Almost the luckiest escape. (method I used for removing broken long series drill)

[thehomeengineer]

Hi All

I took on a private job to fit a decompression valve to a 2 stroke motor bike head that the old boy couldn't kick over. Then the dreaded words came out "can't get another one of these cylinder heads anywhere as they don't make them anymore, so we have only one shot at this" (WE?) (I should have walked away at this point).

However the customer had marked up the position where the hole was to be spot faced and drilled. The material was only aluminium, and this had been preformed on other heads, so what could go wrong!

The head was set up on the mill and aligned to the marked out position and the job positioned under the spindle. Slideways then locked off and proceeded to centre drill and drill. The hole was only 3/32" diameter and a long series drill was going to be required to finish the hole.

While drilling there was a funny crunching noise so stopped the drilling process. Nothing obvious so I wondered if it was porosity? Went slightly deeper and it then stopped. Then disaster! with only a 1/16" of depth to go to break through the drill end broke off in the head. The cold sweats and that sinking feeling quickly hit me and the words "so we have only one shot at this" were ring in my ears.

So first things first how do I remove a broken long series drill without causing anymore damage

It was impossible to drill from the other side so a had to come up with another idea . Lets take the broken drill bit and place it in a battery drill and run it backwards to see if I could dislodge the broken part of the drill. To my surprise some swarf was dislodged, but the drill was still firmly in place. I couldn't use a magnet so was stuck having to confess to the damage to the head with no solutions to solve the problem.

Then another moment. I thought maybe just maybe could I could hydraulically remove it .
So the hole was filled with oil and the shank end or the broken drill used as a piston. The hole was charged and refilled several times until a good pressure was felt/achieved and a slight tap with a hammer this moved the broken part of the drill and I was able to then blow it out with the airline.

Total relief short lived!

WHY DID OIL look like it was coming through to the coolant passage A torch was then used to shine light into the coolant chamber and to my horror there it was I had drilled into the coolant cavity. The casting wasn't flanged as I was told but was hollow. So the porosity I thought I had broken into, was actually the coolant cavity. The drill must have then wondered on re-entry to the casting and broke off the end of the drill bit.

The dreaded phone call was then made and the customer said he would pop over straight away. When I said "you showed me a YouTube video of this modification and they didn't have any issues with it braking into the coolant chamber": His response " that might be because that was a different head assembly from the one I gave you. The video was for a Honda not a KTM".

To overcome this I will need to sleeve and weld the head so coolant cant enter the piston.

What a nightmare.

Thank you for reading and sharing my pain.
The Home Engineer