It was dirty work today, lapping.
I will just explain something that I haven't shown.
When the unlapped piston was pushed thru the bore, I noticed that about 2/3rds the way up from the end (opposite to the flange end of the cylinder) I noticed there was a slight binding up between the two parts. If I had had not been so lazy and used my bore gauge from the start, rather than using an internal mic at each end, I would have picked this up and taken a few more non feed cuts when I was boring the cylinder. As it was, I used my bore hone to straighten things out. There was most probably only about a tenth to be removed, and it only took a couple of minutes to hone it out.
All operations were done on the slowest setting my lathe will go, 65 RPM.
After that, it was just a matter of starting out with a bit of diamond paste gently spread onto the surface of the piston. I wouldn't recommend using diamond if it was a non ferrous bore or piston, as it tends to permanently embed itself into the soft metal surface, so if it isn't all completely removed, it will carry on wearing away the bore as the engine is running. After a good wash down with thinners, I used a commercial chrome polish, cleaned down again, then finished off with rouge paste.
These operations took just over an hour, gently rotating the sleeve up and down the over the piston in a sort of figure of eight movement until things start slackening off and getting easier with each different compound. I recharged each compound about 3 or 4 times, when it started to get filthy loaded with cast iron sludge. You have to make sure that the whole inner surface of the bore is lapped to the outer surface of the piston.
In the beginning, the sleeve had to be held fairly tight to stop it rotating with the piston, but as things went on, it got easier to hold. The final lap was actually done with just one finger pressure moving the sleeve up and down.
I tried to get a flash shot showing the semi chrome finish on both the bore and piston, but failed miserably. Because I am scared of boogie men, I won't go out to the shop in the dark to take another picture.
Anyway, another take my word for it, they were s-m-o-o-t-h as silk.
In fact, just by gently sucking and blowing on the flanged end of the cylinder, the piston went up and down in the bore.
I have taken a shot of the bits and bobs that I used for the exercise, and I will try to explain how the hone works.
On the left is my commercial cylinder hone, designed for things like brake cylinders and small i/c bores. I paid less than 20 squid for this off the net, from a motor factors. If you buy one from a model engineering supplier, they will cost you double that, for exactly the same thing (or even less contents).
This one will work with bores from around 3/4" up to about 3.5". You can buy extra stones, both harder and softer grades in different lengths, to cater for different materials and depth of bore. I have found the ones supplied have done a great job on all the different bores I have honed.
The way they work is that when the stones are fed into the bore, they lie flat against the inner surface, and by gently rotating in the bore, and moving it in and out, as I did my lapping, it will gradually make the bore perfectly parallel and round. In use, you keep it well lubricated, I use my general purpose hydraulic oil that I use on my machine for the gearboxes and local lubrication, and by adjusting a thumb screw at the back of the spring, you can make the stones cut harder or softer. The closer you get to perfect, the softer you have the setting.
These hones are not really for getting a very smooth bore, but they will do if adjusted correctly, as I said, they are used for getting things straightened out. You will find that after use, if you have followed the movement regime I mentioned earlier, there are minute scratches in the bore face. As the engine is running in, oil will get trapped in these tiny scratches and help prevent the piston seizing in the bore as the two bed in together over time. Eventually the scratches will be worn away and you will end up with highly polished bore and piston.
From certain model engineer suppliers, you can buy casting kits that when made up will do the same thing for the outside of the pistons, they are called external hones.
The lapping I have done does away with this engine running in period, and the engine should be able to get up to full speed from the start.
On the right hand side of the pic are the three compounds I used for lapping. The green stuff at the back is my own diamond lapping paste that I made up a few years ago, and it is used to get most of the sleeve to piston undulations removed. Next one down is a commercial chrome polish. This is for getting things started to be really smooth, then followed by the jewellers rouge which imparts a nice polish to both surfaces.
People use all sorts of different compounds for lapping, these are just the basic ones I use. If I was doing a non ferrous bore or piston, I would actually use an aluminium oxide grinding paste (the stuff used for grinding in valves on a car) instead of the diamond paste. The ali oxide actually breaks down into a harmless sludge during the lapping process, so is easily washed off, and causing no further wear to the parts.
So that is the cylinder bore and piston finished, except for drilling a few mounting holes. That means I can get back onto making things.
So you have to imagine what I look like now, hands filthy, embedded CI dust in the pores, the front of my white t-shirt covered in the same stuff, and a ring of dirty oil around my lips, from sucking the piston in and out, and of course, once John S finds out, he will be there with all the Al Jolson quips, as usual.
Bogs
