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Originally posted by TechNoir
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Hello to all my friends in the AMCA. It has been over a year since I did any updates on this thread (or any updates on the bike) but I have now unlocked the impasse (I hope).
First a recap so folks dont have to go back through the entire thread to bring themselves up to speed and remind themselves of how we got here. Bear with me while I do the recap, the progress update will follow.
I bought this bike in pieces in late 2015 and managed to put it together with the help of the folks on here and CAIMAG. It had been disassembled in 1988 after having stood in a shed since 1936. The engine had been rebuilt in 1989 before I got it but it turned out that the cylinder bores were in need of attention but hadn't received any so I needed to address this.
I was advised to go to someone with experience of blind cylinders and found a guy who I was told was experienced with these old bikes. He had the cylinders bored out until the pitting was just gone and after some debate advised me to stick with cast iron pistons similar to the original ones. I had new, oversize pistons cast and the guy machined them for me. However it turned out that he didn't machine the gudgeon pin holes (wrist pin holes) in line with each other. The pin would go into one side of the piston but not through the other side. Needless to say I was not happy.
I pondered my options and things stalled for a bit and then I got some more castings made. Whilst my preference would have been to machine them myself I didn't have a mill so this time I gave them to a company that make Steam Engine parts to machine which they did. At this point things stalled again due to a desire to move house. This first entailed doing a million DIY jobs that curtailed any screwing around with old bikes. In any free time I did get I did a total rebuild from the ground up a new (to me) lathe. I had another reason for this which was the need to get rid of my old lathe so it would be one less lump of old iron to shift if we ever do move home. I also made a start on sorting out an old horizontal mill that I had acquired.
By the end of last year I had put most of the other stuff in order so my attention now turned back to the 20F. First the pistons and I noted that I had asked for a tight clearance between the rings and lands because I wasn't certain, when I asked for them to be machined, of the best figure so I wanted to leave room for adjustment. Since then I had decided on the correct figure and the adjustment is a simple job on a lathe. When I went to chuck the pistons to add the required clearance for the rings I noted that the crown sounded thin when I tapped it to seat it against the chuck. It turned out the crowns were only 0.030" thick (at most) compared to the original pistons which were 0.130". Back when I had them machined I had specified the key dimensions such as diameter and clearance for the rings but had just asked for the other features to be "similar to the original pistons" and provided one as a sample. I hadn't checked the crown thickness when I collected them.
Gutted!
In late January I commissioned some more castings. While they were being done I set to starting another bike project as the new castings were going to be 8 to 10 weeks away. They were ready at the end of March but then sat on a shelf for about 4 weeks as I was concentrating on the other project I mentioned. However, when that got to a suitable point I paused it and turned my attention back to the 20F.
Early May 2019.
Something that I have always known is that if you want something doing properly you need to do it yourself and this sorry tale proves the point. I don't have a fully functional mill (which was always the stumbling block) but I do now have enough functionality to sort out these pistons.
So I first chucked the casting in a 4 jaw taking time to get the best alignment. Being a casting, all surfaces are rough so it a matter of finding the best average. I did try to get the two pin bosses parallel to the cross slide so that when bored the pin would be parallel to the crown and through the centre of each boss.
Once the main dimensions were cut I left the crown thicker then needed (about 0.200") so that once all the holes were cut I would be able to match the weight of the pistons to the weight of the previous pistons to maintain balance.
I made what I will call "U" blocks to clamp the pistons in a vice to bore the holes.
One thing that I had thought about was how to align the the pin bosses along the axis of the mill. I had thought long and hard about this issue and made a tall thin vee block that fitted snugly inside the skirt. This transferred the alignment of the bosses to above the piston skirt. It was then a simple job to indicate the block.
It was then a simple matter of boring and reaming the holes.
This was another issue with the first guys work. The holes were off centre on the same side of the piston so even if aligned it would have natuarally imparted a turning moment on it. Here is a picture of the off centre screw-up
I then transferred the piston to a rotary table to do the other holes.
Back in the lathe I cut the ring grooves. Because I was holding an open cylinder in a chuck it was impossible to get the TIR better than 0.0003" to 0.0004" but that is good enough.
The last job was to take the crown down to thickness whilst also keeping an eye on weight to maintain balance.
Pistons now sorted out it was a simple matter to assemble the top end.
Stay tuned, more to follow.
John
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