Pinball Gasket cutting and fixing a leak

I've had a small water leak since the car has been built; a drip onto the clutch housing.

After assembling the thermostat housing & mounting it to the engine I did check back on Paul's build, Paul was building roughly at the same time as me, and he took good advice from GBS to clean up the mating surface between the housing and the connecting pipe - I hadn't on mine & thought it would be fine - and turned out (touch wood) that area was fine for me.

However;

The other mating surface between the black right angle pipe and engine head was not flat enough and should have been cleaned back to bare/flat metal. The powder coat on this face was too lumpy for the gasket to seal properly & hence my leak.

Took the whole assembly off & cleaned up this face  to be completely clean of powder coat and smooth to the touch & flat - looks much better now:

Somewhere online, after I cut my other gaskets, I saw the trick to cutting bolt holes - rather than cut them out with scissors or knife a sharp tap with a ball hammer will cut the holes in exactly the right place using the edge of the hole itself.

Of course I don't have an engineer's hammer - but - inspiration - do have a pinball (used to have a Judge Dredd machine for some years - another high maintenance toy which I can recommend if you like playing & tinkering with solenoids & bulbs :) -

Given a tap from a rubber hammer the ball had the right effect - clean chads & holes produced:

Re assembled, gasket in place with a smear of instant gasket paste. I'll let it go off overnight then re-fill and see if it made any difference.

Update - Now my water temp sensor, the dash one, is working properly. Penny just dropped, the dash temp sensor relies on an earthed body, with the powder coat between the thermostat housing, bolts and engine in place the housing (& attached sensor) had a bad earth & thus the sensor was always reading low.

No effect on the ECU temp sensor, cooling, fan etc - the ECU temp sensor has its own ground feed.