Let’s see what an exhaust opening of
88 degrees after top dead center (184
degrees exhaust duration) and a transfer
opening of 118 degrees after top dead
center (124 degrees transfer duration)
looks like. The stock cylinder is a 34 mm
bore cylinder and the modified cylinder
is a 36 mm bore. From the 29 mm stroke
tables below we set the exhaust slug to
16.3 mm (.642”) (16.1 x .2 mm squish
clearance) and the transfer slug to 23.2
mm (.913”) (23.0 + .2 mm squish
clearance). The stock cylinder has a
smaller diameter set of “slugs” set for
similar timing, but different dimensions,
from the 34 mm bore tables.
PROPWASH
6
April 2016
As was discussed in the last article,
the intake side of the piston skirt needs
to be trimmed between .040” (1 mm)
and .060” (1.5 mm) to increase the
intake timing. The modified cylinder in
this example was done by Brian Buaas
and run in my gas scale boat. In this
cylinder the intake port was lowered
rather than trimming the piston. This can
be done by lowering the bottom of the
intake port using a simple “slug”. It is set
to the intake port table height plus the
piston skirt length and the squish
clearance. The intake port can then be
angled down to the “slug”. The picture
shows a slug set for an intake opening of
80 degrees after top dead center (160
degrees open duration). In this case the
piston has had the crown polished so the
skirt length is 29.2 mm (1.150”). Stock
skirts are close to 30 mm (1.180”). The
slug is set to 43.4 mm (1.709”) (14 +
29.2 mm skirt length + .2 mm squish
clearance)
(Continued from page 5)
Zenoah Porting 101
Notice that a degree is about .25 mm
(.010”) in the exhaust port height and
about .2 mm (.008”) in the transfer port
height. The modified cylinder used as an
example had .2 mm (.008”) squish
clearance, a little tight for many engine
builders. Twice that is more common.
The “slugs” are set to the desired height,
placed in the cylinder and locked in
place with the bolt through the spark
plug hole. The ports then can be cut to
match. Be very careful not to grind
through the cylinder wall in the eyebrow
area of the rear transfer passage or the
top of the exhaust port. The thickness
over the one transfer is about 2.9 mm
(.114”) in the Zenoah 260 cylinder
pictured. I have ground the eyebrows as
deep as 2.5 mm (.100”). The stock
Zenoah exhaust angles down about 5
degrees. The modified port should angle
down between 15 and 20 degrees at the
bore.
Transfer slug
Zenoah wall thickness
Piston skirt measurement
Intake slug setting
Intake Slug at Bottom of Intake Port
Exhaust Setting
Transfer Setting
