As a long-term project, I am refurbishing a Subaru H6 motor, with the intention of swapping it with the one currently in my 03 Outback. Some parts of the replacement motor are heavily deposited with carbonised oil; these deposits are proving difficult to remove. 
I was hoping to soften and remove the carbon by soaking it in kero or shellite, and, whilst this has softened the carbon to a degree, it still requires forceful removal with a wire brush and/or wet and dry sandpaper. Internal passages that I can't reach would still be gunked up.
In the picture, the LHS rocker cover was soaked in kero for a week then worked on - the RHS rocker is as it came off the motor.
Can anyone suggest a liquid I could use to dissolve the carbon? It can't be acidic, as that would pit the alloy.
Thanks in advance. 
Used to be able to buy a liquid called "Decarb" from PeerlessJal worked wonders on carbies and parts encrusted with old oil deposits. Not sure if it still exist? Try to see PeerlessJal on the net!
If you want them to come up absolutely mint, and don't mind paying a few bucks, get them vapour blasted.
Otherwise maybe carby cleaner?
I've used oven cleaner before on non-critical parts in the past, but pretty sure it will eat ally, so may damage sealing surfaces if you aren't careful, i'd say a no-go for your rocker covers.
Another option is if your local workshop has an ultrasonic cleaner, that may help accelerate the process, not sure how well it goes with heavy carbon deposits tho.
Thanks, swoosh; thanks, felix. I'll pursue the PeerlessJal and vapour blasting options, and let you know what transpires. 
Aluminium safe oven clean. But must be aluminium safe. Wrap the parts up in newspaper and rags.
Repco had the brake cleaner on special this week, 4 cans for $10. Best stuff if you don't want to use kero and degreaser. I use it on any work I do around home, as it hasn't the odour of most fluid type cleaning products. Just did inside of rocker cover on 4.2 precom diesel engine after 250,000 k's, one can, and spotless. 
I worked in a engine reconditioning workshop for many years .
Best option is an alloy acid bath overnight , washed in parts cleaning machine (hot water washing machine)next day.
If you have a local with these facilities the end result is "AS NEW".
Should only cost minimal for professional finish and no risk of damaging as pressure cleaning can do!
Repco re-conditioning workshop should be able to quote, or your local CAT or DETROIT dealer.
