1999 Scambler 400. I blew the plug out of the head twice in the same day.. "NO" damaged, to the threads in the head. Once the plug blew on a cool down ride after an hour plus of real hard riding. The second time was at idle after a 1/2 hour of hard riding. Again no damage. Screwed the plug back in and kept riding. The plug also blew out once about a year ago. I use 20 ft.lb when tightening the plug. One suggestion I got was to go to 40 ft.lbs. I hate over tightening into the head. I know 2 strokes can blow plugs, but I do not know the "why"? ( I'd really like to know the "Why" ) Another comment from a respected source was that the original owner may have had a SplitFire plug in it at one time and that the SplitFire has a "slightly" different pitch that may have affected the threads in the head. Anyone else had any plugs blow out ? Would really like some answers.

 

I dont want this to sound like a stupid question, but are the washers on the plugs? Not only acting as a sealing ring, they help keep pressure like a lock washer. I got a whole box one time missing the rings on them. The torque specs on plugs are: New spark plug from 9-11ft/lbs, and used plugs go 17-20ft/lbs. If all seems OK, you might want to either heli coil the head, or the simplest thing would be to use the most mild form of loctite available. Hope this helps,Bubba

 

There are no stupid questions. The compression washers were on the plugs. I agree with your spec's on tighning the plugs. The 40 lbs. just seemed extrememly high. Thanks for the reply.

 

Wow, I've had alot of different machines over the years and have never blown out a plug. Granted it must be loosening and backing itself out do to the compression force and vibration or else it would have to pull the threads out with it. Does it fit in the head loosely? Does it wiggle around alot if you thread it in till it almost seals on the head. I know it is not uncommon to feel a little woobling like that but I'm just wondering if there is more damage done to the threads than it appears. My cousin was working on an old motorcycle one time and he put the plug in by hand and took it acrossed the field for a test run. The plug vibrated out it ignited some fuel from a leak he hadn't fixed yet. He came out of it alright but with a little less hair than he had before the little test run.

 

Hey HOSPORTSMAN,
Good to see another S.W. Michigan rider around here.

Brian

 

The plug screws back in like there was never a problem. No wiggle. Nice and tight. Feeling the threads on the head with the tip of my little finger feels like they are in great shape. Rode the Scrambler hard this past weekend and no blow out. Talked with a mechanic at a Yamajha shop and was told blowing the plug is not common, but definetly not unheard of.. The "why" is what bothers me.

 

Dusty-- Go to www.thumperpage.com or just key in Thumper Page. Lot's of Michigan riders on the message forum.. Most of them are ice racing.

 

Dusty,
Yes, its always good to know your not the only one around.