Topic: Crooked Helicoil?

[mgilvary]

This is maddening. After rebuilding the top end of my 550, I discovered crossed threads on spark plug hole #4. Fine. I pulled the head again, got the helicoil kit, drilled it out, tapped and installed the helicoil. While the head was still off, I screwed the plug into the hole to make sure it worked and was sitting flush in the combustion chamber.

I removed the plug and finished reassembling the motor. Got everything back together, adjusted valves, etc. Last step was to put the plugs back in.

When I started with #4, though the plug turned easily, it appeared to be going in crooked. Sure enough, when I put my spark plug wrench on it, the wrench hit up against the head. I was able to screw in the plug with the wrench a bit cockeyed, but then the plug stopped very suddenly.

I pulled the plug out again and shone a light into the hole. The threads appear to be crooked. That is, the beginning of the coil appears to start one thread LOWER than the top thread 180 degrees away. Maybe that's some kind of optical illusion, but what's certain is that the plug is going in crooked, and then it meets a hard stop when the shank of the plug hits up against the raised edge of the head, with the plug only about 2/3 of the way screwed in.

How is this possible? I checked this when the head was still off? How could the threads become cockeyed? And if they're cockeyed, wouldn't the plug just not screw in at all?

And what am I to do about this? I really don't want to pull the head again. I thought about trying to pick out the coil and insert another one, but will that do any good? If the coil is somehow crooked, isn't the problem the tap itself?

Help.

[Nikkisixx]

All may not be lost,  I have seen a helicoil "jump" a thread or two.  It may have stuck to the plug when you pulled it out during the dry run on the bench test and got itself out of the grove. With the first couple of threads misaligned, this may have been what the plug threaded into during your final assembly.  It would explain the crooked and abrupt stop.


 

[mgilvary]

So should I try to pick it out and install a new coil?

[TwoTired]

You may wish to try a time-sert, instead.

Terry in Australia just posted a part number earlier today in another thread.

[Nikkisixx]

Can you get a good look at it - does it look like the top two or three coils are out of the threads?

First -You can try to fiddle with the coil that is in there.  Use a dental pick and maybe an old plug with smooth threads to check progress.  Keep in mind the coil is spring steel and not prone to reforming easily.   I'd not spend a lot of time on this approach, but it is worth an honest effort.

Second - if that fails I'd pull that one out and screw in another.  Can you get pliers in there to pop out the bent end that need to be removed after you screw in a new coil?  Don't want that falling into the combustion chamber.

[754]

Dont blame the tap, blame the tapper.. whoever steered the tap, made the mistake if it is out.

 I can post how to remove helicoil, but trying to retap a hole straight and use a helicoil, is asking for trouble.. any deviation from proper fit will cause problems.

 A common problem with helicoils, in not tapping deep enough or in this case through, which causes the helicoil to taper on lower end. They are I believe tungsten alloy, and usually will not cut with a tap..

 to remove, file in a few notches with a 3 corner file, and get a file or scraper in those notches, and wind it out to outside of head..

 good luck..

[mgilvary]

I am the tapper and I accept all responsibility. But I'm unclear on how I could've screwed it up. I tapped it all the way through and, when I installed a plug to check the fit, it was straight.

A cursory attempt to pick out the thread was not successful. I'll have to get a 3 corner file or a removal tool and try again.

[754]

If plug went in straight at first, something else is wrong.. did you spin in with fingers, but now no room for wrench.. seems strange... BBL

[Nikkisixx]

While the head was still off, I screwed the plug into the hole to make sure it worked and was sitting flush in the combustion chamber.

I checked this when the head was still off? How could the threads become cockeyed? And if they're cockeyed, wouldn't the plug just not screw in at all?

I'd blame the heli-coil.  It sounds like the initial taping went fine since the plug bottomed out and went in straight while the head was on the bench.  The coil itself is just hosed up now, either from cross threading the plug after assembly or the test plug getting hung up on the last few coils on the way out.  

[mgilvary]

Weird update: I tried installing the plug again and torqued it past the point where it was sticking. It seems to go in all the way now. It's definitely going in slightly cockeyed, but appears to settle down flush against the head once it's all the way down. I pulled it out and did it again to make sure. Same result.

Maybe I drilled out the old threads at a slightly off angle? But I would've thought the drill would self-correct once it was a few mm into the hole.

I guess the real test will be to start it up and see if it's actually sealing.

[Nikkisixx]

Yup, this is one of those learning opportunities you hear so much about    Glad it seems to be going your way.  I would recommend some antiseize on that plug.

You said the hole is straight and well tapped, but is is possible the top of the hole is a little loose, conical, or oblong?  This would allow the threads of the coil to wander and the sparkplug to do what you describe - been there myself. 

[chickenman_26]

Weird update: I tried installing the plug again and torqued it past the point where it was sticking. It seems to go in all the way now. It's definitely going in slightly cockeyed, but appears to settle down flush against the head once it's all the way down. I pulled it out and did it again to make sure. Same result.

Maybe I drilled out the old threads at a slightly off angle? But I would've thought the drill would self-correct once it was a few mm into the hole.

I guess the real test will be to start it up and see if it's actually sealing.

I don't know about the Helicoils for spark plug repair, but the TimeSerts have a portion in the bottom that expands to lock the insert in place as you run the insertion tool all the way down. And I've found it's almost impossible using hand tools to drill and tap while maintaining the exact same angle as the original hole. For that reason, the TimeSert tools include a seat cutter to square the seat with the new hole. Black out the seat area with a magic marker, then tighten a plug into place. When you remove the plug, the seat should have a consistent circle of black removed. If there's a gap, the plug isn't seating squarely on the head.

Stu

[mgilvary]

Good call on the magic marker tip. Plug is definitely not sitting flush. I wonder if I can grind that seat down a bit, like with that timesert tool. Is there a danger of the electrode reaching too far into the chamber?

[mgilvary]

Another thought: is it possible to re-tap the hole and insert a new coil, or is that just begging for trouble?

[chickenman_26]

Good call on the magic marker tip. Plug is definitely not sitting flush. I wonder if I can grind that seat down a bit, like with that timesert tool. Is there a danger of the electrode reaching too far into the chamber? Another thought: is it possible to re-tap the hole and insert a new coil, or is that just begging for trouble?

Begging for trouble, IMHO, to attempt re-tap, especially with the head installed. Grinding the seat flush with a TimeSert tool probably won't work in this case, because that tool undercuts the top several threads in the hole, and the Helicoil would get in the way. So long story made short, I'd take the head to a machine shop to sort out at this point, or get another head. A TimeSert kit for 12mm plugs runs around $120, and you could probably source a good used head for a lot less.

Stu

[mgilvary]

(sigh). Bummer.

I am the worst mechanic in the world.

[chickenman_26]

(sigh). Bummer.

I am the worst mechanic in the world.

Not true. Don't beat yourself up over it. Jobs like this require a lot more experience, both with the job and with the tools, than most ads or testimonials would lead one to believe. They make it seem like a child could do it. Oh wait....how old are you?  

One thing I saw in your first post - you said you drilled and tapped? Helicoil kits for spark plugs use a step tap to keep the tool square with the original threads - no drilling. Is it possible that you used an ordinary 12mm x 1.25 Helicoil kit?

Stu

[Nikkisixx]

(sigh). Bummer.

I am the worst mechanic in the world.

No, not possible.  That title is shared by everyone that turns a wrench at one time or another.  Most often it is held by the hillbilly PO of whatever I am trying to bring back from the grave   

Just to be clear, you are now sure that the hole is taped sideways?  The crush washer on the plug can take up a little bit of slop if the plug doesn't seat just so.  There are also thicker/softer washer used for indexing the plugs that might help.  Can you still easily screw a plug into the hole?  If so why not try this:  put a plug in the hole, spray a bit of soapy water around it, and kick it over a few times to see if it blows bubbles. 

The bad news is you can't just re-tap for a helicoil, and you need more that just a hand held drill to reapair what you got going on there if the hole is crooked and the plug won't seat.

[xenoscr]

A TimeSert kit for 12mm plugs runs around $120, and you could probably source a good used head for a lot less.

Stu

You wouldn't happen to know the right length of insert for a CB750K?

XenosCR

[chickenman_26]

For the D8EA plugs, 15 mm.

Stu

[mgilvary]

One thing I saw in your first post - you said you drilled and tapped? Helicoil kits for spark plugs use a step tap to keep the tool square with the original threads - no drilling. Is it possible that you used an ordinary 12mm x 1.25 Helicoil kit?

Apparently so. See? Like I said. Worst mechanic in the world.

I'm just frustrated because I had this bike sitting in pieces for 3 years before I finally had the time and money to rebuild the head and get the cylinders bored out to 605cc. Then, on reassembly, I installed the overbore kit rings upside-down and it was smoking oil (I had a thread about that, too). So I pulled it apart again and corrected the rings only to discover that I'd cross-threaded this plug hole (probably with my compression gauge). Since the head was already off, I figured it'd be an easy fix to put a helicoil in there. So I did that, put the head back on and managed to strip the front 6mm bolt (had a thread about that, too). Pulled the head off again, helicoiled the 6mm bolt and reassembled. The 6mm helicoil worked great. The spark plug? Well, we all know how that turned out.

Worst mechanic in the world, 2010.

Just to be clear, you are now sure that the hole is taped sideways?  The crush washer on the plug can take up a little bit of slop if the plug doesn't seat just so.  There are also thicker/softer washer used for indexing the plugs that might help.  Can you still easily screw a plug into the hole?  If so why not try this:  put a plug in the hole, spray a bit of soapy water around it, and kick it over a few times to see if it blows bubbles. 

I just did the bubble test on the plug as is. It bubbled slightly. Not a lot, and not on every revolution, but it bubbled.

The plug is easy enough to screw in, so I'll try a new, thicker washer.

Is it too late to install Timesert? I can't afford it right now, not after all the machine work I just did on this bike. But I would like it to be repaired properly.

[chickenman_26]

Is it too late to install Timesert? I can't afford it right now, not after all the machine work I just did on this bike. But I would like it to be repaired properly.

TimeSert can't be installed in a hole that's already been tapped for a Helicoil. A machinist may be able to square up the seat with the off-camber hole. Forget about trying multiple washers. I doubt they'll eliminate the gap. And leakage isn't the only issue. There's also a loss of heat transfer that'll tend to overheat the plug. If I were in your shoes, admittedly being the worst mechanic in the world, I would not attempt things like this again. Like Eastwood said, a man's got to know his limitations.    Consult a machinist. Good luck with it.

Stu