CJ love

[turbohunter]

Thanks Brent you’re right.
All part of the learning process. So much info is coming in that I lose the category it belongs in but I’ll get it all figured out.
Gotta remember to use the .001/inch rule first.
So, when the HN1’s get here I’ll rip the slugs out that I have installed, clean ‘em and redo the clearances.

[turbohunter]

Got a question for you piston guys.
What is the purpose of the indented land here.

[turbohunter]

I just looked it up.
It’s called an accumulator groove and is designed to reduce pressure between the rings to maximize ring seal.
Please feel free to add or subtract from above statement.

[gt350hr]

    The accumulator groove came about around 96 or 97 as a result of Nascar development. The concept was two fold. First was to give extra space for any pressure getting by the to ring to go before equalizing the pressure between the rings that would allow "ring flutter" ( loss of top ring seal to the bottom of the groove.) We later improved on this by going to a larger second ring end gap that the top.
   The second part ( and IMHO MORE important) is we were seeing some rubbing in the middle of that land and that could lead to scuffing. By reducing the potential rub area we actually were able to increase the second ring land diameter to use the second land to reduce piston "rock" and loss of ring seal during the directional change at the top of the bore wher it is far more critical than at the bottom of the bore. Due to thermal expansion , we "used to be" as much a s.036 under the bore diameter. With the accumulator groove , we routinely use .030 under and have gone to .026 on aluminum block engines.
    Some companies have different views on the style and width of the groove and others don't use it at all. IMHO it is a good thing and I always recommend it if it isn't standard on the piston you need.

[turbohunter]

Thank you Randy.

Well the bearings from Brent got here and I immediately went to work. It was easy to pull all the pistons and install the STD/HN1 mix. Gave me a chance to look over the rings again just to make sure I had no overlapped oil ringage. All was well.
I am amazed at how dead on the bearings are. Of course it’s my inexperience with them but damn, they are all but one dead nuts on. All a solid .0025 with one just waaaay out at .00245 Rotation is nice with no hard spots.
Rotating assembly done!!!
Maybe now I’ll stop second guessing myself. I’m really amazed at the rock solid readings. Quite different from the way this deal started out with the old crappy bore gauge. But it taught me a lesson so I ain’t complaining.

[Barry_R]

To expand on Randy's comments...

The big gap and accumulator groove concepts came about as a result of computer modeling at Dana in the early 90s.  They were working on OEM development for emissions and mileage, using a pretty high end computer.  Scott G. worked at Speed-Pro, which was a Dana division prior to being acquired by Federal-Mogul.  He talked them into running that simulation on a NASCAR model and the resulting improvement was verified, and subsequently tried by a few selected teams.  I always try to make sure that Scott (I am certain that Randy knows him) gets credit for bringing this concept to the race community.

[chilly460]

Thank you Randy.

Well the bearings from Brent got here and I immediately went to work. It was easy to pull all the pistons and install the STD/HN1 mix. Gave me a chance to look over the rings again just to make sure I had no overlapped oil ringage. All was well.
I am amazed at how dead on the bearings are. Of course it’s my inexperience with them but damn, they are all but one dead nuts on. All a solid .0025 with one just waaaay out at .00245 Rotation is nice with no hard spots.
Rotating assembly done!!!
Maybe now I’ll stop second guessing myself. I’m really amazed at the rock solid readings. Quite different from the way this deal started out with the old crappy bore gauge. But it taught me a lesson so I ain’t complaining.

Ditto, I've been checking clearances with my Mitutoyo bore gauge and it's been MUCH more consistent.  I also found my Starrett 2-3" mic, which I bought used, was slightly bent which was resulting in some goofy readings.  Guess someone tried to use it as a C-clamp.  My backup Harbor Freight mic was fine for establishing journal size, and I just bought a Mitutoyo 2-3" Mic which is also fully repeatable.   Lesson learned, buy the good stuff for this type of work. 

[gt350hr]

    Yes Barry , I do know Scott and you are correct ( obviously) that I was approached by a couple of teams to do the grooving for them. We did try several different designs , one of which was like a 7 , one was a radius only, and a variety of angles and depths. Scott's basic concept was the best and what we went with. Scott was also one of the first to "back me up" when I developed ( for Nascar use as it already existed on import 4 bangers) the conical and spherical dishes that went against the "mirror image" dishes they were used to using. He is a very smart man not afraid to think outside the box.  Scott also convinced  this  "set in his ways" guy to open up the second ring end gap for the same reason. Guess what , he was right! We have had conversations after that as well. "I" still learn every day!
   Randy

[Barry_R]

And you were the guy that talked me into running the spherical/cone shaped dish in my first EMC entry.
We still have a hard time selling that concept to the masses....but it damn sure works

[turbohunter]

Love reading this stuff. Thank you both.
While you’re hanging out, I’m curious about the step above the top ring. I’m assuming it’s to allow the compressed gasses a better path on to the top of and behind the top ring for better sealing. My question is why a hard corned step and not an angle?

On edit as I think about it more, as we’ve seen before in matching manifold to head, most of the time the air really doesn’t care if there is a step, it tumbles over and in anyway.

[gt350hr]

   The "gas distribution groove" is used by several companies to put a slight amount of additional downward pressure on the top ring to keep it seated. It works and we incorporate it in 99.9 % of our pistons as  a couple of people didn't want it.The hard corner distributes the pressure around the top of the ring more evenly.