I drive a 2006 Focus sport 1.8 TDCI 

Several months ago the car was overheating for approx 10 - 15 minutes after a hose came off, a few months later it began lossing water at the rate of about 1 cup per week, 

Looks as if the head gasket may need to be replaced, only symptom is water loss of about 1 cup full per week from coolant bottle. no other symptoms, no power loss as I'm aware, also no oil /water mix and no smoke out the exhaust 

 

 

My friend says the only machine work it would need is a head skim. he said it will not need machine work (other than head skim) because

 

A. It is a cast iron engine

B. It is not overheating and he said if the head/block was cracked then it would be overheating which it is not doing. 

90% of engines are Cast iron. Doesn't make any difference. My 1.4 Peugeot engine was cast but head gasket still went and needed skimming.

Although you haven't seen it overheating, it may of. For the sake of maybe £100 to get a block skimmed, I'd do it to be sure, best to do a job right once than twice

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Not sure where 90% came from, there's an awful lot of ally engine blocks around now...  1.4 TU in the Peugeots is a good example, you must have an old one for it to be cast!  Just done the HG on a 1999 one with an ally block.  Worst HG failure I'd ever seen lol.

Anyway, I wouldn't skim the block, not even sure that's a common practice as I haven't seen it done, but the head will probably need at least a light skim, be about £50ish.