach because the shaft is too fat for the hole.

[Ixion]

How deep do the original threads go ? I've gotten away in the past with using a longer bolt. The top of the trhead is stripped but often the hole is tapped deeper than the bolt reaches , so if you use a slightly longer bolt it can reach undamaged thread. Have you tried screwing the bolts in without the cover, to see if there is more thread?

[xwhatsit]

Very interesting idea, that! Will try with the spare rocker cover/head to see how deep the bolt reaches normally, and if there's more room. However the aluminium is like chocolate, so not sure how permanent it would be. Would be wonderful if somebody has a set of super-long taps that I could borrow for a few hours, but I won't hold my breath.

Cheers for the advice.

EDIT: Looks like I've got 5-6mm of thread left to play with. Shall ring up ENCO tomorrow and ask if they've got some longer taps before I try that, though (looks like they're called `long shank taps', can't see any on their website but will ask anyway). Would rather do it more permanently. The smell of burning oil (four-stroke, that is) is really playing on my nerves

[Ixion]

They're called pulley taps, but they are rare, I've never seen one in the metal. I'm also not sure if the "outer" thread of the helicoil is a standard metric thread or not.

[xwhatsit]

Ooh ok, doesn't sound promising. It's probably not only the outer diameter, but the depth of the `teeth' (don't know proper term there) as well.

Oh well I'll ask them anyway. Surely people must come into these sort of problems all the time.

Could I get my existing tap remachined? Have a lathe cut down some of the shaft where the drill grips? Certainly don't need the entire length of that, not even close. Although I bet that's bloody hard steel.

[xwhatsit]

Alright, no cigar at Enco.

Is the tap too hard to have part of the shaft lathed down in diameter? If not, where's the normal sort of place to have something like this done? Shouldn't be very expensive, should it?

[Ixion]

Too hard to turn I would think. It would have to be ground. If it's not a lot to come off you could do it by hand just with a bench grinder, the shank doesn't have to be perfectly round.

But some taps have the shank smaller than the teeth - some have it bigger, it's a maker by maker thing. Might be worth asking to see a few brands, on the off chance.

[Madness]

Helicoil Taps are specific only to the inserts, standard taps intended for what would be the internal thread of the coil match but the outer thread is oversized and a long series Helicoil tap would be ex overseas.

In Welly here there is a business called PG2000, they are specialist Grinders and would probably charge $50 or so to reduce the shank diameter. There's probably someone like them in Auckland, Tervor Lindsay comes to mind (Linbide Tools).

If it were me I'd be talking to this guy.

Edit:/ No offence Ixion.

[Ixion]

None taken. I questioned earlier whether they were a standard thread .