284 ci Flathead with 5-6 lbs boost. Street and strip use. Ross Forged: 2618 Alloy (part: 80578). Metric rings (1.5, 1.5, 3.0) Top ring moly coated Icon Forged: 4032 Alloy (part: IC626). Rings (1/16, 1/16, 3/16) Top ring moly coated * Rings listed are supplied with pistons Installation Recommendations. Ring clearance .005 per inch. Same for both pistons 2618 alloy .001-.0015 additional bore clearance. Differences: Ross piston: stronger alloy, recommended for boosted applications, bigger bore clearance, piston slap when cold, additional friction and wear. Icon piston: softer material, more prone to detonation issues, smaller bore clearance, less piston slap and wear. Uses very wide oil ring compared to other Flathead forged pistons **Scat rotating assemblies now come with the Icon forged piston and rings listed above. No customizations are available. Thoughts
I've never used Icon, but have had no issues with Ross. How much is a little noise at startup going to bother you, and are a tiny percentage more friction and wear really going to matter, unless you're driving tens of thousands of miles each year? How much difference in price?
I don't think there is much difference between the piston selection; I have known guys that ran stock pistons and head gaskets in sbc with roots blowers at those boost levels. I would worry about the bottom end more. I assume you are using a Scat rotating assembly. BTW I recommend forged pistons on any of my engines. I think I would use Cometic MLS head gaskets.
Good recommendation on using Cometic gaskets for a blown application. It is very important that your deck/head surface finish is correct for these head-gaskets - talk to Cometic before you have your machine work done.
Basically either one of them should work fine in a street application with only 5-6 lbs of boost. For that reason, I think I would go with the one that doesn't have piston slap when cold. Its always going to be annoying to hear a noise coming from a sizeable investment, and piston slapping is never a "good" thing even if its tolerable. If you get detonation with either piston type, its likely to cause damage although the stronger one may take a little longer. As long as your engine is tuned properly and you don't have detonation......either should be fine. Thats the logic I would use with any engine, not just with a flathead. If it was a racing engine, then I would go with the stronger piston because of the extremes that those engines endure. Note: I'm not implying anything wrong with using the Ross piston, but you will find people tend to ask you what that knocking sound is. Friend of a friend had a beautiful truck with show quality everywhere. Engine had piston slap upon startup. He couldn't stand it, and had the engine rebuilt. Its more about what works for your type of needs.
I've found that most of the flatheads I build have loud exhaust systems (many with no mufflers) - so I can't hear any potential piston slap anyway! LOL
I've always been a firm believer of letting the engine get up to temperature before driving, so it shouldn't really matter one way or another, about the piston slap... JMO.
It sounds similar to a loose rocker arm tapping..........but after you probe around and try adjusting the the rocker arms, its still there. Sometimes you can tell with a stethoscope that its lower in the engine, but you still adjust the rockers and hope thats what it is.............
Yep, but instead of a "ticking" sound that loose valvetrain makes, it's more of a heavy "clacking" sound like every piston pin is loose, but it only lasts for 30 seconds or so and then the pistons expand enough to tighten up the clearance, the assumption being that the pistons are fitted correctly.
Start up an early 5.3 (LS) Chev in cold weather and you'll know what piston slap sounds like! (Apologies for the OT reference but it's so common it's the easiest.)
I agree with your explanation, but its not necessarily all pistons in an engine that you can hear. The one I mentioned above only sounded like one piston, and it did not go away quickly.......it continued for as long as I was trying to trace the noise .....which was about a half hour on a lift. I turned the repair job down because I was afraid I'd scratch the paint. I'm just pointing this out so that the OP will know that generally the noise subsides like you said, but there is also a chance that it won't.........and if the car is driven a lot the clearance will increase. If its a race car and gets no real mileage, it should be fine.
That sounds like one wasn't fitted correctly if it continued to make noise for that long. Hence my previous comment "the assumption being that the pistons are fitted correctly." I use a machinist who is very anal about fitment and engine clearances. He creeps up on each cylinder size a half thousandth at a time with the hone when he gets close to the clearance he wants, measures to the ten-thousandths, and charges accordingly. Well fitted forged pistons (to me) are a joy to listen to at startup. On a cold startup I don't move the car until the symphony is over.
What is the price difference between the two? If it was me, the fact that the Ross uses metric rings is what I would use.
My LS5 is claimed/spec’’ed with forged pistons, I’ve never heard “piston slap” when starting it. Maybe a factory engine is different from aftermarket parts?
Call 'Ed' in sales at Ross - good guy and will work with you to help you get exactly what you need. Make sure you put the crank and a rod/pin in both banks and measure the exact compression height (current height - will usually vary side to side). In the end, you need to achieve a squish of .040 to .045. Contact Ross about dome height - usually it is .187. Do your math calculations so you know the deck height on each side. Usually, I square deck the block to ensure that both sides are exactly the same height - this is then used to determine the piston pin location (compression height). Standard stocking pistons for a given stroke will rarely get you exactly the compression height you'll want - which is why the above is necessary. I usually tune my compression height to within .002 of the final number of .040.
The Jalopy Journal
