Engine Construction Part IV
Crankcase, Crosshead Guide & Bearing Cap
9/14/2009, last updated
Crankcase HC-100: The crankcase castings were
waiting for me when I returned from a 5 week trip in July. The first
patterns were made over 3 years before so it's been a a long wait. I
was so anxious to get at the machining I forgot to take any photos of
the plain castings.
The crankcase design information is on the
Design II page
The pattern maker had to add some draft to the joint between the
two halves so the draft has to be machined off that surface. I shimmed the part of the casting under the clamps
in the photo to compensate for the draft and then took small cuts to
smooth the surfaces that mate with frame bar to make a reference
The amount of draft can be determined by
measuring the width of the flange --- it is wider near the bottom..
|The next step was to flip the casting over, use the
reference surface just created and machine off the draft on the
joint between the castings.
The casting is really hanging out there so fine cuts were the
order of the day.
|The top of the joint flange between the castings was
smoothed with a file to eliminate any high spots. The setup at the
right was then used to machine the surface where the main bearing
cap mates with the crankcase.
|Once the joint between the castings and the surface
that mates with the bearing cap were squared, the surfaces that mate
with the frame bar were squared again. This gets four
reference surfaces on each casting.
|The next step was to make the hole for the
crosshead guide. I used the inexpensive adjustable vise
that came with the mill to hold the casting at a 45 degree angle.
The vice jaws had to be removed to accommodate the casting.
Recall that the crosshead guide is 15/32" off center such that
there is 15/16" between the two crosshead guide center lines.
This is the most critical measurement.
The holes were started with a 1/4" drill and several larger
drills were then used, the largest 1 1/4".
|The next step was to enlarge the
hole using the boring head. Small cuts, about 0.03" at a time
were taken. Once the boring bar is cutting into the flat
vertical surface, it is possible to verify all the measurements as
well as the angle. It is possible to make small adjustments at
that time before finishing the boring.
|The two casting were joined together next. The
screw holes were drilled in one half and that half was then used as
a pattern to drill the holes in the other half.
The casting pair is secured in this position
so that the flange in the top half of the front and the front half
of the top can be be cut off and milled flat. Note that there
are no screws in the front half of the top and the top half of the
The piece is resting on a couple
aluminum blocks which substitute for the frame bars.
|The next step was to make the main bearing caps
(HM104 - Engine Design
1) and attach the caps to the casting pair with 6-32 SHCS.
The screw heads are recessed on the bottom so that the bottom
surface is flat. (These screws will be later replaced with
The setup shown on the right was used to bore the hole for the
main bearing in the crankcase castings and the bearing cap.
|Another view of boring the hole for the bearing.
|The setup on the right was used
to turn the shoulder on the bearing cap. The edges of the
bearing cap will be rounded so that it to resembles a casting.
The nearly finished crankcase
with bearing caps and crankshaft is
|Crosshead Guides (HM 105):
The crosshead guides were tuned from cast iron bar stock using
a procedure similar to that used to make the cylinder bushings.
The photo shows boring the inside of the guide.
The two guides are identical except for the
openings in the sides. Both guides have a slot in the front
side and a round hole on the back side.
|The 1 1/4" diameter hole for
the rear side was drilled first and then then the crosshead guided was rotated and two 1 1/4" holes
for the front side were drilled. The holes were initially
drilled 1/4" and then enlarged with several intermediate
drills before finishing with the 1 1/4" drill. Doing the
drilling in steps minimized the forces on the crosshead guide.
Note that the holes for the lower head screws have been drilled in
the flange. This helps with aligning the crosshead guide for
this drilling operation.
The material between the two holes on the front was cut out using
an abrasive cutoff disk in a small rotary tool.
|The last step on the holes was to mill the sides of
the slot even with the sides of the holes at each end as shown in
the photo. The result of this operation looked great.
A caution, the edges left by this milling
operation were surprisingly
sharp. I smoothed them using a 1/2" sanding drum in the in the
This same setup was used to mill a 3/16" wide slot in the top of
cylinder for the fin. The slot is 15/32" off the crosshead
guide centerline. The orientation is reversed between the left
and right sides such that both fins are on the crankcase centerline.
It is wise to verify the correct location of the slot several
times before making the cut (I
|The crosshead guides fit really well in
the crankcase casting. I decided to attach the crosshead guide
to the crank case with six 6-32 SHCS from the bottom. The
screws are oriented near the edge of the crosshead guide cylinder
and the holes pierce the inside of the cylinder. The holes are
to the outside of the path of the cross head so as not to disrupt
the bearing surface.
The crosshead flange must be aligned at the correct distance from
the crankshaft and at the correct angle so that the cylinders are
positioned properly, A hole was tapped for a 6-32 screw along
the crosshead guide centerline next to the edge of the opening in
the crankcase. The screw visible in the photo is used to
hold the crosshead guide after it has been
|The photo above shows drilling one of the holes for the
attachment screws. A tap drill was used for all six holes and then
the crosshead guide was removed and the clearance drill and screw head
counter bore were used on the crankcase.
The drawing at the right shows the locations of the 6 attachment
screws from the crankcase into the crosshead guide.
The locations aren't critical except that the holes into the
crosshead guide should be be outside the crosshead path.
The drawing also shows two 2-56 screws through the fins, one into
the cylinder and the other into the rectangular oil reserve.
This is to hold all the parts together when they are soldered.
The oil reserve is made from 9/16" rectangular bar left over from
the frame. A 7/16" hole was drilled in the end of the bar to
make a cavity which will be packed with something to retain oil.
A 1/16" hole was drilled from the bottom of the reserve into cylinder
--- the plan is for oil to slowly drip onto the crosshead-crosshead
guide bearing surface.
A hole for a 4-40 SHCS was is also drilled through the crankcase into the fin to
help secure the fin.
|The fin is made from 3/16" plate and cut to fit.
Recall that a slot was milled in the crosshead guide to position the
fin. An adjoining slot was also milled in the top of the
After all the holes were drilled everything was thoroughly
cleaned and assembled the joints between the fin and crosshead
guide, fin and crankcase, and fin and oil reserve were fluxed
and silver soldered. A solder with the ability to fill a
larger void was used since these joints were not very precise.
The photo on the right was taken right after
the soldering operation.
|After the soldering was completed, the crankcase
halves were joined and the rocker arm base and rocker arms temporarily
attached. The places where the rocker arms hit the front
of the crankcase were then identified and the interfering material
was removed on the mill. The photo shows this operation.
|The crosshead guide and the crankcase half were
single castings on the prototype. LPS Strong Steel Stick
composite was used to make fillets between the crosshead guide and
crankcase so that the assembly appears like a single casting.
The photos above show the finished crankcase-crosshead
units with the fillets made with the strong steel stick. The
cylinders are also attached so it really resembles the engine at this
point. After it was painted the composite fillets look
like casting fillets.
Making the connecting rod was next --- see
Engine Construction V