So I now have the head off my 2.5 petrol (cast iron head) to replace the valve stem seals, everything else seems in good order, but naturally I want to de-coke the head/pistons while it's in bits. It's so long since I did such a thing how do you do it? Chisel and great care, brass wire brush and electric drill? Dremel? Don't want to damage the pistons. What is the best and/or easiest method to clean it all up ready for re-assembly?
|| So I now have the head off my 2.5 petrol (cast iron head) to replace || the valve stem seals, everything else seems in good order, but || naturally I want to de-coke the head/pistons while it's in bits. || It's so long since I did such a thing how do you do it? Chisel and || great care, brass wire brush and electric drill? Dremel? || Don't want to damage the pistons. || What is the best and/or easiest method to clean it all up ready for || re-assembly? || || -- || Regards || Bob
Bring each piston to TDC in turn. Smear a ring of grease round the edge of the piston (to stop any crud going down into the bore) and then scrape off the carbon. Don't use anything metallic, as a scratch on the piston crown could create a hot spot. I use a set of hard plastic scrapers meant for delicate wood applications - a strip of hardwood sharpened to a chisel edge would do. Finish off with fine wire wool to a nice shine. Light oil seems to help the process. Don't do anything where the grease is, as you need to leave a thin ring of carbon round the edge of the piston to help the seal with the bore (at least, that's the accepted wisdom and I've always followed it). Bring the piston back to TDC if it has moved, and carefully wipe away the grease and crap. If there's a lot of carbon round the top of the bore, you can use similar methods on that (while your grease seal is still there), but again leaving a bit is supposed to help the piston/bore seal.
I would advise against using power tools, and leave things like chisels and wallpaper scrapers back in the shed - you could easily do more harm than good. The important thing is to leave a smooth regular surface on whatever is left, as any "peaks" can start to glow when the engine heats up, causing pre-ignition (pinking) or running-on after you switch off..
HTH
That's what I used to do, so nothing has changed then. :-)
Only one of the valves needed gringing in because of a few marks the rest were perfect which was rather lucky. Done the head today, decoke, new stem seals, so it's the pistons tomorrow then rebuild.
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