February 11, 2012

Crankcase Flowing

It helps a bit to have the crankcases of your two stroke engine reworked. Removing castings burrs and making some radius on sharp edges is the general consensus.
Other than the above, it helps a lot to temporarily install the crankshaft and reed valve in place, as these parts will reveal some more work needed.

It is not rocket science but needs a bit of experience with a few different crankcases so you can have a broader and more complete view of what needs to be done. Key words are "flow" and "direction". These are the most important aspects when reworking a crankcase. Removing rough casting burrs will aid flow, reshaping the paths of flow will aid for better cylinder filling.

While you are there, matching the mating surfaces among crankcase halves and barrels, is a good idea.

Here are a couple pics of pre-post state of a Nsr250 Mc21/28 crankcase that I just finished working on.
First photo is where the LH cylinder bolts on. I may have exaggerated a little bit on opening up the area to the transfer ports though this is what "racelab" stands for... lol

Second photo shows flow to the RH cylinder, notice the casting burrs on the left side of the pic. Both "pools" were filled up with epoxy putty and I then redrilled the oil holes for the bearings. Final finishing were done with #120 grit sandpaper. 


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