Spark plug won't go in more than a few turns, is this normal?

2001 XR400

I was changing my spark plug, and it only took a few turns to get the old plug out.

The new plug goes in a couple of threads by hand, and then a couple more with moderate force (< 13 ft-lbs).

Is this normal, or should it go in all the way until the metal seat of the plug makes contact and compresses (like a car spark plug would)?

I took the tank off, laid the bike on its side, and blew all the dirt out before removing the old plug, so I don't believe there is any dirt in the threads.

Edit: The spark plug threads don't seem to be deforming, and there is no Aluminum on the plug when I pulled it back out, so I don't think it is cross-threading either.

does it tighten up somewhat, then turn it some more and it gets easy again all of a sudden? Anybody know what causes that??

thanks

It tightens up, and I can work it back and forth and get a little farther, but still only four threads or so. Don't really want to push it too far and strip something.

I put the tank back on, and the bike starts and runs OK. Starts better than before with the new plug in. :ride:

Well you should be fine then, Ive heard it only takes hand tightening then 1/2 turn. But they really dont need any torque.

I also dont really understand why some bikes (mine;)) run so different and smooth with a fresh new plug but then it goes back to how it was.. I really dont know if my bike runs lean or rich, I was thinking lean because it has stock jets, etc. But my bike starts with no choke first kick or 2nd, and the plug is a black color, and the plug is under 1 month old. Could be because she smokes and needs a fresh topend. Would the plug color be from that or maybe too rich?

anyway, sorry for kinda hijackling the thread. Hope your bike gives you no probs! :thumbsup:

It sounds strange. I can turn it forever. How long threads do you have on your plug?

Most likely, the plug is cross-threaded and is binding up rather than going all the way in. I recently repaired an XR250 with the same problem. If you don't get this taken care of, the plug is very likely to blow out of the hole at some point, probably out in the middle of East Peckerhead, miles from anything.

It sounds strange. I can turn it forever. How long threads do you have on your plug?

There are about an inch of threads on the plug. I take it the plug is supposed to thread in all the way, and not just seal with the threads.

Both plugs are NGK DPR8Z

Most likely, the plug is cross-threaded and is binding up rather than going all the way in. I recently repaired an XR250 with the same problem. If you don't get this taken care of, the plug is very likely to blow out of the hole at some point, probably out in the middle of East Peckerhead, miles from anything.

Cross-threading is what worries me. I don't want to make things worse if that's the case. I wish I could get a good look at the threads on the engine to see what's going on. The old plug was only threaded in a couple/few threads when I took it out (just bought the bike), and it was black/fouled. It could be there is gunk built up in the threads since the PO didn't have the plug all the way in. Would it make things worse to just keep working the plug in (either stripping the snot out of the threads or cleaning out the threads with the plug) and leaving it in until this winter when I have time to fix it right, or just leave it like it is and just carry a spare plug with me incase it does work loose? I'd like to ride rather than strip down the bike. What would you do?

The bike misses a little when cold, and seems like it could have some more power when warm. That's why I pulled the plug in the first place. This could be caused by the plug tip not being in the right position, no?

Don't try to force the plug into the head, the threads will be ruined for sure. If the plug hole in the head is not completely stripped out, you can try this. Go to an auto parts supply and get a tap ( I am pretty sure it is a 10mmX1.0, but you'd better check). Put a glob of heavy wheel bearing grease on the end of the tap, insert it into the spark plug hole, be sure it is started straight, and carefully work it into the hole with a back and forth motion, a little at a time. After a couple of full turns, back the tap out and clean it off, the aluminum chips will be stuck in the grease. Repeat this process until the tap threads cleanly all the way into the hole. Have the piston at TDC so that the tap will not accidentally fall into the cylinder if you screw it in too far. When you install the plug, put a blob of anti-seize on the threads, this will prevent future problems.

I agree with Creeky. You need to run a tap thru it. The plug does not just seal with the threads. It should screw all the way in and seat with the crush washer.

2nd creeky and Trailrider,

Must be cross threaded !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

3rd, almost sounds like someone ran it with too short a plug in it and the combustion process either eroded the lower threads, or they're gunked up with soot.

You can make a poor mans tap by filing a groove across the threads of a spark plug, as deep as possible. Know what a self-tapping screw point looks like?, File a groove like that across the threads of a plug. The groove needs to be filed in such a way that the thread face is somewhat undercut and has a sharpness to it so it will cut as you thread the plug into the hole. It will clean up minor damage of the plug hole and clean up any carbonization, but for major thread damage of the hole you'll need a tap.

Sounds crossthreaded to me also. Try to never remove a sparkplug, from an aluminum head while the engine is still hot. sometimes it ( the plug) will remove some of the threads. :thumbsup:

Wow, thanks for the great replies. You rule! :worthy:

Would pulling the head so I can actually see the threads be necessary (or helpful) to tap the spark plug hole? Could I then start the tap from the underside where the threads are not chewed up. I don't believe I would need a reverse threaded tap to go in backwards.

Doesn't look like it's too bad of a chore to get the head off in the service manual, and I want the best chance for success.

Also, am I doing any damage running the bike with the plug only partially installed (or could I damage the motor if the plug fell out while riding)? I'd like to ride for now and fix this during the snowy season. Or, should I suck it up, and tear into the bike this weekend?

I have successfully run taps through several spark plug holes with the head on. If the threads are damaged top to bottom, the tap may not do any good and the head will need to be removed and an insert installed in the plug hole. It is not a good idea to run the engine with the plug partially threaded in. Firstly, the electrodes are not in the proper position for good combustion, secondly, the plug could blow out of the head.

Checked the plug, and it's 12mm X 1.25

I really hate the idea of trying to tap the head without being able to see the threads.

How much work to pull the head?

If you can find the proper tap, I wouldn't sweat not being able to see the threads as you tap it. All you need be concerned about is making sure the tap starts correctly and it will take it from there. It would probably be best to use a starter tap instead of a bottom tap, but most taps I've seen available that size are all bottom taps.

There's a trick I've read about of setting the piston at or near BDC and filling the cylinder thru the plug hole with shaving creme. Run your tap and any shavings get caught in the creme. When you're done with the tap, you kick the engine over and the piston blows all the creme and any shavings out the plug hole. Probably makes a mess but saves getting anything in the engine.

I need to write the Mythbusters and have them try this.

If you can find the proper tap, I wouldn't sweat not being able to see the threads as you tap it. All you need be concerned about is making sure the tap starts correctly and it will take it from there. It would probably be best to use a starter tap instead of a bottom tap, but most taps I've seen available that size are all bottom taps.

Since I can thread the plug in a couple of threads by hand, I don't think I would be able to use a Plug Tap since it would be forced to start wrong and would just chew up the rest of the thread. A Taper Tap (starter taps) is rounded off for the first 8-10 threads and would help allign the tap much better. Sounds like the best solution.

Here's a link I found about the different types of taps:

http://pergatory.mit.edu/2.007/Resources/manufacturing/tapping/tapping.html

McGuckins in Boulder carries Taper Taps.

I think I'll pass on the shaving cream thing and just use some grease. ;)

If using a tapered tap, I concerned the TDC thing would be a issue due to clearance of tap (at full insert) to top of piston? Just a note for caution.

Since I can thread the plug in a couple of threads by hand, I don't think I would be able to use a Plug Tap since it would be forced to start wrong and would just chew up the rest of the thread. A Taper Tap (starter taps) is rounded off for the first 8-10 threads and would help allign the tap much better. Sounds like the best solution.

Here's a link I found about the different types of taps:

http://pergatory.mit.edu/2.007/Resources/manufacturing/tapping/tapping.html

McGuckins in Boulder carries Taper Taps.

I think I'll pass on the shaving cream thing and just use some grease. ;)

If using a tapered tap, I concerned the TDC thing would be a issue due to clearance of tap (at full insert) to top of piston? Just a note for caution.

Yes, I think I'll get both the tapered, and the plug taps. Start it with the tapered not at TDC, and then try the plug tap at TDC. Taps are cheap.

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