Sunday, March 5, 2017

Fitting a spark plug thread insert

The latest in my 'never quite right, barrel of woes, one day it will be perfect' Gold Star's compression related issues was a damaged spark plug thread.

I had known for a while that the thread was not perfect but it held fine and I left it be. Turns out it was very much not fine at all and was the source of a small leak and consequent drop in compression. The escaping exhaust gasses had slowly blown the thread further and then began to melt both the threads on the head and the plug itself.

One burnt out and buggered spark plug.

It may seem that it should have been very obvious that the bike had this issue but you have to believe me when I say that it wasn't at all and only came to light when I was about to take the head off for the umpteenth time to try and nail the drop in compression.

The Goldie had suffered from low compression for a while previously and I had stripped down the top end to inspect, found nothing, re-assembled it and it came good. It may well have been the plug at fault back then and torquing it down some more on re-assembly gave a temporary seal.  

One damaged CB32 head.

Searching around for a fix it seemed like an insert would be the best bet. I had thought that more than likely I would have to get the head built up with weld and re-threaded but there was nothing to lose in giving a quick repair a try.

Shopping around and the insert kits are either very cheap or very expensive, I couldn't find any middle ground. Naturally I went for the cheap.

Thread kit and a couple of the inserts. Reamer tool is useless
but even so the price of the kit was small, it came with five
inserts and they worked out at a pound a piece if you throw the
tool away.

The kits come with a combined reamer and thread cutter and several inserts of different lengths. I decided to do the job with the head on. I greased the reamer to try and keep a hold of any shavings from dropping in to the head and figured that if anything was left in there at the end of the job it shouldn't be a big trauma as it will find its way out through the exhaust pretty quickly.

First problem was that the reamer / thread cutter tool was absolutely useless. It did not ream at all and the tap part of it would not take. Thinking around the problem I measured the outer thread on the inserts and luckily had a matching tap. The thread was a UNF. I very lightly counter sunk the plug hole and the decent quality tap went down fine.

Proper job. Quality UNF tap.

Next problem is that of course there was not an insert of correct length to match what I needed. Not a biggie - I just cut the insert down to length.

To install the insert I first put it on to a plug with the threads copper greased. Then applied loctite to the outer thread of the insert and put it in to the head. I torqued it down hard and gave the loctite a while to set. The plug came back out without the insert just fine and the repair was good.

Against my best intentions quite a lot of swarf had entered the head so I brought the piston up to centre slowly (so as not to blow any swarf past the valves) and picked out the head shavings with a screwdriver that I had dipped the tip of in grease. It took a fair while to completely clear the head but it beat taking the thing off.

Removing swarf with a greasy screwdriver.

End result is compression back and a properly fitting plug. Still remains to be seen if the repair will last, the worry is that the insert will expand slower that the head and loosen off but I am fairly confident it will be ok. Fingers crossed that the Goldie motor is finally sorted!

End job. A fairly decent repair.