Piston reset tool worked

Dear Roddie,

I am not sure that the big end's orientation on the crankpin is critical in the highly precise Cox engines. After all, one can buy a new piston and there is nothing about how it should go. With a wrist pin design, perhaps front/back of rod is important in that the piston has been previously worn in to the cylinder in a particular orientation. I think that I have fumbled on this at least once. I have tried to keep it the same on ball/socket engines just in case it made some difference. On one Cox engine I did notice that there was more wear inside the hole in the con rod at one end than at the other, not really wear, but polish. I read somewhere that if the big end is drifting back on the pin and scratching the backplate, reversing the rod on the crankpin may cure it due to complicated dynamic considerations that can only be resolved by test, not by measuring. Don't remember if this was for Cox engines or for any specific brand. If it were a wrist pin engine, perhaps one should check the backplate after very little running, so as to do the flip before the piston got used to the cylinder in a disadvantageous orientation of the con rod on the crankpin.
WD40 takes off both Laundry Sharpie and BIC White-Out very fast, just while handling parts. Sharpie does stand up to dry re-setting handling if keep renewing it since there is usually a little oil or WD40 around or on fingers.

Dial gauge. This is a machining measurement tool. I happen to have one to go with a Unimat, though I have not yet done enough work on it to justify the equipment. A dial gauge is very helpful, for instance, for setting up a part exactly centred in a 4-jaw chuck. One full rotation of the dial is 1 mm on mine, ie about 40 thou. One can put the dial gauge in a vise, it has a projecting flange designed for this. One can put the piston top on a sheet of wood under the gauge, and let the gauge's probe rest on the bottom of the con rod's big end. Then as one raises and lowers the rod in the socket, the dial gauge tells you how much play. .002 is about 5 divs on the 100 division dial, easily seen. At this tightness the rod would still easily fall if piston held horizontal. It was no more stiff than when play was .040". After lubricating the finished socket the play seemed even less, but still finger pressure would not be needed to move the rod. Maybe this is too sloppy, I don't know, but am reporting what I, an ignoramus, noticed, and assumed was satisfactory because the instructions say that .001 to .003 is acceptable. If you don't happen to have a dial gauge, maybe you could do something similar with a vernier caliper or micrometer, but the gauge is so sensitive and the other methods might require 3 hands or someone to help. Otherwise I don't know. A head gasket is .005. If you could somehow gently hold the piston against a stop, maybe somehow strapped with an elastic band, with the rod at max play extension just touching a piece of glass. Then one could see if the gasket would fit under when one pushed the rod up so that the ball was as far into the socket as it would go. Household aluminum foil is about .0007". Two thicknesses .014. Hilroy recycled notebook paper .0028. Bond paper .0038.

Now I am starting to wonder whether I am mixed up and there was 40 divs play (~.016") on the dial gauge, not 40 thou., which would be a full revolution of the needle, that I don't seem to remember. In any event the final result was for sure 5 divs, ~.002" and initially it certainly felt very sloppy.

There is also a special Sharpie that one can use to label CD's that won't bleed through and affect the data side.

Roddie's comment made me realize that I might seem to have been promoting my procedure as an advanced technique, whereas it was the cautious approach of a first time user of the Cox Intl. reset tool and piston holder. If he can keep the piston in the holder and continuously monitor his progress by feel, that is much faster and he probably keeps better track of how he has rotated the tool in the piston while tapping. When I come back from having made a dial gauge measurement and replace the piston in the holder, I have forgotten how the tool was aligned in rotation with respect to the piston for the last tap. I was rotating the holder and piston together under the tool because I was too butterfingers to reliably rotate the piston in the holder. The masking tape helped me to do the rotations my way since the piston would not slip and rotate relative to the holder and I could keep better track of each fractional rotation between taps. I wasn't very good at gluing strain gauges either, low manual dexterity.

There are probably some who can judge the amount of play present quite accurately by touch, maybe even estimate what a measurement would be. Thinking of factories where someone had to try out many piston/cylinder pairs to get matches, something like Roddie's sense of touch must have been needed to get the job done in reasonable time.

I noticed that one can lean the tool a bit and wondered whether it was a good idea to vary this as well as rotate, or to try for vertical every time. The photo in the instructions shows the tool leaning away from the rod. Some taps felt better than others. Maybe they are better for the ball/socket. Holding the cup firmly on to the centre of the vise anvil seemed to give better taps, as well as firmly holding the striker, which requires learning a skill of doing that deftly with just one hand available, or at least taking one's time to set up properly for the next tap.