Friday, 9 December 2016

CBF250 Clutch Fix - Part#3 - Spark Plug Thread Repair

After the spark plug thread in the cylinder head stripped I staggered about holding my head and moaned NOOOOOOOOOO!!!! for a while then started to think about how to fix it.

the threads have completely gone
I could sort things by installing an insert. This means cutting a new, bigger, thread and then installing an insert with a 10mm x 1.0mm internal thread. There was no way of doing this with the engine in place, the frame is in the way, so out came the engine. It seems possible to do the work without removing the cylinder head.

Removing the engine is pretty easy on a small bike like this. I made this timber frame to hold the engine. I clamped it in my "wallmate" to hold it steady.

I bought a thread repair kit consisting of a tap and inserts of various lengths by German company BGS Technic.

I connected the tap to a 3/8" drive socket, extension & tommy bar.

The insert

An NGK CR8E spark plug  and a tube of Loctite 272 high strength, high temperature thread lock fluid.

A problem is the type of spark plug the CBF uses. It's a CR8EHS - the means that it is 10mm in diameter and had a 19mm reach. But the reach is only partially threaded. The top 6mm is unthreaded. This means that it won't fit into a 10mm insert that is threaded all the way. My solution is to use a CR8E plug that is the same size & heat rating but threaded full length. The unthreaded part of the original plug is tapered to provide an additional gas seal. In the new arrangement the seal will be by the gasket only.

Yes, I know that the old plug looks really rough. But, in my defence, its only been in the bike two years. This shows what the ravages of the Scottish winter can do.

When cutting the new thread it is very important to have the tap exactly in line with the hole. It is difficult by hand. I've done this before by mounting the tap in my post drill but this would be tricky with the engine. So I came up with another idea. I make a template in aluminium attached to the cam cover by the cover bolts with a hole to guide the extension shaft attached to the tap.

I first cut threads when working in an engineering factory about 40 years ago. The threads were in actuator valves to control fluid flow in a pottery casting machine. The machine was exported to Iran, I wonder if it's still working? What I learned was to rotate the tap back and forward and clean the swarf out regularly.

Here I go cutting thread. If I get this wrong I've wrecked the engine - so no pressure then.

It's not a good ideas to allow chips of aluminium to enter the engine. To try to avoid this I coat the tap with grease and clean it frequently. Once the job is done I'll blow the cylinder clean with my compressor allowing any chips to exit via the valves.

The tap quickly starts to cut the thread. I work it backwards and forwards in increments and regularly remove the swarf and apply new grease.

The completed new thread - it looks good.

I apply thread lock on the outside of the insert. Hopefully this will hold it in place permanently.

The finished job with the insert in place


  1. Well done Stuart! I had to do this for all 4 spark plugs on my CB400 Four this year. The previous owner had used araldite to keep them in! I took the head off to do it due to poor access and the fear of shards in the pistons.Was a good opportunity to polish up the valves. The kits work well, not sure if they will come out over time but we will see!

  2. The CBF has the narrow 10mm spark plugs. The standard torque is 7-10 ft lb so very easy to overtighten. I pretty sure that the grease held all the swaft and I blew out the cylinder with my compressor when I was finished. I'm not worried if a small piece of aluminium was left in there - it's gone now! The insert should stay in place because I used a high-strength thread locking agent. It's not the end of the world if the insert were to come out in the future.