- If you've calculated the proper mean squish velocity
for your engine (you are using the software programs
indicated on my web page for doing this aren't you - the
software is free) you may be able to achieve the desired
head or top of the barrel milling/decking by careful work
using a granite surface plate, some 3/8 thick glass, some
various grades of wet/dry sand paper, some 2" wide
masking tape and some valve grinding compound. This is
the poor mans milling machine - it is more time consuming
that having it done at a shop but once you buy the tools
you could do a bunch of them if you wanted. Additionally,
lapping heads, barrels, reed cages and just about every
engine mating surface will almost insure that you'll
never suffer a leak due to a warped surface. Simply put
when things are VERY flat there is no place for a leak to
occur. You'll soon find yourself lapping everything in
your garage.
-
- A small 2" to 3" thick by 14" square toolroom grade
granite surface plate (or the largest size you can
afford) can be had at some of the catalog tool sales
depots for not a lot of cash. I have had a great one for
many years. The trick to using them and making them last
is to never grind against the granite. That's what the
wet/dry paper and 3/8" glass is for.
-
- If I knew I had to remove .005" or .010" from the
surface of a head I would first measure its thickness at
the bolt hold down bosses and record it. Then I'd take
out my surface plate and put it on my bench to attach a
piece of about 240 grit paper to it using 2" wide masking
tape. One inch on the paper and one inch on the granite.
Spray it liberally with WD or similar. Carefully,
smoothly and slowly - use rhythm - move the head across
the paper taking care to not damage the masking tape -
since it will make it release. The idea is to move the
piece in circles and rotate the piece to be sure the
material is coming off evenly. Avoid rocking the piece so
it doesn't get rounded off. Let the paper do the cutting
- there is little to gain from pressing hard against the
plate, in fact this could make the proces take longer. If
it's not going fast enough you can use a rougher paper if
there is sufficient material to be removed - then go back
to the smoother paper.
-
- Remove it and check its thickness periodically - say
every 100 rotations to get an idea of how fast the
material is coming off. When the dimension comes to
within .001" of the destination, switch to 400 grit and
"take it home." Many parts will need no further
smoothness than what this provides. Stop the procedure
when the dimension has been reached and there are no
surface scratches. Aluminum cuts faster that
aluminum/iron - that is to say if the job requires the
barrel be decked a few thousanths, it is wise to start
with rougher paper - say 120 and work it there until the
dimension is within a couple of thousandths. It is more
difficult to work with rougher paper since it wants to
bite more at first. Pressing evenly and rotating the
piece back and forth with help. Then switch to 240 for a
thousanth, then to 400 for the rest.
-
- Some parts can be further enhanced by lapping against
glass attached to the surface plate. In this case it is
necessary to use valve grinding compound as the cutting
medium. Many companies sell their compounds in a small
set of "course" and "fine." Parts like reed cages usually
require the fine compound only. The glass will wear out
after a while so it's a good idea to get several pieces
made at once - I also use 1/4" thick glass with good
results when I can't get the thicker stuff when I want
it.
-
- Any part that should be flat can be made really flat
using this technique. It is an old one and is still
performed on every component of every professional race
bike on the track. It is a proven problem reducer and
adds the benifit of becoming "one with" your engines
pieces. I always lap heads, barrels, the reed cage
surface on the barrel, the surfaces of the reed cages,
and I'll radius the edges of the reeds themselves against
very fine paper and then finish off their corners with
the paper in my hand. If an engine is disassembled
further I'll lap with compound the lower deck of the
engine. It is necessary to remove head or cylinder studs
before performing the procedures. Double jam nutting them
will work if you don't have a stud puller.
-
- Rick
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