Machining piston

[Daryl Jones]

"Cam grinding" allowed full skirted pistons room to expand along the piston pin (gudgeon/wrist pin) axis.

Modern 4-Stroke High performance "slipper" pistons  are designed to keep the rings supported and square to the bore, while minimising friction.


They might need a little bit of taper, but there is no reason the small thrust faces cant be perfectly round, to fit the bore.
The taper would accommodate the bottom of the skirt, being the least supported part, flaring out a bit when it gets to operating temperature.


Cheers, DJ

[Mike52]

Model "T" pistons were cam ground , tapered and had a split skirt.
You can see the split skirt in that Chrysler picture.

Some of the drag racing stuff used a bunch of teflon type plugs in the piston skirt and the piston did not touch the bore at all.

I have done engine reconditioning most of my life and have used a piston grinder when I was an apprenti.

[Daryl Jones]

I was an apprenti.

Italian engines?

[HeavenVMX]

No not the new VHM pistons are nearly perfectly round. Yes the NR began as a 500 then went to 750

So by definition that means the VHM piston is not perfectly round and therefore 'cammed' to a degree even though less than others

[Daryl Jones]

VHM solid billet piston

"The piston features a special skirt shape which will result in a cylindrical shape during operating temperature with the optimum clearance for less friction giving the best performances".

Woo Hoo! About time some-one thought of that

Cheers, DJ.

[Lozza]

VHM solid billet piston

"The piston features a special skirt shape which will result in a cylindrical shape during operating temperature with the optimum clearance for less friction giving the best performances".

Woo Hoo! About time some-one thought of that

Cheers, DJ.

Let me know your ideas to get a 47hp 125 up to 48hp within a tightly controlled class. No changes to pipe, ignition or carby     

No not the new VHM pistons are nearly perfectly round. Yes the NR began as a 500 then went to 750

So by definition that means the VHM piston is not perfectly round and therefore 'cammed' to a degree even though less than others

What is your problem? Why are you trying to insinuate I said the pistons are perfectly round?

[HeavenVMX]

my problem?

[Daryl Jones]

Let me know your ideas to get a 47hp 125 up to 48hp within a tightly controlled class. No changes to pipe, ignition or carby     

Hi Lozza,
I haven't suggested that VHM pistons don't have any benefits, merely that their "Marketing Claim" is identical to what Chrysler's was in 1947.

If I wanted another horsepower, within your limits, I might try to get a few extra revs, 300-350 should do it. (If the ignition is not rev limited).
In order to facilitate this, the billet piston could be "machined on every surface allowing for all unnecessary weight to be removed".

If I am rev limited, then I would need to increase the torque. Perhaps this might work:
"A new twin-ring piston design which increases compression stability with less blow-by under extreme conditions".

Cheers, Daryl

[OverTheHill]

can't get over how short those modern big bore 'no stroke' pistons are. Hell don't do what i did with my trusty king quad300. Being a tight arse i had a 1st over old piston & realised they are cam ground but thought--bit of extra clearance will do it, [wrong], or maybe i just didn't give it enough overall. Headed off down to the farm 100 yards but on the tarseal & it just locked the wheels & i sailed up [nearly] over the bars [he says with a high pitched voice. Thought F£$%k, that'll teach me, but left it for a minute then away it went & have been wary 'since' but never stopped since & that was about 4 years ago [minimal use though]. Got a piston kit plus cam & rockers off ebay back then--must put it in lol. Bottom line--'Don't do it.

[Daryl Jones]

By pushing the piston pin, as high into the piston as possible, the amount of 'Rock' around the pin is reduced.
The skirts can be smaller and shorter.  (Don't expect 100.000k between rebuilds)

Modern Design, Engineering & Production is truly impressive in terms of quality & dimensional accuracy.

We can forget that it was not always this way.

Burt Munro cast his own pistons for the WFI in Jam tins. Correct composition of the alloy was more important for his engine.
From the depression era and immediate post-war era there are plenty of stories of pistons turned from hardwood providing 'acceptable' service.

For much of the 100+ year history of racing 2-stroke engines it has been necessary to remove pistons and rub down the high spots to prevent (or correct) seizing.

Cylinders full of ports (particularly air cooled ones) cannot help but to be, anything but perfectly round, as they heat & expand.

If you had the Desperate Need, the Time and the Talent, you could turn up a circular piston, preferably with some taper. Then take a file and some emery cloth and remove a couple of thou. on the gudgeon sides.   Start the motor and bring it up to operating temp. Then.... (This is the step OverTheHill missed)  pull it down again and file or sand down any obvious (shiny) high spots. Repeat the process, until there are no more high points.

Not really an option for "World beaters" (Except Burt Munro.),  but it could keep the wheels turning on some 'worthless dunger' that's valued less than the cost of a new piston kit & rebore.

Note: Kids, DO NOT try this at home.   Your individual results may vary.