Date: Tue, 31 Oct 2006 17:25:53 -0800
Reply-To: Jake de Villiers <crescentbeachguitar@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Jake de Villiers <crescentbeachguitar@GMAIL.COM>
Subject: Re: AMC heads (part 2)
In-Reply-To: <4547EA74.4000705@charter.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I don't know about everybody, but Shane and Eddie at The Bug Shop in BC hand
lap every valve as you described.
If the exhaust valves don't have enough contact with the seat they can't
transfer the heat they have picked up from the 1200 degree exhaust gases and
will overheat and distort pretty quickly. As a young racer I was taught to
make a 1/8 line on exhaust seats for heat transfer and a 1/16 inch line on
intakes for a higher pressure seal.
Still makes sense to me.
On 10/31/06, John Rodgers <inua@charter.net> wrote:
>
> Ben,
>
> Interesting stuff on your website.
>
> Before I get into this, let me say I AM NOT by any means an auto
> mechanic, not even close. So what I have to say may not even be quite
> right or even have application to an automotive engine. Guys (and gals)
> who work them all the time know what is what.
>
> I come from that aircraft mechanic perspective. I was taught on piston
> aircraft engines the critical nature of valve specs to the engine, it's
> operation and longevity. In that training we were taught that the intake
> valve was cold, the exhaust valve was hot, and that told the tale.
> Exhaust valves suffered a lot from the heat of combustion, because they
> didn't have the benefit of the cooling of the fuel-air charge coming in
> past it for combustion as with the intake valve, but the exhaust valve
> got the full brunt of the heat of the burnt fuel going out of the
> cylinders. Thus it was that often exhaust valves stems were sodium
> filled to help transfer some of the heat up the stem to the oil
> circulating around the cylinder top. The sodium would get hot, liquify,
> and slop back and forth in the valve stem first toward the valve face,
> then to stem end, each time transporting heat from the face to the stem
> end protruding into the circulating oil system.
> Crucial to the life of a valve was the alloy. It has to be tough, hard,
> and capable of transferring the heat. Also, such was the case for the
> valve seats to take the hammering they get under extreme conditions.
>
> On the aircraft engines, it was also crucial that the valve face and
> seat be ground to mate properly. "Properly" for an aircraft engine was
> a valve face ground with three different facet angles on it, and if I
> recall, the valve seat was similar. The real trick was to hand "lap" the
> seating surface of the valve face to the seat. Very fine grinding
> compound in a moderate lubricant was applied lightly to the valve face,
> and then with the head at the right angle so the valve stem would be
> vertical to the floor, a tool was used to rotate the valve back and
> fourth 90-180 degrees. The valve was then lifted, rotated 90 degrees,
> then set back in place, and oscillated back and fourth again. This was
> continued until there was a continuous, non-metallic-glistening lapped
> circle of the ground surface on the valve face and on the valve seat.
> The "lap-line" was specified to be approx 1/8 inch wide, and located in
> the center of the center third of the ground valve face. . The final
> test of the proper seat was to remove the valve, wash of all grit,
> re-insert the valve in to the valve guide - with out benefit of springs
> to seat the valve, The weight of gravity provided the force of the seal
> between valve and valve seat. The cylinder was then filled with
> gasoline. If the gas did not leak, then the work passed the test. If the
> gas leaked, woe be unto us - we got our fannies kicked., an "F" for our
> grade, plus the luxury of doing a re-grind to get it right.
>
> I don't know if auto shops go to this much trouble for car engines. For
> an airplane, you had to have it right. It's a bit difficult to park on a
> cloud when there is a malfunction. But I think if auto work was done
> with such attention to detail, a lot of our V-gon problems would
> disappear. To me, a proper valve job would include hand-lapped valve
> seats. It would involve also a check to ensure no variation in the
> length of the valves - even new valves.
>
> Ben, a question(1)
>
> Can one take those valve stem retainer halves and simply grind them
> down on a surface plate with 400-600 grit wet/dry paper and take off
> enough to get them to fit with the tiny gap-o-sis that is needed??
>
> Question (2)
> Do you know what metal is used for the valve guides in todays
> engines.Aircraft engines use to use bronze guides. That may have changed
> by now. Curious to know about modern auto engines.
>
> Regards,
>
> John Rodgers
> 88 GL Driver
> Chelsea, AL
>
> Benny boy wrote:
> > So lately i have worked on some engines with somewhat old AMC heads...
> so of
> > course, i have seen other scary stuff... after some e-mail exchange with
> Bob
> > Donald about those problems... i have uploaded my page to show you how
> bad
> > AMC valve are... or can be, because it's not always the case, that is
> the
> > real main problem!
> >
> > http://www.benplace.com/amc.htm
> >
> > Cheers, Benny boy (working like a dog)
> >
> >
> >
>
--
Jake
1984 Vanagon GL
1986 Westy Weekender "Dixie"
www.crescentbeachguitar.com
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