Piston and Ring Issue

[Ishiftem]

This is a stroked 409 Chevy. 4" stoke with a 6.385 rod. I have two issues that have been ongoing. Piston rock and rings butting. Start with the rock issue. I originally had one set of custom pistons. On assembly I could rock the piston at tdc .032 up and down with rings installed. After running it, they rocked bad enough that they hit above the top ring and actually put a slight indentation in the cylinder at tdc where the piston hit above the ring and at the the opposite side of the piston where the bottom of the skirt hit the cylinder. So I sent that piston to another place explaining the problem. They said that the barrel was in the wrong place on the skirt and made a new set. These were better and rocked .015 at tdc. After running those, they still hit above the top ring. I have talked to several other manufacturers and really don't get anywhere as far as solutions. The piston is VERY top heavy and lopsided due to the 74 degree deck angle and 13.3:1 compression. I really don't think they appreciate just how much these want to rock due to the deign. They are just thinking along the lines of a 90 degree deck engine. They say to cut back the piston above the ring but that just treats the symptom. It's like cutting off your arm because it hurts. I ask about the skirt flexing or using a longer skirt and get answers like "that shouldn't be necessary". Or I get " no, the piston rides on the skirt and not above the top ring" when I'm holding the parts in my hand with the wear above the top ring. I ask about offset pins and they say that's not how they make their race pistons. I know oem does it to reduce piston slap because it softens rock at crossover. They won't even entertain it as a possible solution. Well, this is a unique situation that may require something different yet they just won't hear of it. No critical thinking. I cant risking spending the money for yet another set only to wind up with the same thing. It's frustrating. I feel like I need to tell them what I want because I don't have much confidence in them figuring it out. I talked to two people who have experience with these 409 race engines.. The one said it's just what it is and you have to take them apart, hone the block out and get new pistons once in a while. Really!!!! That's the solution? The other said they make the top ring groove shallow and let the ring hang out so the top ring stabilizes the piston. On what planet is that a good idea? I guess those are good solutions if you are spending other peoples money.
The other problem is ring butting. It has always been a problem since I went to the high compression. I started with gaps of .028 on a 4.3xx bore on 112 c12 and they butted. Went to .032 and the same. Now I am at .040 with a gapless ring and they still butted. I can see the shiny spots on the ring ends. That's with Q16 (116 MON) and I even added squirters! Timing is 34 degrees. At 35 degrees the e.t. is about the same and at 36 the e.t. starts to fall off so I run it at 34. I put a data logger in this last time with two O2 sensors. Readings are 11.8 at the beginning of the run and slowly lean out to 13.6 at the end. I believe I found the issue to be a restrictive rollover valve vent to the cell. I haven't tried it to know for sure because the engine is out....again. I can't see that is being lean enough to detonate on Q16. I'm thinking the large crevice volume with that huge dome plus the counter bore at the top of the cylinder may be causing detonation and that is why the rings are butting. I am really scratching my head as to the cause and solution. Anybody have any suggestions or experiences? I can't keep throwing money at this thing and hoping.

[jred]

how much piston to wall clearance do you have,, also check the area where the rings are that area should have about .030 smaller then the piston that part of the piston should never come in contact with the cylinder wall also check the counterbore on the block that should be .060 bigger then the bore itself ,, who's pistons custom??? and what was the clearance recommended,, a short skirt piston would probley rock more then a longer skirted piston but not enough the the piston would touch at the top..

[Tom68]

Gapless rings....yuk.

Got a picture of the inside of the piston crown ?

Piston weight ?

Maybe the piston is holding too much heat ?? May need a wider ring too help transfer heat too the cylinder.

Piston cooling oil spray like the small journal sbc's did with the con rod cap part line grooves.

[Mark O'Neal]

I can see several things I'd change in the piston design. Might I ask who made that set.

The land stack should be .030 (or so) smaller than the skirt, but from about .150 above the top ring it should step in another .040.

The rings shouldn't be butting at .028 in the top, so you need to figure out why before you through a bunch more money at it.?

N/A..Nitrous, boosted, and what material are the pistons made of?

[Ishiftem]

how much piston to wall clearance do you have,, also check the area where the rings are that area should have about .030 smaller then the piston that part of the piston should never come in contact with the cylinder wall also check the counterbore on the block that should be .060 bigger then the bore itself ,, who's pistons custom??? and what was the clearance recommended,, a short skirt piston would probley rock more then a longer skirted piston but not enough the the piston would touch at the top..

Piston to wall .0055. These are CP. Diamond was worse. Isn’t touching the counter bore. Last time after honing I did the line2line coating to make up for the over after rehoning and to hopefully get a better fit. The coating failed. Flaking layers off and worn off to the point it now has has .006-.008 clearance. That’s after only some dyno time and about 20 runs.They rocked too much before though when the pistons were new at .0055 clearance.

[Ishiftem]

Gapless rings....yuk.

Got a picture of the inside of the piston crown ?

Piston weight ?

Maybe the piston is holding too much heat ?? May need a wider ring too help transfer heat too the cylinder.

Piston cooling oil spray like the small journal sbc's did with the con rod cap part line grooves.

No discoloration under the crown. 670 grams. They are fully profiled underneath. I put squirters in.

[Ishiftem]

I can see several things I'd change in the piston design. Might I ask who made that set.

The land stack should be .030 (or so) smaller than the skirt, but from about .150 above the top ring it should step in another .040.

The rings shouldn't be butting at .028 in the top, so you need to figure out why before you through a bunch more money at it.?

N/A..Nitrous, boosted, and what material are the pistons made of?

I’ll take more measurements when I get home. These are CP. This is N/A. They are 2618. The butting is sure is crazy. .040 gap on Q16 at 13.3:1 with 34 degrees of timing and doesn’t appear lean. It’s crazy!!

[ClassAct]

how much piston to wall clearance do you have,, also check the area where the rings are that area should have about .030 smaller then the piston that part of the piston should never come in contact with the cylinder wall also check the counterbore on the block that should be .060 bigger then the bore itself ,, who's pistons custom??? and what was the clearance recommended,, a short skirt piston would probley rock more then a longer skirted piston but not enough the the piston would touch at the top..

Piston to wall .0055. These are CP. Diamond was worse. Isn’t touching the counter bore. Last time after honing I did the line2line coating to make up for the over after rehoning and to hopefully get a better fit. The coating failed. Flaking layers off and worn off to the point it now has has .006-.008 clearance. That’s after only some dyno time and about 20 runs.They rocked too much before though when the pistons were new at .0055 clearance.

What did Line2Line say about the coating flaking? I know guys with 100k on their L2L coated stuff and upon inspection it looked like it did after it came off of the dyno.

[rebelrouser]

I have only done a few 409 and 348 engines, none of the race, just mild street engines. For what it is worth, being able to rock the piston with your hand is not really a diagnostic tool. Piston manufacturer cold clearance is to set the piston so when it is hot and running it performs like it should. Since the combustion chamber is in the block and the head is flat If think you need a thinner piston crown, working on deck height and rod length is the way to solve that issue. I race NSS with several 409 cars and they have high compression and do not have the issues you talk about, so I am pretty confident the issues can be overcome. But not a lot of 409's around so the knowledge base is small. On the ring end gaps, If the rings are getting hot enough to expand and butt, combustion chamber temps have to be a major player. You mentioned on the dyno O2 readings, but do you have exhaust gas temps? I know a lot of builders don't use them anymore, but they are important sometimes, they can show up a weak cylinder, and for our issue hot cylinders. Just from my experience high combustion temps, if mixture and timing are right, are usually caused by camshaft design or not installed properly. Hate to ask a dumb question but is the timing pointer properly indexed? Guy brought me a SBC blower engine last week, and the pointer was 10 degrees off. He wanted it dyno'ed because it popped and banged. This forum may hook you up with a 409 guru http://www.348-409.com/

[Ishiftem]

how much piston to wall clearance do you have,, also check the area where the rings are that area should have about .030 smaller then the piston that part of the piston should never come in contact with the cylinder wall also check the counterbore on the block that should be .060 bigger then the bore itself ,, who's pistons custom??? and what was the clearance recommended,, a short skirt piston would probley rock more then a longer skirted piston but not enough the the piston would touch at the top..

Piston to wall .0055. These are CP. Diamond was worse. Isn’t touching the counter bore. Last time after honing I did the line2line coating to make up for the over after rehoning and to hopefully get a better fit. The coating failed. Flaking layers off and worn off to the point it now has has .006-.008 clearance. That’s after only some dyno time and about 20 runs.They rocked too much before though when the pistons were new at .0055 clearance.

What did Line2Line say about the coating flaking? I know guys with 100k on their L2L coated stuff and upon inspection it looked like it did after it came off of the dyno.

I sent an email with pictures and I’m waiting to hear back.

[Ishiftem]

I have only done a few 409 and 348 engines, none of the race, just mild street engines. For what it is worth, being able to rock the piston with your hand is not really a diagnostic tool. Piston manufacturer cold clearance is to set the piston so when it is hot and running it performs like it should. Since the combustion chamber is in the block and the head is flat If think you need a thinner piston crown, working on deck height and rod length is the way to solve that issue. I race NSS with several 409 cars and they have high compression and do not have the issues you talk about, so I am pretty confident the issues can be overcome. But not a lot of 409's around so the knowledge base is small. On the ring end gaps, If the rings are getting hot enough to expand and butt, combustion chamber temps have to be a major player. You mentioned on the dyno O2 readings, but do you have exhaust gas temps? I know a lot of builders don't use them anymore, but they are important sometimes, they can show up a weak cylinder, and for our issue hot cylinders. Just from my experience high combustion temps, if mixture and timing are right, are usually caused by camshaft design or not installed properly. Hate to ask a dumb question but is the timing pointer properly indexed? Guy brought me a SBC blower engine last week, and the pointer was 10 degrees off. He wanted it dyno'ed because it popped and banged. This forum may hook you up with a 409 guru http://www.348-409.com/

I’m one of the 409 guys you race with so they do have issues. The other 409 guys don’t do there own work so they don’t know. I talked to the two major w builders and they do have the issue with rock. Another guy who has run high compression 409s and does his own work has had the rock issue with no solution. The crown can’t get any thinner and deck height can’t be changed. TDC has been verified several times. Afr readings were on the track. I never checked exhaust temps. When I had shelf Ross pistons at 9.7:1 compression and 5/64 rings with this same cam there were no issues. That had stock length bbc rods instead of 6.385.

[Tom Walker]

Any chance of trace or occasional detonation even with that fuel? Are all the holes experiencing this situation equally?
Could it be you are fighting the same demons many have fought when pushing the W engines with compression and RPM?
Anyway, they are truly eye candy and hot rod legends and if their owners have a deep enough wallet, then it is glorious for the rest of us to get to watch. Good luck and thanks for keeping the legend alive!

[PackardV8]

We feel your pain. That is probably among the worst piston designs ever put in a race engine.

The land stack should be .030 (or so) smaller than the skirt, but from about .150 above the top ring it should step in another .040.

What did Line2Line say about the coating flaking? I know guys with 100k on their L2L coated stuff and upon inspection it looked like it did after it came off of the dyno.

As usual, Mark knows his pistons and his suggestions as to dimensions and coatings would solve much of your problem. If the coated pistons are machined to his specs and the Line2Line coating is as good as we know it can be, there can be no piston rock, because even that abortion design can't rock at zero skirt clearance.

[Dan Timberlake]

The shop I worked at 1973 to 1996 sometimes "pin balanced" 409 pistons.

Put two wrist pins in so they hung out the sides.
Put the assembly on knife edges, at which point the thick dome side rolled downward fast.
Then milled out material on the THICK dome side until the piston stayed level.
I don't really recall if we could make them stay level without going too thin.
Worked to Jenkin's dome thickness recommendations in "The Chevrolet racing engine."
With generous radiuses everywhere
By SA Design ?

How do the ring sealing faces look under magnification ?
The butting in the top ring picture would have me checking to see how parallel the filed faces are.

If they are parallel, Maybe put .002" taper on the ends so the gap is a little wider where the shinies are now.

I'd think some local relief of the top land just to remove the scuffing wouldn't hurt anything.

[miniv8]

Are the water jackets filled in the block?