Pistons problemo!

[tonyfoale]

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

Nice looking bike.

I knew Syd Lawton, they were the second importers for Aermacchi, a company whose name started with a "W" were the first. I do not recall how Syd took it over maybe the "W" owner died? Dick Linton more or less took over at least the racing side. Dick blames me for his involvement ;<)

 

[Pylonracer]

I did not know any of those people, sadly they were selling Reliant cars when the shop and workshop burnt down.

 

[tonyfoale]

I did not know any of those people, sadly they were selling Reliant cars when the shop and workshop burnt down.

Maybe a Reliant owner had a box of matches?

 

[Pylonracer]

Maybe a Reliant owner had a box of matches?

Having had a Scimitar in the past and driven a Regal van I consider a box of matches as essential.

 

[graham-xrf]

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.

Thanks for that tip. You can bet that I will be playing that sequence through until it is completely understood, and I am sure to first try a "practise run" on some old bit of pipe.

I think, in theory, the taper part and the offset eccentric parts would be done together. The small angle on the compound provides the taper, but only gets applied at a certain point from the top.

I have handed the beat up, but thankfully oversized piston to a pal who is well experienced at TIG welding, and has all the kit. He also takes apart airplane engines. He expressed some misgivings, but said he would give it a try. The plan is to thoroughly clean it up, with final acetone clean just before the weld, and then weld up the ring slots with 4032 filler. Also, under where the skirt oil ring is to go, it needs the thickening on the inside, so the oil ring can be cut.

This whole scene is driven by not having any easier way out, and not much to lose in the attempt. It still feels like madness!

 

[graham-xrf]

There is the possibility of making a CAD model, which can output G-Code, and then having some CNC firm make it. Is this a hugely expensive way?

 

[Pylonracer]

Maybe a Reliant owner had a box of matches?

Did you know a chap called Dave Farmer who raced Mondial bikes in the late 50's/ early 60's. He told me that he was a works rider, he also came from the Southampton area.

 

[tonyfoale]

Thanks for that tip. You can bet that I will be playing that sequence through until it is completely understood, and I am sure to first try a "practise run" on some old bit of pipe.

I think, in theory, the taper part and the offset eccentric parts would be done together. The small angle on the compound provides the taper, but only gets applied at a certain point from the top.

I have handed the beat up, but thankfully oversized piston to a pal who is well experienced at TIG welding, and has all the kit. He also takes apart airplane engines. He expressed some misgivings, but said he would give it a try. The plan is to thoroughly clean it up, with final acetone clean just before the weld, and then weld up the ring slots with 4032 filler. Also, under where the skirt oil ring is to go, it needs the thickening on the inside, so the oil ring can be cut.

This whole scene is driven by not having any easier way out, and not much to lose in the attempt. It still feels like madness!

Be very careful with this much welding on a piston. The welding itself can be very difficult because cast aluminium items, particular of this vintage, can be a bit porous and years of sitting in oil tends to fill the pores with oil. Then no matter how well you clean them externally, they start weeping oil as you start to weld and then that's the end of it. Even with the right rods the piston will be softened unless heat treated after welding and heat treating can be difficult unless you know the alloy that was used originally.
I tend to think that your best bet is to look for new pistons that are close to what you want. There are a few piston makers in the UK, Omega for one who make specials. There are a whole stack of them in the US, probably a few in Germany too.
Have you tried asking around on classic bike forums? Try asking on this one https://www.facebook.com/groups/367147517553916/
it has nearly 50,000 members so maybe there is someone there who can help. Maybe there are groups specialised in Douglas?

PS. machining businesses with CNC machines often charge what may seem to be exorbinent prices but they have to often break down whatever they are set up for, then maybe make some tooling to hold a special job and then put their machine back to what it was. However, there are many Chinese companies that offer very low volume CNC machining at very good prices even with shipping. I have friends who have used these and the quality is good. I know that there is an Italian company that does the same I will try to find their details. There are two alloy grades that are used for billet pistons and I will try to remember to dig that out also.

PPS. Another option. Rather than welding the pistons I think a better option would be to machine the grooves a touch wider and get some rings made. I have seen adverts for making rings from centrifucally cast cylinder liners. Which is probably what Douglas did originally.

Here is a tip for holding pistons for machining in a lathe if you decide to machine what you have.
https://www.homemadetools.net/forum/work-holding-lathe-90665#post201593

 

[matthewsx]

There is the possibility of making a CAD model, which can output G-Code, and then having some CNC firm make it. Is this a hugely expensive way?

Have you contacted Weisco? They may have something on the shelf that would be close enough.

John

 

[tonyfoale]

I checked my notes and the main alloys used for machined from bar pistons are 4032 and 2618.
4032 is probably the best choice for your engine. It has a very high silicon content which reduces expansion so you can run tighter fits and have a quieter engine. 2618 is more suited to extreme use in race engines etc.

I could not find where I had details of the Italian CNC place that I mentioned.