piston skirt relief

[petertha]

I'm just about to make the pistons for my 5-cylinder radial. The plans call for a cylindrical profile, the bottom skirt portion is shelled out and a further deeper milled slot for the link rod. I noticed the Edwards radial is much the same. But I've noticed almost every RC 4-stroke engine I've examined (and the odd hobby designed engine) has a further scallop or milled reduction in the skirt like the OS-56 piston on the left. I don't think its for any kind of clearance & I doubt the weight reduction is of consequence. So wondering if its to provide more direct flow access to mist lubrication. Any thoughts?

 

[petertha]

The other thing I wanted to mention is the commercial pistons are typically cast or forged aluminum as opposed to my bar stock. When I milled my tester piston, I detected about 0.0005" of deviation to skirt diameter. This could have been stress relief or could have been from V-block clamping pressure which was about as light a grip as I dared without slipping during milling. The next piston was 7075 instead of 6061 & seemed to stay stable. There is 0.0025" piston OD cylinder bore clearance so hopefully enough. Anyways just wondering if milling a window might open up the closed hoop & the diameter of the remaining wings might drift some more. I wasn't planning on lapping the pistons to some defined ID tool, but I guess that's an option too.

 

[Ken I]

That scallop is for weight reduction or to clear the throw of the crank counterweight webs (or both). Neither are usually an issue on models.

On real pistons only the ring land area is round - the remaining skirt area is both tapered and oval - so that at operating temperature gradients the skirt approximates a cylinder.

The nominal piston diameter is as measured at 90° to the wrist pin at the wrist pin centreline (its all over the place anywhere else).

Cast or forged pistons are often thermally cycled ( aged ) in an oven so as to maintain shape consistency when machined - sometimes going as far as a second ageing after rough machining for the fine tolerances usually employed.

Regards, Ken

 

[rick9345]

Scallops reduce weight, and also allows thermal expansion without a full skirt seizing in bore

 

[petertha]

That scallop is for weight reduction or to clear the throw of the crank counterweight webs (or both).
Regards, Ken

Ah, totally forgot about counterweight clearance & the more compact rod assembly on the typical single cylinder RC-4S. Thanks for the comments.

 

[Peter Twissell]

The reason for oval piston skirts is distortion of pistons under load. With pressure ditributed equally across the crown face, the piston will tend to flex about the axis of the gudgeon pin. This flex results in distortion of the skirt, reducing it's diameter when measured at 90 degrees to the pin axis.
I designed pistons for my radial to minimise this issue. There are no sides to the skirts, only the front and rear parts which stabilise the piston in the bore. The remaining material provides a web which provides stiffness against the distortion described above.

 

[awake]

Peter, that is an interesting design - never seen pistons done like that!

 

[josodl1953]

This is what modern pistons look like (Mahle)

About piston distortion: you should bear in mind that the piston rings provide the seal in the cylinder, not the piston circumference itself. Therefore it is not necessary to lap the pistons in the cylinders, the pistons need (very little ) room in the cylinder to allow expansion.

Jos

 

[Peter Twissell]

Absolutely right Jos.
Piston clearance needs to be just sufficient to allow an oil film to be maintained and to allow for differential expansion plus any distortion.
In small engines, the relative stiffness of the piston will be high, so distortion is likely to be small.
My radial at 2.3 liters is a behemoth by comparison.

 

[petertha]

About piston distortion: you should bear in mind that the piston rings provide the seal in the cylinder, not the piston circumference itself. Therefore it is not necessary to lap the pistons in the cylinders, the pistons need (very little ) room in the cylinder to allow expansion.

Thanks Jos. Yes I understand the clearance issue. What I was referring to was (avoiding) lapping the piston to its correct finished diameter in order to correct any eccentricity or stress relief resultant from milling the non-symmetrical pocket, skirt window etc. So for example if the bore is 0.9450" and the piston OD should be 0.9425, but in reality came out 0.9430, then I could finish lap it back to 0.9425 with a separate lap tool.

Initially I thought I would hold the piston in an upright collet bloc for millingk, but there wasn't a good way to pre-align the wrist pin hole for the pocket milling orientation. So I gingerly held it in 2 V blocks with tape protection/grip. I think the 6061 tester piston became eccentric by 0.0005" as opposed to over/undersized turning on the lathe. But so far looks like the real ones made from 7075 are remaining true.

I guess its getting about high time for me to start an actual build thread! LOL

 

[josodl1953]

For milling the inside of my Edwards pistons I made a fixture ( one of the many for this build). The I.D. is slightly undersize, the screw in the middle is for opening the clamp so the piston can be placed. This screw is removed prior to clamping the fixture in a vise or whatever tool is needed for machining. The outer screws are for clamping . In this way, there is minimal distortion of the piston.

 

[petertha]

Yes, my initial plan was to use my 5C collet block. But in order to have the wrist pin hole exposed for alignment, it would only grip top crown portion of the piston. The V blocks have 4 lines of contact & the tape provides some protection & grip. But its concentrated stress so a fixture is a better solution.

 

[Cogsy]

Yes, my initial plan was to use my 5C collet block. But in order to have the wrist pin hole exposed for alignment, it would only grip top crown portion of the piston.

Since I've started using a 5C collet block I make sure I leave a spigot on the top of the piston blanks when I machine them. Then I can hold the spigot in the collet block nice and securely while I machine the wrist pin hole and the rest of the internal features. Once it's done the spigot goes back in the lathe and the piston is parted off, then it's back in the chuck for a light facing of the top. As long as you're making flat top pistons this works well.