Thursday, July 18, 2013

The Pistons and Cylinders

Take note where all of the guide pins are (pic3).  The head broke loose from the cylinders really easily.  At this point there were lots of things to notice.  Valve seats; corrosion, wobbliness, color.  Cam chain adjuster was locked, frozen into the head, so the chain was slack and scraped off a track into the metal.  The cylinder was much harder to break free from the lower crankcase.  More patience and gentle knocks with a rubber hammer and a piece of wood to spread the impact onto several cooling fins, instead of just striking the cooling fins and risk cracking them off.  The cylinder walls were in pretty good shape, but a little glazed.  You can still see a hint of the crosshatch, so that's good.  The pistons were in pretty good shape too, some minor skirt wear and carbon deposits on and around the head.  Rings were in pretty good shape, but when I removed one to check the end gap, it snapped on me. So I removed them all and the gap was within clearance specs.  When removing pistons, take care to not drop the clips that hold the piston pin in.  With all of this in mind, it seemed that the valves were most likely causing the majority of compression loss.  I had a reading of 95 in one and 110 in the other.  It's supposed to be at least 150 psi.  So I took the head/valves to get a machinist to properly grind a bit so the valves seat correctly.  He said my valve guides were ok (causing the wobbliness) so i left those alone.  The machinist also honed my cylinders and drilled out the cam chain tensioner bolt that was seized up in the front of the cylinder.  I guess this bolt was a design flaw in these 70's honda twins.  Once drilled out, I found a new bolt and made sure to put on some anti-seize.  And since I broke one of the rings, I replaced them, the pistons, pins and clips.  Finding the std size rings was a huge issue, it took me about 3 months to find them. I ended up finding them on ebay from Israel.

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