Inline Twin Engine Ep. 9

The cylinder head is one of the more complex parts, requiring a number of different operations. As usual, I’ll let the pictures do most of the talking as I’m going trough each step.

Started with a facing cut:

Inline Twin 124 (800x450)

The outside contour is milled to full depth:

Inline Twin 125 (800x450)

Spotting and drilling:

Inline Twin 126 (800x450)Inline Twin 127 (800x450)

Milling the combustion chamber:

Inline Twin 128 (800x450)

Milling the recess for the O-ring, sealing the coolant passage from the block to the head:

Inline Twin 129 (800x450)

Drilling and reaming for the valve guides:

Inline Twin 130 (800x450)Inline Twin 131 (800x450)

I’ll be using Rimfire spark plugs. Since even the long reach plug is still too short, the combustion chamber is getting an “extension” with a ball end mill:

Inline Twin 132 (800x450)

First side of the cylinder head complete:

Inline Twin 133 (800x450)

The cylinder head is turned over and faced to final thickness:

Inline Twin 134 (800x450)Inline Twin 135 (800x450)

The coolant passage is milled:

Inline Twin 136 (800x450)

The second side complete:

Inline Twin 137 (800x450)

The head is then set up on its side to drill the holes for mounting the exhaust manifold and carburetor mounting. The location for the inlet and exhaust ports are only spot drilled. They are angled and will be drilled later.

Inline Twin 138 (800x450)Inline Twin 139 (800x450)

To finish up the cylinder head, I needed the valve guides as the inlet and exhaust passages are drilled trough the guides. The material I used for the guides is O-1 drill rod.

Inline Twin 140 (800x450)

Valve guides installed in the cylinder head (press fit):

Inline Twin 141 (800x450)Inline Twin 142 (800x450)

The cylinder head is clamped in the vise at the correct angle to drill the exhaust port.

Inline Twin 143 (800x450)

The location of the hole is picked up with a center in the spot drilled location marked earlier.

Inline Twin 144 (800x450)

Using an end mill, a flat is milled to be able to center drill and drill the hole on the angled surface.

Inline Twin 145 (800x450)

Inline Twin 146 (800x450)

Inline Twin 147 (800x450)

Inline Twin 148 (800x450)

The same process was then repeated three more times for the other ports:

Inline Twin 149 (800x450)

Inline Twin 150 (800x450)

Inline Twin 151 (800x450)

To finish off the cylinder head, it needed a cover to seal the coolant passage.

Inline Twin 152 (800x450)

The finished head assembly:

Inline Twin 153 (800x450)


Thank you for visiting my shop.

If you have any questions about the engine or any of the other projects, please leave a comment below, or email at yogisworkshop(at)

For other projects from the shop, please check out the Projects Page in the top menu.



  1. Looking good Yogi!

    I made the mistake of showing my wife your work. She’s not very impressed with my threaded barrel tenons anymore.

    Hope all is well on your end.



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