Cb350 stripped spark plug hole, use one size bigger plug?

So after buying a cb350 72' I noticed the plug hole is stripped. Can I simply use a spark plug with larger diameter threads? What is this one called?

Stock is b8es, is there a bigger diameter plug and I can cut with a tap and die kit?

irk miller

You've been mostly-dead all day.
They make Heli-coil kits specifically for spark plugs. I've used them with great success many times.

irk miller

You've been mostly-dead all day.
Why not just tap a slightly bigger threaded spark plug in?
You then have to find the right length of the threaded part too. IMO, the best option is the Heli-coil. Then, you're sticking to the stock plug configuration. Why complicate things by changing them up?


I prefer Time-serts over Helicoil but either one is better than going to a larger plug IMHO. A different type of plug for each cylinder? Yeah... no.
This guy did one size bigger, using a normal tap and the new plug.

If using two different size plugs on each side of the engine is bad... Could I just tap the other side, the good side too? That way they are both even.

This seems like a superior and cheaper way than ordering a timesert or helicoil plus there's less moving parts, no worries about a helicoil backing out coming loose overtime.

What is the next size up from b8es?


Active Member
How bad are the spark plug threads? Missing altogether? Totally effed or just can't get a plug in straight? I've had success with this amazing Backtap. https://www.amazon.com/Powerbuilt-6...ocphy=9051997&hvtargid=pla-561833142948&psc=1 B8's are maxed at 14mm already from my understanding (10mm, 12mm or 14mm are the standard sizes). It says 14 x 3/4 and I measured the one I have in front of me and it's 19.20mm (.75 inches so it's correct) from the end of the threads to the BODY of the plug (not the washer) Honestly, there's only a couple ways to go in my opinion. I'd timesert it if it was me if the backtap can't possibly fix it. It's just about as good as you can get or better since they likely won't strip out again. However, as you note, the cost of the tool is high. If you are buying and working on motorcycles for a long time, this is honestly a fantastic purchase, as I'm sure you'll use it in the future. Here's a similar cheap product https://www.autozone.com/test-scan-...ls-spark-plug-thread-repair-system/141511_0_0 Helicoils work but aren't as sturdy as the timeserts. They've been used millions of times and a few come out from poor drilling/tapping here and there. Still a viable option. So, since there isn't a "larger" spark plug possible...you can't go that route...
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Well-Known Member
It should match your spark plugs as Irk already mentioned. They are either 1/2" or 34" and the plug diameter is either 10, 12 or 14mm. Measure them.

I don't mean to sound mean, but you have all the input you need. It's time to stop and think about it and learn from your own inspection of the parts. You call yourself KickRacing Motorcycles as if you build racing motorcycles and it's good to be aspirational, but a healthy dose of reality and initiative help. People love to help those who help themselves, but aren't really interested in helping people who seem to be playing some sort of cosplay with motorcycle builds and who want others to do all the thinking for them.

It's the modern equivalent of catch a fish and feed a man for a day. Teach him how to fish and he can feed himself. Same idea - less fishing. :)

I'm really not being mean. I just want you to stop and think about what information you have in front of you already. Do some research on line about spark plugs. Check the NGK site for example and some of the missing pieces will start to drop into place.

irk miller

You've been mostly-dead all day.
Being this a thread about a stripped spark plug hole, I will also add that you should always install spark plugs with a torque wrench. People often ignore this, but the torque is relevant to the plugs ability to cool itself as well as prevent stripping the threads. Most plugs torque around 20 ft/lbs, which isn't a lot, but check specs for your engine.


Active Member
If the threads are .75 inches (3/4 of an inch) as I stated above in my post...then you have to get something slightly shorter than that per your post #9 above. As Teazer is saying, read everything that has been said, it's all in there for you. I even gave it to you in millimeters just in case (19.2mm) You should have the info you need to get this sorted out and more. As a side note...since I'm sure you won't pull the head to fix this, DON'T put the cylinder at TDC on that side as you'll drill through the top of the piston when you are drilling and tapping. LOTS of grease on the taps. DO create some sort of vacuum attachment (like a piece of fuel hose) to suck out any bits that make their way into the cylinder. Doing it with the head attached to the engine has it's negatives, but usually the aluminum bits are soft enough to pass through the exhaust valve...usually...ha. Good luck.

If I do buy a timesert what size should I get? I hear you don't want the timesert to be too long at all. Ideally 1 thread shorter than the head. M14x 1.25 x ?

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