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Recently, I submitted a post on here regarding a broken ground electrode on the spark plug of my 2005 Suzuki RM125:

Bottom piece of spark plug broke off in motorcycle engine

Well, I found out what caused this. The piston is traveling too high in the cylinder and actually running into the spark plug. After riding with a new plug (NGK BR9ES), I took it out and noticed that the ground electrode had been bent all the way to the center:

enter image description here

What could be causing my piston to travel so high and slam into the spark plug? My best guess is something in the bottom end (crank, rods, etc.) is worn or out of place. Am I going to have to replace the entire bottom end? Or maybe just the crank bearings. Any ideas?

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My best bet is that the spark plug is too long in the first place. I don't know what the stock spark plug should be for your bike, but the NGK you are showing may/may not be completely within spec and therefor is extending down into the cylinder longer than the spark plug should. If the bottom end was worn out enough to allow the piston to travel that far up into the cylinder, your whole rotating assembly would be shot. Realize that tolerances are measured in 1/1000's of an inch. To come into contact with the spark plug would take 1/10ths of an inch to accomplish. You'd be hearing a lot more banging around happening of this was the case.

RockAuto.com is showing the following sparkplugs for your bike:

  • AUTOLITE Part # XS4063 (Iridium)
  • CHAMPION Part # 89041
  • CHAMPION Part # 686 (racing plug - non-resistor)
  • AUTOLITE Part # 4063 (copper resistor style plug)

I would go to a power sports store with your plug in hand and see if these plugs are noticeably shorter than yours. I'll bet you'll be surprised.

Another thing you might be able to do is to put a second sealing washer on there. This may or may not allow a perfect seal, though. It would definitely pull the spark plug out far enough to keep it out of the path of the piston.

  • Funny you mentioned that last point. I actually ended up putting two more washers on the plug and the bike ran great! I updated my question with the spark plug type. I'll do some more research, but I was pretty sure this was the right spark plug. Thanks for your help. – hgwhittle Jan 13 '15 at 15:54
  • @hgwhittle ... you wouldn't even need to buy one, but just take it to a store and check yours against one of the ones I showed above. I'm betting these would be shorter than yours. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '15 at 16:16
  • I ordered the Owner's Manual recommended spark plug (NGK R6918B-8) and compared it to the spark plug I had been using (NGK BR9EG). The height not noticeably different, although the diode shapes are different. – hgwhittle Feb 10 '15 at 15:01
  • I'd still be using at least one washer on it, then. Keep it off the piston. – Pᴀᴜʟsᴛᴇʀ2 Feb 10 '15 at 18:13

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