51 replies to this topic

[1977 Loud_Mini]

Hi all, as some may know i had problems with my 1380cc engine being that the pistons and rings were too small for the bore.
At college i was talking to my tutor and he suggested making some pistons on the CNC milling machine they have there along with a lathe. I've drawn up a design in software called SolidWorks which means the drawing can be transfered to AutoCAD and then converted into code (G-codes) for the CNC milling machine to recognise.
I've been researching the required material which is 2618 Aluminium so that the piston can cope with the stresses of heat and combustion etc, just don't know where to buy it if it can be bought anywhere.

I was thinking to get some 74mm piston rings and then file them down to the correct ring gap so that they will be sort of custom rings for my application.
If anyone has any other information it would be great to here from you.

Thanks, Evan.

[mini13]

Pistons are a funny shape, tapered and eliptical, so that when they are hot they end up the right shape to fit the bore.

feel free to prove me wrong, but I bet that if you make your own they will size in the bore pretty quick.

[jamie@thefatgarage]

Not easy. They may look like perfectly round cylinders with straight sides but they are not. Also the design is dependent on the material and its expansion properties. So while it is possible, it is probably beyond what anyone without the relevant metallurgy, engineering and physics knowledge would successfully pull off. I have specified pistons for projects and had them custom made, but that didn't include all the actual making it fit the bore under load and temperature. It was just crown height and design / skirts etc.

[Cooperman]

Pistons are cast or drop-forged. Do you have the equipment to do either of these?

[1977 Loud_Mini]

I have read somewhere that the pistons aren't round. I haven't really looked into it in any detail yet but i believe they taper intowards the top of the piston. It's the piston skirt that is usually where the measurements are taken (across the gudgeon pin not along it)?

[mini13]

yep, the measurement is taken at the point of least expansion/wear

you are looking at a taper in the order of 15 thou depending on material, the ovality is the harder part to cope with, the pistons will expand differently depending on the shape of the pin support.

Also, the ring grooves need to be ACCURATE!


far cheaper and easier to get another set of pistons and another block.

[Dan]

I can't say I know for sure but surely there would be a fairly limited life to a billet piston. You'll be chopping through all the structure of the billet to machine it down, that's why production car pistons are cast. AFAIK billet pistons tend to only be used for wierd engines that aren't designed to last long, like tractor pull engines and that kind of thing.

[andydclements]

My father once encountered (this will show his age) a lorry with a home made piston.

It was during the war (he was an apprentice then) and so parts were hard to acquire and obviously the engine had damaged a piston and the previous owners needed to get the lorry back on the road to sell it.
The engine wasn't making the power output it should, it wasn't missing on any cylinder as such, but one cylinder didn't seem to carry the weight the others could (pulling plug lead didn't alter revs as much as any other plug lead would). They found the compression was down on this cylinder. When the engine was pulled apart they found the top of the piston had a thin metal plate on it, secured with nails. Can you guess yet?

The previous owners (a wood yard) had turned down a piece of wood to match the piston dimensions, including grooves for rings, fitted a sheet of tin plate to the top so that when it fired (and it did) it didn't set fire to the piston.



Obviously some cam make pistons at home, but perhaps not as well as a specialist manufacturers.

[MRA]

Making pistons is easy, just follow my simple steps below.....

1) spend £450,000 on the correct machinery
2) Invest in trained operators
3) spend a large fortune on development.....

No seriously a proper piston as above is not round, is usually pressure die cast or forged and machined to very high tolerances...

That said you could make your own pistons out of billet and machine them to suit, but they would be round... well as round as the available machinery will get them anyway, you would also find that they would "slap" in the bores.

But yes you could make your own the old fashioned way...

[mk3cortina]

i once knew a guy who bought a morris minor that had a solid oak piston.

[MRA]

Hi all, as some may know i had problems with my 1380cc engine being that the pistons and rings were too small for the bore.
At college i was talking to my tutor and he suggested making some pistons on the CNC milling machine they have there along with a lathe. I've drawn up a design in software called SolidWorks which means the drawing can be transfered to AutoCAD and then converted into code (G-codes) for the CNC milling machine to recognise.
I've been researching the required material which is 2618 Aluminium so that the piston can cope with the stresses of heat and combustion etc, just don't know where to buy it if it can be bought anywhere.

I was thinking to get some 74mm piston rings and then file them down to the correct ring gap so that they will be sort of custom rings for my application.
If anyone has any other information it would be great to here from you.

Thanks, Evan.

Evan, Solidworks is a much superior cad package than Autocad..... why not create the G & M coding in SW ?? and yes I know where to get 2618 ally from if you really want to make 1 or some etc....

[Shifty]

My father once encountered (this will show his age) a lorry with a home made piston.

It was during the war (he was an apprentice then) and so parts were hard to acquire and obviously the engine had damaged a piston and the previous owners needed to get the lorry back on the road to sell it.
The engine wasn't making the power output it should, it wasn't missing on any cylinder as such, but one cylinder didn't seem to carry the weight the others could (pulling plug lead didn't alter revs as much as any other plug lead would). They found the compression was down on this cylinder. When the engine was pulled apart they found the top of the piston had a thin metal plate on it, secured with nails. Can you guess yet?

The previous owners (a wood yard) had turned down a piece of wood to match the piston dimensions, including grooves for rings, fitted a sheet of tin plate to the top so that when it fired (and it did) it didn't set fire to the piston.



Obviously some cam make pistons at home, but perhaps not as well as a specialist manufacturers.

I love this story!!

Nice one

[icklemini]

I was thinking to get some 74mm piston rings and then file them down to the correct ring gap so that they will be sort of custom rings for my application.

74 mm rings in a 73.5mm hole wont be 'round' - it will be oval... not good...

changing the ring gaps doesnt change the diameter of the ring...

Edited by icklemini, 25 January 2011 - 09:55 PM.

[1977 Loud_Mini]

I and the college don't have any forging or casting facilities which is a shame but there is an assignment in my course which requires a project to be made/carried out and i thought that making a piston would be a great one.
The ring gaps etc that i've designed are within the specified tollerances that i found on JEPistons website as is the piston to bore clearance. Whether or not this is accurate i'll never know but a bit more research may be needed. I've followed some dimensions of piston length and height from the centre of the gudgeon pin to the crown etc the same as the 21253 pistons i think.
I'll have to measure my 1380 powermax pistons accurately and determine the taper etc.

Mini13 - 15thou you say?

[mini13]

15 thou is about what my 73.5 powermax's were, only measured with a vernier though so not terribly accurate.