I have a set of Ross (#425) like-new pistons that came in a 427 I recently bought. They have a ridge (step) about 1/8" down from the top. The top of the piston measures 4.18. Below the step the measurement is 4.20.
I don't recall seeing this on any other 390 piston I've ever bought, but maybe I just don't remember well. This is my first 427 if that's relevant.
See the little 'arc' above the valve reliefs? That's on every piston in the set. I think it was assembled incorrectly and the engine was never run after initial startup.
I forgot to send the valve assy to Barry when I sent the side bolts/spacers the other day. Will mail it tomorrow. The pistons are really nice; too bad they won't fit in the stroker. I think that little indentation would cause no harm if ever reused.
Re: Harm? Hell no, it's just a nick. Many of my TP pistons are.....................
August 9 2012, 9:48 AM
Funny as I had a exhaust valve head pop off in Detroit and made a perfect X in my Fairlane.Didn't hurt it enough to replace it as I had the tunnel wedge kit in and would have to have the new one balanced to match it.Ran it for two more years and then sold the car but I had replaced them with Venolias.
I think he is referring to the piston diameter being smaller in an area above the to ring grove. I think I remember seeing some rebuilder pistons back in the day that had a small step down around the top. Maybe supposed to clean any ridge in cylinder without boring ??
Yes, I was referring to the perimeter just above the top ring on each piston. Interesting, about cutting a piston to clear a ridge; never heard of that before. This block had absolutely no ridge in it, as is it had not been run-in at all. Max taper in any cyl was .0025
more clearance above the top ring for expansion. When looking at a used piston, you will see a shiny spot above the intake valve area. That is the last place for the mixture to react, (hottest) and the most swelled as they are elliptical to some extent when running. Some have a (heat groove) cut into them as well. Dykes as well as headland rings were developed from these observations for a more even pressure gradiant as a result.
If you dont stand for something, you will fall for anything
It looks like a pure-performance feature that is specifically beneficial during high-horsepower racing: As the piston crown heats up and thermally expands during the time of maximum horsepower, the step-down prevents skirt skuffing of the cylinder bore as the piston crown heats to maximum temperatures, though the step does increase crevice-type pollution emissions (improves exhaust fragrance) when the piston is operating at reduced power levels, since the crevice is larger than convention and therefore contains more fuel and air. The enlarged crevice, which helps prevent scuffing at maximum horsepower, doesn't permit the extra fuel/air mixture which gets compressed into this chilly volume to fully combust. The added crevice volume is sufficiently small that virtually no fuel economy difference will be measurable, and resulting pollution emissions in these max-horsepower applications are driven more by the high valve-overlap associated with max-horsepower camshafts than by crevice volumes, so the step has little in the line of negative side-effects, so long as horsepower is the primary goal.
With that said, there may actually be no additional crevice volume associated with this racing piston, and therefore no additional exhaust fragrance, and no negatives at all (other than machining cost), if it is determined the step permits closer clearances than convention when below the step and above the top compression ring. Most/all FE pistons are stepped down further in diameter above the top compression ring than below the top compression ring, specifically to address expansion and distortion on the piston crown when power is high. Perhaps Ross was focused on only stepping the minimum necessary to get the job done for racing, so they may have spent a tiny bit more to locate an additional step above the top compression ring.
I'm always super-happy to participate in a post with Kevin High-Riser, even if this response only provides my usual theoretical fiction.
This message has been edited by daveshoe22 on Aug 9, 2012 10:05 PM This message has been edited by daveshoe22 on Aug 9, 2012 9:50 PM
The step style reduction up there is definitely a heat related feature. Probably an effort to reduce or eliminate top land scuff as the hot piston rocks over at TDC. Race pistons often operate at very large skirt clearances and this was an area where we used to see contact wear from time to time.
Crevice volume is actually a big deal in race pistons, just like in passenger car stuff. While we don't get as concerned with the emmissions aspect of having unburnt fuel trapped above the top ring, we are very concerned about the missed opportunity to convert that fuel into energy.
Current practice is to reduce that crevice volume as much as possible working against design and material limits of pistons and rings - the ring groove gets pretty thin around valve pockets, and that inactive area above the ring protects the popular really thin rings from full combustion heat - something of a trade-off.
This type of design idea is probably not the best solution given the crevice volume impact, but it was a decent and simple effort to solve a the-known problem. Piston skirt and ring land designs have taken some very sophisticated turns these past 20 years as we learn the impacts of temperature and load on the machined shapes. Things like vertical uptilt of grooves are old concepts that are far more highly defined now, as are multi-dimensional skirt profiles and even "non-round" pin holes in some stuff..
Come on guys, you're car guys. Almost all pistons have
August 9 2012, 2:53 PM
that ridge around the top to allow compression gasses to get to the rings. There has to be an expansion area for heat build-up above the rings, and if the piston were straight cut, then it would cause numerous problems. The measurement of a piston for fitment is taken approximately 1" below the pin boss on the skirts. It will be larger than the top of the piston. Joe-JDC.
Not sure what I would do with them. My initial intent of this post wasn't in selling them, but can't run them in this engine. The block was +.030, so that'll be what the pistons were for. No markings on the slugs anywhere except for Ross-Banks, and 425, on the underside.
There was a note I found with the parts stash that had a scribbled note: "Ross, #425, 580g,11.2"
They're mated to C6-A rods at present. Some surface rust on the rods, the pads have been ground at one time or another for balancing, buy I doubt you'd use these rods nowadays anyway.
I can measure the pistons (accurately) if you need more info.
Thank you for letting me know. I have a standard mb;pcl, and don't want to overbore it if I can help it. It wouldn't need more than 10 maxz anyway. Used to be able to find pistons in 5 over at one time. Doubt that there are any sets left out there.
Have a great day,