Ai, this doesn't bode well, but all ain't lost. Let's assume your assessment on face value, botch job on sealing the tappet cover. My advice would be to clean the motor up and do the tapper cover seal properly. Remember that if the head is the problem, symptoms will either continue or be severe, which you would have picked up on already! Once clean, you can identify the root of the problem, if any exists - you can only fix what is evidently broken.overambitious wrote: From the quantity of gasket sealer around the tappet cover, and the fact that the head has a cleaner shade of oil on it to the block, I assume the head had been repaired/replaced at some point.
Do you know if there is any way to tell if an Amc head was fitted or if its still the standard Mitsubishi head?
I wouldn't jump to any conclusions yet. Normally any issues on the head reveals itself rather evidently; water loss, smoke, power loss, oil loss, water and oil mixtures, oil or water leaks. If the head is gone, its gone, if uncertain, drive sedately and go for an inspection at a head specialist - they do all the normal inspection type checks, but also do specialized checks, i.e. CO2 in the water, etc. If the head has an issue, just replace it with an AMC head. Pressure test of the head is close to 2k already, and even if done, there might be hairline cracks inside which they can't detect, and secondly, those heads are so hard, they are nigh impossible to skim. The head is really the only weak point of the 4M40 motor, if looked after, you can easily run 100's of 1000's of miles
If you want to know if you already have an AMC head, I would advise to visit a head specialist who installs AMC heads and ask them, of the cuff I don't know the external differences. HeadZone in Brackenfell sells them and confirmed all the above when I had to make tough choices 12 months ago.