GDB4 – Inline 4 Cylinder, 4 stroke IC engine by George Britnell
I am kicking off another engine build and as my last was a twin, I thought I might move up to a 4 cylinder. George Britnell has always been a hero of mine and has made available plans for an inline 4 cylinder that meets my criteria as a next build. George is a true craftsman and does incredible work. I am very fortunate that he has taken the time to create plans of his masterpieces so that I can continue to build my skills as a hobby machinist and fabricate one of his engines. I know I will not be able to meet his exacting standard, but I hope to end my quest with a running engine.
First, several photos of George’s engine to get motivated.
This will be the smallest engine I have ever built, so I will be challenged by the smaller scale. My last engine used 6-32 screws to secure the oil pan to the crank case, this one uses 0-80. Yipes.
I looked over the plans of the crankcase, as good a place to start as any, and established that the most critical aspects of the design is the relationship between the crankshaft and the camshaft. They need to be perfectly co-linear and the proper distance apart since a set of timing gears reside between them. I have decided to let the camshaft be my first major datum.
Working from CAD models helps me visualize the engine and allows me to develop my machining strategy.
Lets make some chips!
My plan is to square up a block of aluminum for the crankcase oversize by .050″ on all sides, then drill and reamed the .3125 hole for the camshaft.
First I squared up the block of aluminum on the mill, then moved to the lathe to drill the camshaft hole. I did not have a drill long enough, so I drilled most of the way through with a standard drill bit, but then finished the job with a custom D-bit made from .3125 drill rod. I had to cut a .0625 groove down the side of the D-bit as air would get trapped inside and I couldn’t push the bit in to cut. If you look closely at the D-bit you can see the groove.
Once I punched all the way through with the D-bit, I followed up with a reamer. Fortunately my reamer was long enough to make it all the way through.
Below is a picture of the crankcase with a .3125 drill rod installed in place of the camshaft. This is now the my master datum and I will create other features from this.
I squared up the bottom and the left side of the crank case to the camshaft on the mill. These two sides are now square to the camshaft and can be used a datum planes for future milling operations. Next I plan to machine the crankshaft referenced to the camshaft.
I took a strip of aluminum with the same cross section as the crankshaft main bearing caps and using a 1/4″ ball end mill, mill a slot down the middle .125″ deep. My intention here is to create a matching groove down the bottom of the crank case where the crankshaft will be centered. This will allow me to place the crankshaft exactly where I want it with respect to the camshaft.
Then I cut a matching groove down the crankcase perfectly positioned with respect to the cam shaft.
Finally I clamp the long bearing cap strip to the crankshaft using a 1/4″ drill rod to align it to the crankcase and drill the crankshaft bearing cap mounting holes.
Once I screw the bearing cap strip down I will ream the crankshaft hole to size.
Hope this all makes sense.