• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Spark plug hole screw up

sonny91be

CB750 Member
Messages
21
Reaction score
2
Points
3
Location
Heerlen
Hey there lads,

I was trying to perform a compression test but noticed that my tester loses compression fast, so I figured my litle screw in did not perform well, long story short i noticed I have a damaged thread in cilinder one. I have no idea how , because before the compression test I was fully able to screw in and out the plug but now I can't get it back in. It will only fit in halfway and then it does not go further. It seems as if a piece of thread has broken off and I need to fix that.

Can anyone tell me what the best approach for this is? The plugsize is M10 1.25 if I'm correct. Retap it with 2 taps a starter and a fulltap ? I probably have to remove the top end and start from the inside out retapping I assume. I tried putting in the plug with sole wd40 and moving it back and forth but the only thing that gave me was some more shavings that came off either way some might have fallen on top of the cilidnerhead so I figure I have to remove it all to fix it?

Any feedback would be appreciated on this.
 
Your compression tester might not be the right thread pitch, and you perhaps damaged the thread by screwing it in harder while trying to get it to seal.

If you're able to get any, pictures of the damaged threads would help.

First step is usually a thread chaser -- a special tap meant to repair existing threads that isn't as long as a typical tap. You can use a normal tap instead if you turn the engine by hand to get that cylinder to bottom dead center. Putting a layer of grease inside the flutes of either one of these will catch most of the metal chips they make. Both of these options assume that you haven't completely broken the threads. If there's damage serious enough that a tap won't fix it, you'll have to decide what kind of thread repair method to use (coiled thread or threaded insert) and to do either of those properly you will likely have to pull the engine out.

Any step that makes metal shavings requires you to fish them out of the cylinder and you definitely already have some in that cylinder. Unfortunately the only proper way to make sure you get absolutely every one of those bits of metal is to take the cylinder head off. However, there are a couple of other sketchy options that people try; it's your bike, so it's your choice as to how much risk you're willing to accept.

Option 1 is to find a vacuum cleaner and adapt its suction end to a very small piece of rubber hose or similar, and suck out all the shavings you can through the spark plug hole after it's repaired. Option 2 is to turn the engine by hand until the exhaust valves on that cylinder are open, remove the exhaust for that cylinder, and try to blow all the shavings out. Both are obviously not great compared to doing it properly but pulling an engine and taking it apart is a lot of work.
 
That makes a lot of sense and I was thinking that, I'm trying a thread repair and ordered one which I will try this week.
I bought the compressiontester as a set. Meaning the connectionshould be the right thread as it says m12 1.25 which is the correct one.

If you look at the pictures I included then you will notice that you can see I think I did not damage the threads on top but rather there are bad thread downwards. My compressionplug only goes in with 3 windings that' it so it doesn't go into the middle or deeper and that is where the damage seems to be and I think a repair tool will take the filth out so it can be used againe, mmost of the threads are still intact so they should do fine for now, they mght need to be replaced once I truely overhaul the engine nonetheless with heli coils as old alloy is not as good as new alloy they told me and threads go bad on these old bikes.

Thoughts senseamid?

https://ibb.co/7k3XqM3https://ibb.co/vhmzqJb
 
Hey, those are great pictures. Exactly what I was hoping for.

A thread chaser tap should work just fine on those. You've got plenty of threads to engage above the damage so even if it breaks off a couple threads it should still allow the correct torque for the spark plug install.

I think a Time-Sert style insert is better for spark plug and oil pan threads than a Heli-Coil because it uses a solid flanged insert.

But honestly, there's no need to redo the spark plug threads or oil pan threads as long as you use an accurate torque wrench. There are plenty of other fasteners in that engine that would benefit from Heli-Coils though, like the camshaft cap bolts.
 
Hey, those are great pictures. Exactly what I was hoping for.

A thread chaser tap should work just fine on those. You've got plenty of threads to engage above the damage so even if it breaks off a couple threads it should still allow the correct torque for the spark plug install.

I think a Time-Sert style insert is better for spark plug and oil pan threads than a Heli-Coil because it uses a solid flanged insert.

But honestly, there's no need to redo the spark plug threads or oil pan threads as long as you use an accurate torque wrench. There are plenty of other fasteners in that engine that would benefit from Heli-Coils though, like the camshaft cap bolts.
You really know what you are talking about, héhé.

Yes I talked to someone who restores bikes in a shp near were I live, he told me the same, he never rethreads but instantly makes new holes. I'm going to try and fix the hole nonetheless, some good new thanks a lot!, will open a valve and blow out remaining residue that way. removing the whole top section is a lot of work and as I just started it is something I'm not experienced enough at just yet I think.

I will try and torque em up later on and have new spark plugs + grease for them (molybdeen).
 
Your fix is pretty easy, I got a '72' with a spark plug broke off @ the threads. I'm thinking I should pull the engine & take the head off to do this right. the engine is fine otherwise.
 
Back
Top