Old farts, like me, would call it ‘fretting’. Wikipedia says ‘Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration.’ With an aluminium (head)/copper (gasket)/iron (cylinder) interface I would have thought there was the potential for fretting and a galvanic reaction. Somebody clever might have the answer…
Now it isn't my specialist subject but I was involved in some fretting research at one time and I've sat through a innumerable tedious presentations so let's see what sunk in . Firstly, as everything should be dry galvanic corrosion shouldn't be an issue with or without a copper ring in there unless, of course, it's parked in a pond in which case that's the least of your problems Fretting damage, fretting wear and fretting fatigue could well occur and is exactly as you say, small repeated relative movement of two faces in contact. The trouble is that there seem to be a very complex set of processes involved with both abrasion and adhesion and it ends up being an entirely empirical subject with lots of rather simplistic experiments to gather information. The gist that I have gathered, and I hope that someone better informed will come along and correct me, is that the fretting gets worse with more lateral relative movement of the faces, increased load for the same lateral displacement, large differences in hardness of the two contacting metals, the number of times that the faces rub back and forth and the ability of the metals involved to form oxides. I think that suggests that putting a copper ring in might reduce the tendency for fretting damage, all other things being equal, as the relative displacement between the head and the cylinder is shared over the aluminium to copper interface and the cast iron to copper interface. Anyway, you can tell if there is a fretting problem as there will be dark stains and possibly scars or even cracks on the contacting surfaces of the head and cylinder. Sorry, if that was a bit of a ramble, and not relevant to the thread
Absolutely relevant. Here are the fretted shuffling barrels. I can feel the edge in a few places where it leaked. In other areas where it leaked I cannot. Between the barrels where it didn't leak it's perfectly flat. Sorry this is the only post carbon scraping photo I have. I almost went for 25ft/lbs on the middle 4 and 27 on the outer 4 but what do I know?
Did I post this already? There's an alarming lack of support for the lower outer studs. The others all have cast bosses through the fins.
I don't think that fretting damage itself is a big problem here; it does look from the photos as though there might be some damage at various places on the sealing face which would suggest that there is relative movement going on but it's not leading to structural failure. The 4 o'clock position on the left hand chamber on the first photo looks particularly suspicious. You might see similar damage on the cast iron at the same location. Incidentally, the harder material can suffer more damage as the wear debris can get embedded in the soft metal and act as a sort of emery paper to wear the other face. With really severe fretting you can get large cracks forming and bits falling out or parts splitting. What it does suggest, and others here are far more expert than I am as a newcomer to the world of aircooled VWs, is that the stiffness of the whole head-cylinder interface isn't quite enough. From a basic engineering point of view, 4 studs around each cylinder does seem a little mean to provide uniform clamping, especially as you point out the lower two don't have much metal around them. My flathead Ford V8 has 21 studs per 4 cylinder bank and Ford deemed that insufficient and went to 24.
Spot on. And milling the deck 2.5mm and the thickest fin 1.75mm, which at its original 4.5mm is clearly more than a cooling fin... can't have helped. The damage at &4 o'clock is yer man slipping with a die grinder, if you look again you can see the tracks across the head. The left one's contact looked generally rough in my estimation, but was blowing slightly less.
But they, actually Buick/Oldsmobile, didn't get the bottom end quite right and there are fretting problems with the main bearing caps and the crank case; Rover had to cross bolt them for the big engines.
Indeed, they did. The heads have 14 holes but I only use 10 of them. An aluminium crankcase with steel main bearing caps wasn’t going to end well although my 3.9l isn’t cross bolted. So far so good but an earlier 3.5l I had did have a lot of fretting on the case and caps.
Are you fretting about the shuffling? Or does shuffling make you fret? Either way, it must be worrying
Started first compression. I put the VS exhaust on because the other was hitting the oil filler. There's enough wriggle on the filler to avoid that but having recently sealed it I wasn't going to mess with that sucker. I bet it still pops, I'll find out later on the supermarket run.
I fitted my new thermostat too. There's a constant 1 cylinder thud at tickover. Not mixtures. Ha, just remembered I left 2 plugs in the head while i scraped carbon and cc'd. I wonder if that's it. I've got another set.
I was (self) deluded, I hadn't really finished. Stupid under tins and the wire on the oil temp sensor left. I'll have to unpack the jack and get it over with. After that there is a split CV boot I'd pushed to the back of my mind. I think a sunny day required for mental stability doing that poopy job without a vice to hold the drive shaft. Damn - I hate CV boots. I should have put this lot on my van thread, but you know how it is, a popping on overrun, next thing you know you've changed the flywheel, flywheel seals, fan hub seal, dipstick boot, refitted the filler, new thermostat, one head off and refitted with new pushrod tube seals, both heads extra-torqued, heat exchangers filed flat... it's probably the cruddy cheap HT leads.
That's done (not the CV boot). I had to take the oil filter off to get at it. Fool. It would have been a doddle if I'd connected it earlier in the scheme of things. That's it. I've had enough for this session.
