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Piston to valve contact - need cast vs forged pistons advice
Pulled my cylinder heads and found some damage to my pistons. I made a mistake a year ago when I replaced the timing gear/chain and had the intake valve timing off. At the time, the engine just ran poorly. I corrected the issue, had my tuner do his thing, did two road track days and thousands of road miles with no issue. My question is, are these pistons usable? I think I know the answer, which is to replace them, but still just curious. Every piston has a nice, almost intentional looking valve relief. There's a little mushrooming around the relief, which I could clean up. Kinda crazy, but doesn't seem like the valves were damaged. Engine had excellent compression when I tested it after. I'm assuming these are going to be risky to run, in case there is some internal stresses or cracking of the piston that would only let go at some point.
So my question is what do I do for replacements? I picked up a new set of heads and a cam which was the reason for pulling the heads. These are PRC as Cast 225cc with a 62cc chamber, and a 228R cam from TSP. With my oem pistons I had calculated a 11.0:1 compression. When I was recalculating I realized I'd actually have a 11.25:1 compression. This seems rather high to me, even though I'd have my tuner do his work - not sure if this is borderline high for a street car that does road track days. I'm going to talk with my engine tuner this week about piston choices, compression, and all of this stuff. Just looking for internet opinions right now.
- I will be tracking the car as often as I can, about 2-4 times a year.
- Cam/head upgrades will necessitate making power around 6000rpm
- Will have a tune done when engine is done
- Will run 93 octane gas all the time
As for piston choices, OEM pistons appear to be about $100 each with rings - $800 total.
A set of 8 Clevite/Mahle pistons matching OEM, with rings, on ebay are $350. These have skirt coatings which seems like a nice add on. In terms of price, this seems like the way to go:
I'm wondering if a forged set would be a wise upgrade considering the increased hp and track time. Doesn't seem like there are forged "true" flat top LS1 pistons, but many have a slight dish to them, which would reduce compression a bit. I don't need the piston to valve clearance with the 15 degree heads, but oh well. What I did find was some from JE with no rings:
I'm sure I could find more choices with a -4cc to -2cc dish which would drop compression down to a more reasonable (in my mind) 10.7:1 or a 10.9:1.
So does the higher compression of oem pistons present a concern, or should I spend the money on forged to reduce compression, and have a more durable piston?
Thanks!
Last edited by KCS; 02-24-2019 at 10:47 AM.
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Yea that's a nice piston for the $. Just need aftermarket rods to go with them. If i hadn't of already bought a set of mahle forged i woulda bought those.
Connecting rods: Summit Racing is the first company to offer each piston kit with versions for the Gen. 3 press-fit, Gen. 4 full-floating, and a aftermarket forged 4340 steel connecting rods. No matter what connecting rod you have, you will find a piston that fits.
You're right, I did post the aftermarket version. These are the ones I think would be the best fit. I'll need to call them to double check, but with the right headgasket, I can probably tweak my compression ratio a bit.
We've mentioned it, but the 5.7L press fit pistons have a circlip groove for use with Gen. 4 rods, if one has those and they hone out the small ends .0015". Clips are currently separate but we may start including them.
Yea that's a nice piston for the $. Just need aftermarket rods to go with them. If i hadn't of already bought a set of mahle forged i woulda bought those.
Im sure its a nice piston , but for the money the Wiseco LS Pro-Tru piston is a great value at $400 . Maybe summit could tell us what their piston has to offer over a Wiseco or others. Wish there was more selection in size too.
No way the valve hit the piston hard enough to knock valve reliefs into the pistons and didn't damage any valves at all.
I'm with you, but compression numbers as measured by me and my mechanic on separate occasions tell a different story. Which tells me the pistons are relatively soft material. Not something I would expect to hold up to more power. It's moot since I have new heads. My engine tuner said when a spark plug blows to bits at the drag strip they just clean up the little damage and move on.
Im sure its a nice piston , but for the money the Wiseco LS Pro-Tru piston is a great value at $400 . Maybe summit could tell us what their piston has to offer over a Wiseco or others. Wish there was more selection in size too.
Agreed with others that the piston doesn't need swapped. The PTS523 series Pro Tru piston is pretty good, but they don't come in standard bore LS1. We have standard bore size and .005 over as well. We sell Pro Tru's for $491 including rings and price match if when you find them for less. Summit Pro LS are $499 and the ring packs are an additional $99.
Rings: Pro Tru's include a basic non-file fit 1/16 plasma moly ductile iron top, 1/16 cast second, 3mm non-file fit ring set. Our ring packs are a state of the art File-Fit 1.2, 1.2, 3mm with Nitride Steel top and Martensitic (ductile) Napier seconds. A lot less friction, better seal, better oil control, longer lasting, and able to take anything you can throw at them. Our piston ring lands are thicker due to narrower ring grooves. Our second land also has a pressure seal groove for better ring seal. Pro Tru does not.
Forgings: Pro tru uses older style of 4032 alloy forging called slab sided or slipper skirt, which isn't as strong or as light as the modern strutted forging designs we use. Pro Tru's weigh 463g, The pin boss span is wide (1.238 in.) which allows the pin to flex more around the factory small-end rod-eye width. Summit Pro LS are 2618 alloy strutted forging which are stronger and they weigh 457g. The Pro LS pin boss span is narrower at 1.142 which captures the factory connecting rod width perfectly...so much less chance of pin bending.
Pins: Pro Tru uses a 2.500 pin (125g). We use a shorter (114g) 2.250 pin. This allows our forgings to clear reluctor rings without notching the pin bosses when used with stroker cranks. This increases strength.
Dome and valve relief: The Pro Tru uses an as-forged dome with symmetrical valve reliefs. Ours is fully CNC'd with dedicated asymmetric reliefs so the dome is shorter for better combustion.
Piston skirt profile. Summit Pro LS pistons will rock less, so wear is better and they are quieter. The stroker pistons are more stable at bdc than others on the market because of this.
Pin oiling: Both have pressure fed pin oilers. Pro LS is a little better here with double pin oilers and horizontal slots for reduced pin wear.
Oil Drainbacks. Pro Tru are forged in and cut open when the oil ring groove is cut. Summit's are drilled and drop below the bottom oil ring wiper. This gives the oil a place to go on the downstroke, so oil control is better.
Weight: Nearly a wash. Combined weight of a PTS523a3903 463 (g), clips (2g), pins (125g) and rings (20g) is 610g. Summit Pro LS SUM-326943903-3 (pictured below) is 457g with 114g pin, 2g clips, 28g rings combined are 601g
I'm with you, but compression numbers as measured by me and my mechanic on separate occasions tell a different story. Which tells me the pistons are relatively soft material. Not something I would expect to hold up to more power. It's moot since I have new heads. My engine tuner said when a spark plug blows to bits at the drag strip they just clean up the little damage and move on.
How do you know 100% certain your valves hit those pistons and left those marks. I don't see it. Not even a little. The valve stems are tiny and bend easy.
I've bent valves before, and all I had on the pistons was witness marks, not deep pocket valve reliefs.
By the way, the pistons aren't that soft. To be soft enough for that too happen they would have to be as soft as 24k gold and there's no way a metal that soft is going to not deform around the wrist pin under all the load and stress, much less the heat in there.
I guess in this world full of weird ****, anything is possible, but I can't see this one.
How do you know 100% certain your valves hit those pistons and left those marks.
Sounds like you're suggesting the piston marks could have come at some time prior to when I thought they happened? Hmm... hadn't though much about that, but I guess it's possible since I bought it used with about 30k. They are now buried under enough carbon deposits that it'd be hard to guess how recent they were. Based on how off the cam timing was, the shop that corrected it said it was off enough that piston to valve contact would have been possible. But yet the engine held compression, made decent power and has held together all this time. That's probably b/c the valve reliefs were already there, and the valves had nothing to contact.
So someone's previous mistake, now saved me a bunch on a new valve job. woot.
How do you know 100% certain your valves hit those pistons and left those marks. I don't see it. Not even a little. The valve stems are tiny and bend easy.
I've bent valves before, and all I had on the pistons was witness marks, not deep pocket valve reliefs.
By the way, the pistons aren't that soft. To be soft enough for that too happen they would have to be as soft as 24k gold and there's no way a metal that soft is going to not deform around the wrist pin under all the load and stress, much less the heat in there.
I guess in this world full of weird ****, anything is possible, but I can't see this one.
I would agree with you .
He stated the heads were PRC as Cast 225cc with a 62cc chamber, so I am guessing some one at some point Fubared the OEM head's and valves at some point. Since there is carbon in there its been run that way for a while and not recent damage.
I agree that much of a dent would deform the the valve enough to cause low compression.
How do you know 100% certain your valves hit those pistons and left those marks. I don't see it. Not even a little. The valve stems are tiny and bend easy.
I've bent valves before, and all I had on the pistons was witness marks, not deep pocket valve reliefs.
By the way, the pistons aren't that soft. To be soft enough for that too happen they would have to be as soft as 24k gold and there's no way a metal that soft is going to not deform around the wrist pin under all the load and stress, much less the heat in there.
I guess in this world full of weird ****, anything is possible, but I can't see this one.
Exactly what i was thinking, Ive bent valves before too and not even close to leaving a mark like that.
And every single pistons looks like that???? someone has defiantly cut those reliefs in there
To be honest you dont really need forged pistons foe what you are doing.
We have complete rebuild kits with everything you need (minus lifters) for $700 shipped. Thats basically the cost of a forged set of pistons and bearings or maybe balancing if needed also.
The point is, they are drop in, super durable, and inexpensive. We have some guys pushing these to 7k with no issue!!
Once I clean up the pistons I'll share some pics. There is definitely some "mushrooming" of the relief, so it seems like it was a valve forced into the piston top, not a machined relief. Came out a sideswiped Camaro with 30k miles.
Going to drop off rods tomorrow to get checked/resized with the ARP bolts - if they are reusable, then I can decide what to do on pistons. I wish I could find a forged 0cc piston for the 3.898" bore and then I'd be able to run 11:1 compression with the heads and cam I have. I could just reuse the pistons, but I'd rather not risk it considering the new $$ 62cc CNC heads I'll be running. I see Forged pistons from Mahle with -4cc reliefs for under $600, and still keep a 10.5:1 compression.
WS6Store, if you can point me to something that might work within the range I'm looking for, I'd appreciate it.
I bet they were flycut. Unless those are the exhaust sides of the piston, then they could be DOD pistons. But then they would be a 4.065 piston and not a gen 3
What engine is this anyway?
Clean the pistons real good and you may find more info on what those are and how they got there. The mushroom could be carbon buildup or from a valve that made contact with the edge of the relief. The relief if flycut, may be off a hair from the right spot and that's how the mush room got there. But who knows man, it was modded there's not telling what happened when and where and I bet some stuff happened before you got it and you're just thinking that when you had the cam issue it caused all the issues. But it looks like there's a story from before you had it.
There's plenty of decent power cars running around with flycut pistons too so if you're concerned about strength they should be fine.
Again, if you got money and time go ahead get some new forged pistons with reliefs, if it's a strain or you'd rather not just run it
Also, don't worry about 11:1 compression. That's stock LS2 compression.
That 228r cam is going to drop your dynamic compression a hair and dynamic compression is the number you focus on to see if you can run pump gas or not.
So I personally would aim for 11:0-11.5 ish. And I wouldn't be afraid of approaching 12:1
02-24-2019, 09:34 AM
Piston to valve contact - need cast vs forged pistons advice 
