So I replaced the engine head, gaskets, oil etc. - a new head was actually cheaper than having the old head skimmed.
A cylinder head skim typically costs around £40. If you bought a brand new head for less than that, I am sure a lot of people would love to know where
I also upgraded ....to the SAIC type of head gasket.
The MLS types of gasket are often regarded to being less tolerant of out of spec liner stand proud than the elastomer type. You need to have measured the stand proud of each liner, and ensured that at the least, they are of even stand proud across all liners. Ideally, they need to be over 3 thou, but you would probably be alright down to one thou if they are all the same.
- Liners -> I'll try compression test to see if they are equal on all cylinders - Is there a better way? Can this tell me if the liners are cracked?
Compression test will tell you if there is a leak from the cylinder - it won't allow you to distinguish whether it is due to worn valves, worn piston rings, HGF or a damaged liner though.
Now, if it is liner sealing failure, is it possible to re-seal the liners, without taking them completely out of the engine?
I would think the sealant is unlikely - it is only there to stop weeping of coolant into the sump. The liners are a fairly snug fit on the shoulder in the block, and wouldn't normally allow coolant to leak past them at any great rate (according to someone who did a lot of test work on the K series during its development). You won't be able to apply new sealant to them without removing them from the block. However, it is usually possibly to remove the liner without needing to completely dismantle the bottom end.
Could I reuse the SAIC head gasket? should I go for the MLS or elastomer type instead?
You won't be able to re-use any of the head gasket types - they all work on a principle of the clamping force between head and block/liners crushing some part of them to form a tight effective seal. You would need another new gasket if you take the head off again. As to which is best to use, you can only judge from measuring the liner heights/stand proud. If the stand proud is below 1 thou or uneven, it is often suggested that the original type SLS/elastomer gasket may be more effective. If the liners are flush with the block face, it is debateable whether any of the gaskets would seal effectively for very long, and if the liners have sunk below the head face (usually the result of the liner seats being damaged/softened due to gross overheating), then the only options are a replacement block, or shim the liners back up to their specified height (Brown and Gammons used to sell sets of 3 thou shims for this purpose, but I am unsure whether they are still available).
All things being equal, liner heights even and 2 thou plus, I would have no hesitation in using the SAIC gasket - I've had one in my ZR 120 for four years and over 25k miles now with absolutely zero coolant loss.
There isn't any way to check the liners for damage without taking them out, but it needs to be said at this point, that cracked liners (although not unheard of) is a rarity on these engines. I would discount the IMG; even if that is leaking, you will be unlikely to get coolant into the sump from that source, and certainly not the quantity you have found after a short engine run (unless the piston rings are very badly worn - in which case you would have bigger problems than coolant loss!).