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Poppet Valve Engine - Crankcase

|Crank | Crankcase | Cylinder | Cylinder Head | Cylinder Liner | Flywheel | Spark Plug | Timing Gears

Nigel Taylor

The crankcase was machined from a cube of aluminium, quite a rough cube that had been laying around my shed for some time.

The first job was to machine the hole for the crank and then rotate the part and machine the hole and the outer for the cylinder location.

This shows the crankcase set up on the rotary table that is mounted to an adapter plate with a bolt through the centre.

The adapter plate is then bolted to the rotary table using T-nuts at the 3 points around the table.

This image clearly shows the bolt pointing upwards through the crankcase.

An 8mm bolt was fixed into the adapter plate and points up through the crankcase, a number of washers were then used and the part locked down with a nut. A dial gauge was then used to centre the part, rotating it back and forth whilst nudging it into location. The nut was then tightened and a second locking nut added.


This milling process was done in two stages, this shows the first stage, the tool will then be lowered for the second stage.

You need to be very careful with cutting direction and be prepared to use the table locking screw as the table has a tendency to rotate with the cutter forces and the vibration.


Sometimes you get complacent.

I was in a hurry and thought for some reason that I could make a hardwood adapter.

What I just decided to ignore were the forces involved in end milling with a 12mm diameter bit and cutting speeds of around 1500rpm.

The cutter bit and jammed, nearly damaging the part beyond repair and nearly doing a lot more damage besides that.


This shows a close up of the adapter plate that was made from a piece of 8mm aluminium.

The bolts were recessed so that the cutter would clear them when machining the second stage that would complete the round body of the crankcase.

An 8mm bolt was fixed into the adapter plate and points up through the crankcase, a number of washers were then used and the part locked down with a nut. A dial gauge was then used to centre the part, rotating it back and forth whilst nudging it into location. The nut was then tightened and a second locking nut added.


A locking nut is important as the table was not large enough to get a fixing it that would stop the crankcase rotating with the milling forces. So the nut was tightened as far as possible and a lock-nut added to remove the possibility of it vibrating loose.

The crankcase after the initial machining.


Crankcase, cylinder and liner.


This is the part that supports the front end of the crank and has the timing gear attached. Not a very interesting part at first.


The nose shown in the background to the main crankcase.


Attached you can see that the nose stands proud of the case.


The flange around the crankcase nose is 10mm larger diameter than the main crankcase. This material will get machined away eventually to leave 4 lugs to attach through-bolts from the rear crankcase cover. The intention is that these bolts will be visible along with the bolts fixing the cylinder head and cylinder head to the crankcase.


The next challenge with the crankcase was to machine the clearance for the conrod. Setting up a part at a given angle on a mill can be quite a pain and this is where the digital inclinometer comes into it′s own.

Place the gauge on the table and zero the scale.


Next place the gauge on the part and rotate until you get to 20°.


Using an end mill I machined the recesses for the connecting rod. Not so easy to photograph, so I have taken the images from a number of angles.



You can see that the clearance has been machined just for the conrod and so sits central in the opening where the cylinder bolts.



A top hat closure for the rear of the crankcase.


I made the flange thicker than that for the front cover as this may be used to attach the engine to some form of mount.

I may change my mind yet as I quite like the idea of proper mounting brackets that are bolted to the side of the crankcase.

As you can see this was designed to come close to the big end and reduce the crankcase volume and at the same time stop the conrod floating off.


Index Page for the Poppet Valve Engine

If you have made a similar engine and would like to share your experience we would love to hear from you - ed.