I just recieved my custom cut JE pistons for a forged iron block engine I'm building and I'm a bit confused. I haven't spent much time with aftermarket internals before, so maybe it's common, but my new pistons have a groove between the first and second compression rings. They're custom cut 2618 pistons and it's going to see a lot of boost (I hope), the first ring is .285 deep...the groove between the 2 compression rings is like it's 45 degrees...has anyone seen this before? Is it something all aftermarket pistons have and I just never noticed it before? I can get a picture of it on monday if I need to.

Other than having never seen that before, everything else about the pistons (the gas ports in the oil control ring, all the machining everywhere...I'm a machinist) is beautiful. Definitely the prettiest pieces of aluminum I've ever bought.

 

Its called an accumulator groove, and it's intended to set the volume between the two rings. The volume of that groove can have some effect on the pressure drop across the top compression ring which can help the ring seal. There are several ways to achieve the desired volume below the top ring. The groove in your pistons is one way, another is to use the diameter of the second land and the distance between the top and second ring.

 

This triangular groove has been called a 'pressure seal groove' by some. Because some combustion gas leaks past the top ring, this groove helps reduce the pressure of that gas by reflecting the pressure waves which stops the second ring from receiving the full force of the leaking gas pressure.

A different view of how it might work, but the same reason for being: better overall sealing.

 

Cool thank you guys...I never knew there was much work with pressure waves and volume between the rings. I'm glad JE does.

That said, does this mean that in a boosted application I should have a piston/ring combo that does a better job of controlling blowby and containing compression? It's going to be a "street" car...but I guess I'm taking that term very lightly at this point.

 

Wiseco did some Pro Stock work with pressure seal grooves I believe. Those engines probably make about as much pressure NA as you'll make blown, so it sonds like you have a good piston.

 

Sounds great. Thank you, I appreciate it.

I attached something I found from the JE catalog in another thread when I searched for "pressure seal groove" too...I hadn't ever noticed it in the catalog before.

 

adding to this, about ring end gap

I've learned the second ring end gap should be larger then the top. (I'd always thought opposit, top ring gets hotter, should have more gap, but no.)

This has somthing to do with preventing pressure build up between the 1st and 2nd ring. the larger gap on the 2nd ring will let pressure bleed downward faster.

 

the surface area also helps in ring land deformation ... ^^^ i like to run a a wider second ring in boosted apps. to aid in ventalation