Pistons problemo!

[Pylonracer]

21mm piston, completely machined on a Mazak 530, the first photo is prior to turn finishing the OD, at this point the OD is 0.25mm oversized.

Second photo shows the piston being machined on the OD with a PCD tool.

But these are round !!

For Graham (the OP), this is all done within a few miles of Saunders.

I would get the Honda piston and hold it in the orientation of this piston and go around the OD with a decent tool to create that tapered oval, I think there is a machine shop at the Totton end of the road that runs past Saunders that could do this. We have no time to take on sub con work like this, too busy doing the stuff in the photos.

 

[graham-xrf]

21mm piston, completely machined on a Mazak 530, the first photo is prior to turn finishing the OD, at this point the OD is 0.25mm oversized.

Second photo shows the piston being machined on the OD with a PCD tool.

But these are round !!

For Graham (the OP), this is all done within a few miles of Saunders.

I would get the Honda piston and hold it in the orientation of this piston and go around the OD with a decent tool to create that tapered oval, I think there is a machine shop at the Totton end of the road that runs past Saunders that could do this. We have no time to take on sub con work like this, too busy doing the stuff in the photos.

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That is a beautiful thing, and so tiny. I have just realised the association with pylon racing. I guess you would not be into the full-sized version like the Red Bull Air Race. I am into sailplane gliding. Think about 3/4 of a ton of high performance flying machine, able to fly 100s of km, staging from cloud to cloud on thermals. It's a very real kind of flying, not at all like regular flying, which is a bit like getting on a bus in comparison. Arguably, I am getting too old for this kind of thing.

Regarding the pistons, for me, the hunt is on for anything that will fit, or can be adapted. It's a simple, vintage engine of 1940's technology. If I can find a old motorcycle type piston set with 5/8" wrist pin, about 70mm (2+3/4") tall, and OK to suit the bore, with about 56mm (2.2" or 2+13/64") that isn't dome, it might work.

 

[Pylonracer]

No RC pylon racing.

Down in Milford One Sea there was a company called FJ Engineering which was run by Dave Fletcher Jones who was ex Wellworthys development engineer.

It is not there anymore but he had a huge number of patterns for pistons which they then machined. Such things as Austin 7 but loads of R&D stuff as well.

It might be worth trying to see if Dave is still kicking.

 

[graham-xrf]

No RC pylon racing.
Down in Milford on Sea there was a company called FJ Engineering which was run by Dave Fletcher Jones who was ex Wellworthys development engineer.
It is not there anymore but he had a huge number of patterns for pistons which they then machined. Such things as Austin 7 but loads of R&D stuff as well.
It might be worth trying to see if Dave is still kicking.

Thanks much for the suggestion.
It seems the place is still there, with that same name, and in Milford. Apparently it's a top-notch outfit in aluminium and stainless steel fabrications.
I am not sure it is still into anything Dave Jones did, but..
.. it may take a while to even get started. This is what the website yields.



I have had one of those passing crazy ideas that totally deserves what will inevitably come, such that I hesitate to even say it, but what if ..
Plate electroless nickel strike onto the cleaned up original piston, masking out the wrist pin bore, the little skirt holes, and possibly the walls of the ring slots, but leaving the bottom of the slots exposed. Without help from an external current source, (and also a different plating solution), the process runs out of chemistry urge at about 0.127mm (0.005").

Then plate copper, about 0.010", or a much as is needed to get to the +0.020 (in diameter) oversize for the bore, but allowing for the hard final plate.

Finally plate with nickel the last 0.005". That final surface would be very hard, and may not be appropriate for sliding in a bore. Maybe some other metal? Anyway, I just wondered what could be done to to re-work a piston into something "a bit bigger".

OK - consider the thought now perished. I still think we might yet find something near-fit suitable.

 

[tonyfoale]

I don't think the oval piston would be a piece of cake at all on a normal cnc lathe. I would have thought that the spindle would have to be synchronised with the tool and that is normally only a function available in threading operations, where the movement of the tool is controlled for threading.

Oval pistons are normally done with some sort of cam machine.

A hobby level CNC machine may not easy turn an oval but real CNC lathes would have no problem. I converted my lathe to software control, below what is normally meant as CNC, but it is a piece of cake to grind cams on it, oval pistons would only be a crumb.
See this : Videos, electronic lathe conversion

 

[tonyfoale]

Machining pistons on a manual lathe is not too difficult. Firstly you turn it circular, then you offset it to one side (make it eccentric by an appropriate amount) machine that and then offset to the opposite side and repeat. Regarding taper, there are a few possibilities to do that on a manual lathe also.

 

[Pylonracer]

Machining pistons on a manual lathe is not too difficult. Firstly you turn it circular, then you offset it to one side (make it a eccentric by an appropriate amount) machine that and then offset to the opposite side and repeat. Regarding taper, there are a few possibilities to do that on a manual lathe also.

Thanks, that is interesting, what bikes are you building and racing?

 

[tonyfoale]

I had suggested a CNC lathe because you might be able to find a jobber that has the capability. The lathe would have to have a synchronized spindle. nut it should be able to make the pistons provided you can furnish a CAD model.

Another thought would be a CNC mill with 4th axis capability. My Tormach 770, along with my 4th axis could definitely machine the exterior.. There may be some difficulty machining the interior surfaces though, depending upon the complexity. The mill might not produce as nice a finish as the lathe would but but it should be acceptable for your purposes.

If you only used the mill for the relieved sides then the surface finish would not matter much. I have machined interior surfaces on manual mills, CNC would be even easier. I do not have a video of doing a piston on a CNC mill but here is a 2 min quicky doing a cam using the methods you described

Here is a magnesium piston that I made from solid bar in the late 1960s on manual machines.

 

[tonyfoale]

Thanks, that is interesting, what bikes are you building and racing?

Classic racing Aermacchis. I only make stuff for my own use now. Lots of pix on my web site in the photo section. https://motochassis.com

 

[Pylonracer]

Classic racing Aermacchis. I only make stuff for my own use now. Lots of pix on my web site in the photo section. https://motochassis.com

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I think the Aermacchi importer used to be in Southampton, Lawton and Wilson, I only remember the workshop burning down !!

Nice looking bike.