In everyone's experience how many miles should an XK 120 or 140 engine run before problems will occur (aside from regular maintenance)?

The only thing I can think of is the valve clearances if the valve seats were to recede slightly due to running on poor quality fuel and pre-ignition which can cause this (best to run on 97 octane and additives)
If clearances change then re-shimming may become necessary

OK, thank you.  What I meant was are these reliable enough to go 100k miles, or beyond that?

 

ron

21 hours ago, ronisme said:

OK, thank you.  What I meant was are these reliable enough to go 100k miles, or beyond that?

Absolutely, they are superbly engineered and there is no reason that with the correct routine maintenance it should go on indefinitely