No-one ever said it would go without incident and it certainly didn't, but I think I have enough thread of sufficient holding power in the block to at least get the car back to the UK (after some reliability testing in France). The jig worked really well and I cannot begin to imagine how I could have done the job witthout it (or something similar). The drilling out process was nervy - no matter what combination of drill speed and pressure I used the drill bit would find ways to grab, but in the end the hole got drilled and it looked and felt perpendicular to the block. I also used the jig to guide the tap in (having drilled out the jig hole to accommodate the tap) and this made the all-important first few threads easy to cut. I was having to use a fair amount of force to cut the thread, but not so much that it threatened to snap the tap. Cast iron feels 'grainy' when tapping, not like mild steel.
Then we came to the business of inserting the Helicoil. The first one I tried, I can only assume I used too much initial pressure on the tool, as the insert went in at what looked like every other thread - i.e. a very coarse pitch thread when it should be a fine 1.25mm pitch. Once I realised this the only way out was to drill out the helicoil and start again. Thankfully I was able to do this (with the aid of the jig), leaving behind the thread to accept the helicoil, which I then ran the supplied tap down 2 or 3 times. The thread didn't seem any the worse for wear and so with much trepidation I put a second helicoil on to the tool and very carefully started the insert into the thread. Thankfully this time the internal thread was a genuine M10 x 1.25, which I verified by carefully running a head bolt down it.
I now had a bit of a dilemma. The inserts are only 10mm deep, which is a long way short of the 20-22mm of thread depth the standard head bolts occupy. So I decided to try and use 2 inserts, one above the other. The first one went in until there was 12-13mm of free thread above it, which I was happy with. So I broke off the tang, and put another insert on to the tool and threaded that in, until it bottomed out against the lower insert. The top of the insert was about 1-2mm below the block deck, so that was ideal. When I tried a head bolt down the newly Helicoiled thread the first part (on the top insert) felt very good - minimal pressure needed, but no slop or wobble. But then when I came to the lower insert it went very stiff, to the point that I wasn't prepared to continue.
I had assumed that the thread would continue without any real separation between the lower and upper inserts, but it appears that when threading the insert in, the coil of thread is actually stretched. albeit not by much. This means that the threads of the two inserts are out of step, as I found out. So I suspect I will need to take some careful measurements aind shorten one of the head bolts so it only tightens against the upper insert and doesn't impinge on the lower insert, as that would throw the torque figures right out. I am told that a Helicoil repair is actually stronger than the original thread, and it looks like I will have the chance to test this out...
So we have progress, but I don't feel that I'm out of the woods just yet. The head will have to be successfully torqued down on all bolts and the engine running properly before I can feel confident.
