A Piston

Always learning, I suppose. Over the past several decades of working on engines, when it came time to install  a new piston I would just order the correct replacement piece and give my local machinist  the piston and cylinder and tell him the clearance that had been recommended to me. When I got it back, I would just set the end gap on the rings and put it all together. It would never occur to me to check the dimension of the bore, or the dimension of the piston or to even check on the clearance with a feeler gauge. I always assumed that the machinist had worked it out. As I have recently found out, this is not always the case. 

After my experience with this cylinder/piston I have changed my ways. In this case, the first challenge was to find a machinist here in La Paz that would do the work. A motorcycle mechanic who I trust told me that no one in La Paz would do a decent job. He recommended a machinist in Los Mochis (which is on the mainland) So, I dispatched the piston and cylinder to the recommended shop with instructions to bore the cylinder to achieve a clearance of .040-.045mm. The piston that I gave him was an OEM Yamaha DT forth oversize (67mm...theoretically). The cylinder came back bored to 67.020mm.  I was advised by the machinist to check the clearances and then sand the piston as necessary to get the correct clearance. As it turns out the piston was undersize (by about .025mm) up on the ring lands and oversize (by about .025mm) on the skirt. I am sure that is why he did not just bore the cylinder to 67.040mm. He knew that would not work.

So I spent a couple of hours patiently sanding and measuring the skirt of the piston to achieve the recommended clearance. It looks like the piston will have a bit over .050mm clearance at the rings, and the skirt will be right at .040mm.

I am still waiting for some parts and tools to arrive to put the engine back together. Meanwhile, I expect to get some more nuts and bolts delivered today which will allow me to do some work on the Bultaco...Oh joy!