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Discussion Starter #1 (Edited)
You may have seen my recent thread regarding a bad fuel pump in my 2009 Vulcan 900 Custom. I got the fuel pump replaced, and the bike was running well. I just rode it in city traffic for about 25-30 miles two days ago, and had zero problems.

When I first bought the bike, I replaced the spark plugs in both cylinders. During my troubleshooting of the fuel pump however, I undid the replacement and put the (presumably) original plugs back in. Those are the plugs that were in the bike when it was most recently ridden, with no problems.

Today I decided that since the plugs still needed to be replaced, I would go ahead and do that. I got the front cylinder (not sure if it's #1 or #2) just fine, but when I did the rear cylinder the spark plug broke. What I mean specifically is that the threaded part of the plug broke off of the rest of the plug, so that the wrench was just spinning the top part of the plug in the hole, but the threaded part (including the tip of the plug) was no longer connected.

After some swearing and a lot of struggling, I managed to get the threaded part out of the hole (with the tip of the plug intact). I took a tiny stick magnet down into the hole to get any tiny metal bits that might have come off when it broke.

I didn't use an extreme amount of force, but figured maybe I overdid it or maybe it was a faulty plug, so I went back to the store and got another. They were out of the exact plugs I'd bought the first time, so I bought another brand that is identical spec. So I know it wasn't just a bad batch of plugs from the same shipment or something.

I got back home, and went to install the new plug. This time I didn't use a wrench, and just finger tightened it (using a spark plug socket and an extension, but no ratchet). As soon as I felt resistance, I got my torque wrench out and set it to 13 ft-lbs (per the service manual). I started tightening the plug, but didn't even get to 13 ft-lbs before I felt it start spinning in place again (just like before). Sure enough I pull out the brand new plug and the exact same thing has happened to it. This time I know I didn't use excessive force. I've replaced spark plugs before in other vehicles so I have at least a little experience and know not to use too much force or you run the risk of stripping the threads in the block.

I'm at a loss. Does anyone know what might cause this to happen twice in a row? Could it be something about the threads? If they were stripped, I would expect the plug to stay in one piece but spin loosely (or be impossible to tighten down). However since two totally different plugs both broke in the same fashion, that points to either the bike, or me.

Any ideas?


EDIT: Okay, I was able to resolve this. The problem was between the seat and the handlebars (user error!)... In the service manual under the spark plug replacement procedures it specifically states, "The insulator of the spark plug may break if the wrench is inclined during tightening." My ratchet extension is just long enough to put the ratchet right next to the frame, which makes it impossible to keep straight when the ratchet (or torque wrench) is attached. Using a longer extension I was able to get a plug in there night and tight, without breaking.
 

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Glad you found your issue.
None of us has had that happen ever. lol
 

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Wait!!! The torque for spark plugs is 13 inch pounds NOT FOOT pounds (just over 1 foot pound).

13 foot pounds is likely why you broke the plug, hopefully you didn't damage the aluminum cylinder head. You need to divide inch pounds by 12 to get foot pounds. Don't need a lot of force to seat spark plugs.

WB
 

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Discussion Starter #5
Are you sure? The service manual specifies 13 ft-lb, not in-lb. It also provides figures in N-m (18 N-m is the spec), which converts to just over 13 ft-lb.

Regardless, I got the plug in without it breaking. The engine runs well and seems to be fine, though I'll need to take it for a longer ride to know for sure.

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Interesting I have two Kawi service manuals, one for LT model says 13 inch pounds, the 900 custom manual indicates 13 foot pounds. The engine is the same so I assume one of them has a typo. Looking at Victory manual it say 10-14 foot pounds so I think my LT manual with inch pounds is wrong.
 

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Discussion Starter #7
Interesting I have two Kawi service manuals, one for LT model says 13 inch pounds, the 900 custom manual indicates 13 foot pounds. The engine is the same so I assume one of them has a typo. Looking at Victory manual it say 10-14 foot pounds so I think my LT manual with inch pounds is wrong.
Strange. 13 ft-lb isn't much force, but 13 in-lb sounds like it would be a little loose. It's certainly possible, there are so many figures in those manuals. Looking around online it seems like most vehicles call for around 15 ft-lb of torque for spark plugs, so I'm guessing this is fairly standard.

In the past when I've done spark plugs I didn't have a torque wrench, but I was told to just make them snug, but not to overdo it. Basically just finger tighten them as far as I could, then give it one more moderate pull with the wrench and call it a day. That's why I was so frustrated with the plugs breaking like they were, since I really wasn't using a lot of force. But I guess I was at just the right angle to cause a problem.

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EDIT: Okay, I was able to resolve this. In the service manual under the spark plug replacement procedures it specifically states, "The insulator of the spark plug may break if the wrench is inclined during tightening."
This SM language only warns about breaking the ceramic insulator though, not breaking the hex socket block from the threaded body. I don't know why you had that at all, much less twice.

I've changed a jillion plugs without a torque wrench, hand torquing them "just snug", which prob. translates to well under 10 ft-lbs.. I've never found one loosened up when checking or changing them.
 

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Discussion Starter #10
This SM language only warns about breaking the ceramic insulator though, not breaking the hex socket block from the threaded body. I don't know why you had that at all, much less twice.

I've changed a jillion plugs without a torque wrench, hand torquing them "just snug", which prob. translates to well under 10 ft-lbs.. I've never found one loosened up when checking or changing them.
Yeah, I thought it was referring to another part of the plug. The part that broke on both plugs is metal. It basically snapped off right where the hex socket meets the threaded body, both in the exact same place. I figure it has to be the slight angle, though I don't know for sure.

I got the new plug into the front cylinder and torqued it to spec with no problems whatsoever, but I don't have the frame clearance issue on that side that I do with the rear.

I most likely won't get a chance to ride again until this weekend. When I do I'm planning to ride normally until it is warmed up, then gradually take it up through various amounts of throttle in a few different gears. Basically I want to make sure it's running right, and that the plug doesn't come out or anything under hard acceleration. I'm pretty positive that I didn't do any damage to the threads in the block, but I figure this will be a good test. Anything else I should check for?

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No, just blow out dust & debris from the plug hole before removing the plug so junk doesn't fall into the cylinder.
 

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Discussion Starter #12
No, just blow out dust & debris from the plug hole before removing the plug so junk doesn't fall into the cylinder.
Yep, I did that when I was changing the plugs. I also took a tiny stick magnet and put it down into the hole (and even slightly into the spark plug hole into the cylinder) to get any metal bits that might have come off during the break or the extraction. I feel pretty confident that nothing is in the cylinder, and for what it's worth the engine runs fine now.i just wasn't able to take it for a ride to really test it on the road.

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I used the spark plug socket which is supplied in the OEM 900 tool kit to change the plugs. Make certain that the socket is completely set on the metal body - using caution, can be done by feel. Waste of time and money - the OEM plugs essentially were in new condition. Installed new ones which I'd already bought, used anti-seize compound on threads, threaded them in by hand without using a wrench until seated, then used wrench only to snug them down by feel - didn't use torque wrench. No problems.
 

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These plugs are tiny small. They can easily break.

Not to mention NGK as a whole is not making the same qaulity plugs I used to remember when I was a kid.

The other aspect is 13 FT LBS is a lot of force for a tiny plug. I prefer to snug them by hand and just check them more frequently.
 

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Discussion Starter #15
I was able to go for a ride and it seems to be running fine, and the plug is still tight. I also changed out the brake lines this weekend (Spiegler stainless lines), so I am going to take a break from working on the bike and just ride it. I've had it for 3 months and have put probably only about 50 miles on it!

Eventually I'll need to get back in there and finish changing the plugs properly though. At least I learned my lesson about tightening them correctly.

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