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Rocky Trail

Stripped Pilot Screw...Any Bright Ideas?

8 posts in this topic

I just picked up a 99YZ400. I haven't had a bike in about 6 years so I'm pretty excited about it. I am also impressed at the wealth of info here on TT. So far, however, though I have searched, I can't seem to find anyone who has had my problem. I got a deal on the bike because it has a stripped pilot screw. The one that goes into the bottom of the carb right in front of the float bowl drain plug. It has tiny little threads and it goes in at an angle to the rest of the machining on the carb. Has anyone run into this?? What did you do about it?? Or does anyone have an extra carb laying around that they would like to sell cheap?? Any help would be greatly appreciated!

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WOW! That could be a tough one.I guess i would start by removing the carb,taking the bowl off and see how bad it really is.If it's just the 1st or 2nd tread maybe you could just Chase the treads and be ok.If it goes deeper than thatI would start looking on ebay for a used fcr carb!Whatever you do be careful!!!!!!

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The carb is off. Float bowl is off. It is so stripped that it is hard to tell where the threads used to be. The hole is at an angle to the rest of the carb, so there is no easy way to hold it straight to try to drill and tap it. The previous owner said that he heard that there might be a helicoil for it, but I have a hard time imagining it would come out straight. I need to check on that today.

You say "be carefull" what do you mean? carefull of getting the right thing on ebay or on fixing this carb??

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Is there any room to put a sleeve in and re-tap it. they also have those I think they are called a helicoil?

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First thing to do is get a new screw so you'll know what you're drilling.

You have to center it up in a drill press. No hand drills here please!

Drill it first with a bit pretty close to the size of the hole to get a good center. Then drill it to size according to a tap chart. Just deep enough to remove the threaded part of the screw. Remove what's left of the screw and spring, clean it out and tap the restored hole. I think it's an 8mm x .75 thread? (Been a while since I've done this.)

If just the slot is gone, you might try a left handed bit. Sometimes you get lucky and the damaged screw will turn out. But if the screw is seized in place, you'll have to drill it like above. Just take your time, especially in set-up.

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I guess I wasn't very clear on this. The screw is out, but the body of the carb is so stripped that there is no evidence of any threads left. The thread on the screw is supposed to be 6mm x .5. I ordered the tap and a new screw today. I have a metal lathe, so I have no doubt that I can make my own helicoil or insert if I have to, but I wondered if anyone else had been through this problem and could share their experience. (hopefully with a simple solution and a positive outcome)

I guess my big question is...

Does anyone know if there is a thread repair kit for this application? (helicoil or the like)

And if there is how do you get it to start straight?

Thanks for the input so far. Keep it coming!

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Each size helicoil requires a specific tap drill and a special helicoil tap. The coil is like a spring with a tang on the end. A helicoil installation tool, which is like a bolt with a shoulder on the end, grabs onto the tang, which will advance it into the hole by compressing the spring (the coil is actually bigger than the helicoil tapped hole, but it fits when it's compressed and won't come out since it expands in the hole). So, getting it in straight is just a matter of getting the helicoil tap straight. Once you get it in, you have to break off the tang, so you'll have to make sure you can get that piece out.

You also may have to make sure you get the coil indexed properly with the right length of threads after it's installed. I just counted 10.5 full turns from lightly seated to screw out on my fuel screw, so if you're right about the thread pitch at .5, which is the distance between threads in mm, the entire threaded portion in the carb should be at least 5.25 mm.

That said, in a very quick search I wasn't able to find an M6x.5 helicoil, but if you have that size tap, I wouldn't say it doesn't exist.

Your other option would be to make an insert, but due to the minimum wall thickness you would need, the male threads may be bigger than the amount of material you have left in the carb (the design of the helicoil allows for the least amount of over boring).

I hope I didn't go on explaining something you already knew, just trying to be thorough.

Good Luck.

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