Angus,

It might be worth trying what I use when ever I have to clean up engines.
A few years ago I used to do a lot of trading on Ebay, before evey one got in on the act, selling s/hand engines, and the best cleaner of all was automotive coolant, glycol, I think thats how you spell it.
I use the concentrat coolant, the coolant you have to add water to when you add it to your radiator, not the stuff that you pour straight in, the most common here in Australia is the green stuff.
You will also need an old electric crock pot, the one that you use to make stews in and has the ceramic / stoneware pot in it, and once you have used it, do not use it again for food processing.
Fill the crock pot with coolant, place the header in it, and switch on and walk away. Leave the crock pot setting on high as you won't boil the coolant.
The next day remove the header and let it cool, but use a pair of tongs or plyers or something as the part will be very hot.
Once cool enough to handle inspect the header, if not clean enough put it back and give it another day, the longest I ever had to cook a part was 3 days.
When clean, just rinse the part under running water, or just let it drain dry.
Coolant won't harm Alloys, brass, copper , steel or rubber, as well as bieng an excellent cleaner it's also a rust inhibitor, it won't harm the inside of your car engine, so how can it harm a model engine.
The only down side is that its fairly expensive stuff and it does deteriate with use, the more you use it the lesser cleaning job it does, and make sure you do it in a well ventilated space. IF YOU WANT A DIVORCE DO IT IN THE KITCHEN. but the bottom line is, it does work.

Will

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