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why you should NEVER skim an aluminum head
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jim beam
Feb 19, 2012, 2:51:18 PM2/19/12
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<http://www.flickr.com/photos/38636024@N00/6904448039>
here we see what happens when you take an aluminum head to a traditional
machine shop for skimming or "cleaning" as part of head gasket work.
in this case, the black gouge marks are shown where i stained a scrap
head that i knew had been skimmed, then used wet and dry paper to cut
off the top layer leaving the gouges in relief.
you will *never* get this head to seal properly. and unfortunately,
this kind of damage is pretty much *inevitable* from any traditional
skimming operation. and what's worse, unless it's really bad and you
know what to look for, you can't see it /unless/ you do what i did.
for the techs that will want to say, "but i get heads skimmed all the
time and never see a problem", i will say "yes, /you/ don't see any
problem because /you/ will never see that car again because the owner is
so pissed at spending all that money only to have it fail again a couple
of months later, they'll either throw the car away or blame you and have
someone else have another go".
anyway, this happens because the cutting tip either picks up grit on the
hole side left over from the head casting operation, or the swarf from
the prior cut folds under and drags. both gouge rather than cut through
the soft aluminum. the only way to re-finish an aluminum head
successfully is by flat bed grinding. very few shops are equipped to do
this.
the best method of dealing with head prep for shop work is to use a thin
bladed scraper like a razor blade [which doesn't gouge] and plenty of
gasket remover. do NOT use abrasives unless on a flat bed, and only if
you know how to clean the thing properly to remove embedded grit. do
NOT use scotchbrite or anything of that nature with spicular alumina in
it. guaranteed to cause premature wear in the whole engine since it's
so hard to remove.
[yes, there all kinds of other condition issues with the head - that's
why it's scrap.]
--
nomina rutrum rutrum
here we see what happens when you take an aluminum head to a traditional
machine shop for skimming or "cleaning" as part of head gasket work.
in this case, the black gouge marks are shown where i stained a scrap
head that i knew had been skimmed, then used wet and dry paper to cut
off the top layer leaving the gouges in relief.
you will *never* get this head to seal properly. and unfortunately,
this kind of damage is pretty much *inevitable* from any traditional
skimming operation. and what's worse, unless it's really bad and you
know what to look for, you can't see it /unless/ you do what i did.
for the techs that will want to say, "but i get heads skimmed all the
time and never see a problem", i will say "yes, /you/ don't see any
problem because /you/ will never see that car again because the owner is
so pissed at spending all that money only to have it fail again a couple
of months later, they'll either throw the car away or blame you and have
someone else have another go".
anyway, this happens because the cutting tip either picks up grit on the
hole side left over from the head casting operation, or the swarf from
the prior cut folds under and drags. both gouge rather than cut through
the soft aluminum. the only way to re-finish an aluminum head
successfully is by flat bed grinding. very few shops are equipped to do
this.
the best method of dealing with head prep for shop work is to use a thin
bladed scraper like a razor blade [which doesn't gouge] and plenty of
gasket remover. do NOT use abrasives unless on a flat bed, and only if
you know how to clean the thing properly to remove embedded grit. do
NOT use scotchbrite or anything of that nature with spicular alumina in
it. guaranteed to cause premature wear in the whole engine since it's
so hard to remove.
[yes, there all kinds of other condition issues with the head - that's
why it's scrap.]
--
nomina rutrum rutrum
AD
Feb 20, 2012, 11:55:52 AM2/20/12
to
hls
Feb 20, 2012, 3:03:25 PM2/20/12
to
"AD" <isq...@gmail.com> wrote in message
news:97cad692-1d66-4a4f...@i2g2000vbv.googlegroups.com...
On Feb 19, 10:51 pm, jim beam <m...@privacy.net> wrote:
> <http://www.flickr.com/photos/38636024@N00/6904448039>
>
> <http://www.flickr.com/photos/38636024@N00/6904448039>
>
> --
So that's your reply to the free porn?
*********
So that's your reply to the free porn?
This was a dumb post to begin with, but considering the source, I guess it
could have been expected.
As far as I know, we in the lower 48 dont use the term "skim" for this sort
of machine work.
Cylinder heads are "machined", "or "milled" or even "trued"
but not "skimmed" around here.
Blocks are normally "decked", or "surfaced" but other words might apply
locally. But not "skimmed".
And it CAN be done and done well, including aluminum alloy heads and blocks.
jim beam
Feb 20, 2012, 3:20:25 PM2/20/12
to
problem with machining sand-cast aluminum like this. the above was done
by a "reputable" and expensive shop used by several dealers.
but your response is typical of people who don't /want/ to know, that
don't know what they're looking at, and who don't take the kind of
trouble i took to get that evidence. they will forever remain in the
dark about this problem. particularly if all their mistakes end up in
the junkyard.
--
nomina rutrum rutrum
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