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rebuilt head, precup protrusion specs?
by
b1rdjx88
on 12 Dec, 2009 22:09
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I had an aaz head rebuilt last winter and am finally getting around to putting the engine together. I've noticed that each of the precups protrudes above the head surface by .2mm on the edge closest to the valves, but is flush on the opposite edge. Is this normal?
The precups were taken out of the head before milling, and the machinist claimed he has done lots of VW diesels and drives one himself.
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#1
by
Quantum TD
on 13 Dec, 2009 00:58
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Yeah, I had that problem on a head I had redone. I never installed it like that. Frankly, there should be NO protrusion. Tell him to redo the pre-cups. Clearly, there is a taper to the head resurface and he botched the prechambers. That's why I steer clear of shops that pull the prechambers for a resurface. You're better off having it milled on a proper machine, or better yet, ground with a stone mill.
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#2
by
b1rdjx88
on 13 Dec, 2009 09:04
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Darn, I was worried that would be the answer. I moved out of Ottawa last year, so the shop I had it done at is quite far away from me.
Would I be foolish to try to fix them myself? I'm thinking, working one precup at a time: mark on the precup where the protrusion exists; pound it out of the head; sand it down with 600 grit on a piece of glass while checking regularly with a micrometer.
Bad idea?
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#3
by
theman53
on 13 Dec, 2009 11:15
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I don't think you would touch it with 600grit. I think they are made of inconel = harder than a woodpeckers lips. You may but it would probably be worth taking to a machine shop.
I thought that it was supposed to take the precups out to machine. Since the cups are harder than the aluminum they machine differently with the same feed and pressure from the mill/cnc? Also I thought a little sticking up was supposed to help hold them in as it has different expansion rate than Al. I am not a machinist, but I read here alot...that and I have stayed at a holiday inn
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#4
by
Rabbit on Roids
on 13 Dec, 2009 11:36
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they are supposed to take the pre cups out, mill the head, then fly cut the pre chamber hole then they put them all back together once the head is milled flat. yes, inconel is some of the hardest metal known to man im sure. compared to aluminum, it takes much longer to mill the same amount off the pre cup. so he probably ground it with the cups in. as the side with no cups got ground, the head angled its self because the inconel precups became the high spot, and the grinder couldnt remove the material at the same rate. so, here is a prime example of what can happen when you dont do it right.
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#5
by
b1rdjx88
on 13 Dec, 2009 17:25
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But in this case the precups were removed prior to milling the head. The milling marks confirm this.
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#6
by
smoken u
on 13 Dec, 2009 20:09
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when i used to work at the local vw shop, we would always remove the precups, and machine them seperately from the head, when they are re-installed there should be no protrusion at all. rabbit on roids is entorely correct if you dont remove them the inconel becomes the high spot and screws up the machining on the head. not alot of shops will actually take the time to do them properly, i had the same problem on my 6.5, the guy in the machine shop left the precups in, and machined the heads on a taper. best place ive ever had a cylinder head done is eurohaus in alliston, been there for 40 years, and all he does is work on VW.