The cylinder head is one of the more complex parts due to the fact that there are 7 different surfaces that need machining, five on the outside and two internal, and they must all align as well as possible. The tools that are used include a 1/4″ end mill, 1/8″ end mill, 1/4″ ball end mill, 3/32″ ball end mill, spot drill and drills. I did some significant redesign of the head making the following changes:
- Used larger, yet more cost effective, CM-6 sparkplugs. I had to juggle their position to clear the head mounting screws.
- I like to use complete bronze valve cages which incorporate the valve guide and the valve seat. the original design has the valves seat directly into the aluminum of the cylinder head.
- Moved the water jacket holes to give as much clearance to the cylinder head holes, that is maximize the amount of head gasket material between the holes, the edge and the combustion chamber.
- Rotated the exhaust flanges so the mounting holes do not hit the seam of the top and bottom halves. I don’t want the flange mounting screws putting a separating force on the two halves.
- Adjusted the sparkplug depth and angle to give good access to the combustion chamber, but not interfere with the valve guides.
- Maximize the water jacket volume without compromising wall thickness.
In order to machine the internal passageways for the air/fuel mixture and the exhaust gases, I decided to fabricate the head from two pieces bonded together.
3D Model of the one half of the cylinder head showing internal passageways
My plan evolved as follows:
- square up two blocks of aluminum, .25″ oversize from front to back and from left to right. Exact dimension top to bottom. Total height of the head is .875, bottom half is .475″ thick and the top is .400″ thick.
- Machine internal passages. roughing will be done with a 1/8″ end mill entering from the outside of the work piece leaving .010″ material. All radii are .1875″, so a 1/4″ ball end mill will be used for the final finishing passes. Total machining time is 34 minutes per half.
- install locating pins located in the excess material
- bead blast the internal passages and the mating surfaces, this increases the surface area for the adhesive.
- Bond the top and bottom halves of the head using structural adhesive and a sprinkling of 80 grit glass beads. this insures there is a micro space between the halves and the adhesive does not get squeezed out during clamping. Loctite EA9340 is used as the structural adhesive. It has excellent resistance to chemicals including fuels and coolant and is rated to a very high temperature. The alignment pins are used to insure proper alignment, but the three .375″ internal passageways can also be used to align the two parts by installing matching dowels. Note: Using the glass beads did not work as they were too big and the two parts just slid around on them like ball bearings and the adhesive would have to be much thicker than I wanted. I ended up just bonding clean parts, but was careful to moderate the clamping force so as to not squeeze out the adhesive.
- After the adhesive is cured, square the parts up to proper dimension all around.
The two starting work pieces next to a 3D Printed model of the head
Roughing out the internal passageways with 1/8″ end mill
Internal passage ways after finish machining with 1/4″ ball end mill.
Cured cylinder head work piece, machined to proper size all around.
Machining of the internal water jacket from the top.
The cooling and mounting holes are machined from the bottom as this is the critical mounting surface with the block. I used a 1/4″ and 1/8″ flat end mills to machine the top. I did not worry about the aesthetics as it will be enclosed and the roughness will not have a material effect of the coolant flow. This is in contrast to the fine finish machining of the air/fuel and exhaust passages that need smooth air flow and thus justified the additional machining time.
Machining the underside – combustion chamber, valve guide holes and water jacket holes. I used a 1/4″ roughing end mill, a 1/8″ finishing flat end mill and a 3/32 ball end mill for the sparkplug hole.
Finished machining the bottom with the head mounting holes complete.
The mounting holes were spot drilled and drilled through using peck drilling. this is where the drill bit enters about a diameter of the drill bit then retracts and clears the chips, working its way “peaking” through.
I was very happy with the alignment of the internal passages and the valve guides.
Simple jig to provide the proper angle for the machining of the spark plug holes.
A 3D model was created for the machining of these holes using the top edge, closest edge as datums. I am not sure about the proper use of the word “datums” in this context, data is plural for datum, any way I used these edges as my zero points for machining.