If on a service a mechanic fitted 4 incorrect 1.6 Zetec spark plugs, onto a fiesta ST 2.0 (56 plate) would this have driven without fault for 2 weeks, or would this fail have instantly ?
Hello, I hope the following is of assistance.
If the incorrect spark plugs were too long so that they impacted the piston I would expect an instant failure. If the spark plugs are the incorrect length but don't hit the piston then it is possible they plugs would fail later due to their incorrect position in the combustion chamber. The spark plug been in the incorrect position in the combustion chamber would affect the temperature in the combustion process which in the end may damage the spark plug or even possibly the cylinder itself.
From memory I believe the 1.6 Zetec plugs are longer than the ST plugs, I'm not sure of the heat rating of the 2 different plugs but if they are different then damage to the spark plugs and possibly cylinders can occur.
I hope this helps,
let me know if I can assist further and I'll reply as soon as I'm able to.