Should/Would you replace a cylinder head gasket on high kilometre cars?

[BrazilianY60]

Fit an Engine Guard for $150 and keep an eye on your engine block/head temps.

I have bought a brazilian made digital temperature monitor device, with a bolt on thermocoupler to monitor engine temperature without depending on coolant. What would be the best place to screw in that thermocoupler, the block or the head?