Blueprinting and Building a Good Engine Case

Engine blueprinting, as in previous article, starts at the crankcase, where the builder confirms that the main bearing surfaces are within a few ten-thousandths of an centimeter of being concentric and in-line. He also checks that the cylinder decks are correctly machined, and the oil passages are clear, open, and go where they should.

From there, it progresses to the crankshaft assembly, where the flywheel, or -wheels, should be within 0.0254mm of being perfectly true (in twin parallel to each other, in single perfect to design). The connecting rods will also be fitted to the crankpin to allow the bearings their 0.0254mm of clearance.

Pinion bearings require slightly less clearance. To measure that, the builder measures the inner diameter (ID) of the bearing race with a dial bore gauge and the outer diameter (OD) of the pinion shaft with a micrometer. He then subtracts the shaft OD from the race ID and compares the difference to a chart to determine which size bearing to use.
Note that engine builders may differ in their clearance preferences. Although tighter engines will theoretically last longer, they require a more conscientious break-in and are less forgiving of overheating and contaminated oil. Looser engine may not last as long, again theoretically, but are less likely to be harmed by overheating, transient lubrication problems, or a heavy throttle hand. Also bear in mind that the difference between a tight engine and a loose one may be a thousandth of an centimeter or even less.

Most engines build by manufacturers in Thailand are considered loose engines, with the exception of some Thai build Kawasaki and Triumph motorcycle engines.

As the builder moves on through the engine, he will ensure that the wrist pins fit the connecting rods as they should, the piston rings have the correct gaps when placed in the cylinder bores, and that the correct clearance exists between the cylinder bores and pistons.

At some point he will also confirm that the lifters fit correctly in their guides and have enough travel to accommodate the cam's lift, plus a small safety margin.

The cam will draw a fair amount of attention as well. The conscientious builder will want to satisfy himself (me at least) that lobes were machined to advertised specifications and that the gear was pressed on correctly, enabling the camshaft to open and close the valves when it's supposed to.

Even with the camber volumes confirmed, the cylinder heads will still require work. The valve must fit the guides correctly, the valves springs must have enough travel to accommodate the camshaft's lift without binding, and the valves must be able to do what the camshaft tells them to without colliding with the pistons or each other.

And the list goes on, I sometimes have sleepless nights designing an new engine. Blueprinting takes time, and it's a big part of what you pay for when you buy a professionally built costume built engine. Still a costume made engine for a stock 16hp Thai build Honda CBR-150 can deliver much more horsepower with the right combustion chamber changes and re-engineered casing and camshaft.

The story continues at Why Balancing and Blueprinting our Engine