Airbox is back on, with a cross threaded spark plug

[kryzhkov]

Alright, so I discovered the cross threaded spark plug when I changed the plugs out when I got the bike, Lord knows how a person could do this.. Anywho, without the airbox, the bike cranked right up, first try, no put puts, all go. I replaced the spark plug, but noticed that it sits 1/4 of an inch too high, because of my cross threaded problem. I put the airbox back on, and now the bike is a bit temperamental when I try to start it up. Would tapping the plug threads so that the plug fits correctly fix this problem, or is there an air to fuel ratio imbalance?

[reelrazor]

Just running a thread chaser or a tap thru that hole may solve your problem. It's about 70/30 that you get away with just chasing the threads (70% chance of that being all that is necessary). If not, a solid thread insert (not a coil type like heli-coil) is your next recourse.

Some guys on here are gonna say you need to pull the head to do this right.

I say get it at top dead center (that cylinder) and use some heavy grease in the tap/chaser flutes and go for it. Then use a long blowgun to clear any chips that fall in. This applies to either chasing the threads or thread insert repair.

[SquadraCorse]

Just running a thread chaser or a tap thru that hole may solve your problem. It's about 70/30 that you get away with just chasing the threads (70% chance of that being all that is necessary). If not, a solid thread insert (not a coil type like heli-coil) is your next recourse.

Some guys on here are gonna say you need to pull the head to do this right.

I say get it at top dead center (that cylinder) and use some heavy grease in the tap/chaser flutes and go for it. Then use a long blowgun to clear any chips that fall in. This applies to either chasing the threads or thread insert repair.

You'd really chase the threads with the head on? If it were I, I'd pull it. I don't see how you could prevent metal shavings from ending up in the cylinder. Even if you blow it out, you have no way of knowing if you got all of them. You can absolutely do it with the head on, but peace of mind alone would be enough for me to pull the head.

[reelrazor]

You'd really chase the threads with the head on? If it were I, I'd pull it. I don't see how you could prevent metal shavings from ending up in the cylinder. Even if you blow it out, you have no way of knowing if you got all of them. You can absolutely do it with the head on, but peace of mind alone would be enough for me to pull the head.

It happens every day, probably hundreds of times. I've turned wrenches for the majority of my working life, and most of my friends do too.

The car shops are seeing lots and lots of this...with the newish "100,000 mile spark plug" replacement intervals. They NEVER last more than about 70k..and by that time, the plugs don't want to come out- and take the threads with them when they are removed. EVERY single Ford truck I have seen in the last ten years seizes the #7 plug in the head if left in there more than about 30k. Removing the heads on some of them is a cab off proposition.

And, the bike shops I have worked in have always seen this....and have pretty much always used the TDC, head in place method. BMW SPECIFIES this as standard procedure on their engines.

When I was 7 years old(1972), I watched my family's VW bus get thread inserts at a gas station in pennsylvania this way while we were on a cross country trip. My Ma drove that thing for another 8 years after that-and then sold it to a family down the block.

At TDC, there is nowhere harmful that any shavings can go-valves are closed, edges of pistons are well shrouded by the squish band. Any couple of shavings that may be left are immediately expelled out the exhaust. With two strokes, you can rotate the engine so that only the exhaust port is open and clear the chips out thru that

Combustion chambers are a very simple, and very durable place when it comes right down to it. Big flakes of carbon are always shedding. A couple tiny flakes of aluminum? Out they go.

[dfuerst]

Yep, this is standard practice for small engines too. Even for installing a thread insert, which is a much more drastic procedure. Lots of grease on the drill bit and tap, and have at it. There is no need to pull the head.

Darren

[kryzhkov]

Alrighty! does anyone know the measurement of the tap needed? I have an 89 fizzer. I'm no machinist, but the machine shop department at work should probably have something I can use.

[DonTZ125]

The plug thread is M10x1.0. If you need to drill and re-thread, the insert instructions will give you the drill and tap size; they tend to vary by brand.