Warbird Information Exchange

It seems to have fins on the back of the piston and several small holes around the side and near the fins. Anyone know what they are for? or any other interesting details.

Thanks

Fins are structural support for the piston dome. Holes should be for oil scavage from the oil scraper ring land.

The 'fins' also promote heat disappation

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Don't make me go get my flying monkeys-

Heat dissipation was what came to my mind. Increased strength is possible but why not just make it solid as it is not that much more weight.

Holes for scavenging oil? Do you mean that the oil goes from the cylinder wall through the piston and back into the main part of the engne?

Correct. As the piston moves down the cylinder and the oil ring scrapes the oil off the cylinder wall, that scraped oil goes back through the holes in the piston and to the crankcase. If your rings were off the piston it would be easier to see how it works.

Here is a Merlin piston out of a Mustang. It's a little bit different design and the oil return holes are under the ring land, but same principal.

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Thank you very much. Now for the other holes. Take a look at the original picture. Four holes can be seen just beside the fins on a sort of shelf. As well, just below that little shelf with the four holes, there appears to be five oil-filled holes. What are they for?

Piston pin lubrication inside the bosses is what those holes provide since it's a 'floating pin' held in place with a turned aluminum plug on each cylinder wall end to keep it centered. Remember, this is a P&W engine and it leaks and uses enough oil on its own.(actually, all radials leak so be suspicious of one that's bone dry on the outside)
The lower cylinders are almost awash in oil whether the engine is stopped or running and the rings don't know 'pump' from 'scrape' just 'up' and 'down' so whatever you can design to assist oil scavenge is to your benefit. Machining the flutes into the underside of the piston dome may not save a great deal of weight, but every bit you can remove from the extreme end of the bob weight (which is what a piston really is) reduces 'reciprocating weight', cuts down on rod stretch, and the dynamic loads on the whole assembly. It also adds to the strength of the piston where it's needed most it's like cardboard, corrugated cardboard is stronger than a flat sheet of the same thickness.
In addition, the flutes assist in carrying away heat from the hot spot on the piston head.

_________________
Don't make me go get my flying monkeys-

The piston has 2 oil scraper rings.
The very bottom ring and the lowest of the top 3 rings. Both of those ring grooves are drilled.
Oiling of the piston pin is the result as mentioned as well as oil control from entering the combustion chamber.
This also removes some of heat generated from the cylinder walls and from the connecting rods. The oil picks up the heat from the walls of the cylinder and is scraped away and splashes onto the rods.
IIRC the upper compression rings have the chamfer down or the widest point of the ring is away from the top of the piston.

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Rich Palmer

Remember an Injured Youth
benstear.org
#64- Stay Strong and Keep the Faith

BOOM BOOM, ROUND ROUND, PROPELLER GO

Don't Be A Dilbert!

Thanks, for the info guys.

After reading a bit more on the net, I was wondering if it is possible that there is an oil scraper ring and an oil control ring, although both rings appear to have holes in their vicinity. Could the lubrication for the piston pins be coming from different holes as the ones visible seem to be far away from where the pin goes.

When you look at the direction the oil sprays out of the holes the upper holes probably squirt on the bottom of the piston which will help cool the piston and the top of the rod with the pin for cooling and lubrication.
The lower holes are oriented so the oil squirts onto the side of the rod for cooling.
On the ribbed area you can see the coking of oil due to heat and oil combining in this area. Particularly above where the pin and top of the rod are.
If I have a chance I'll look in the 1340 parts book and see if the P/N and call outs for the rings are the same or different for the oil rings. There are also various 1340 models and changes over the years so it is possible not all the pistons are the same.

_________________
Rich Palmer

Remember an Injured Youth
benstear.org
#64- Stay Strong and Keep the Faith

BOOM BOOM, ROUND ROUND, PROPELLER GO

Don't Be A Dilbert!