Hey folks, I have a motor at the machine shop at the moment, it had less than 1000 miles on it when I was forced to tear it down. Here are the specs as far as I know (it was handed down to me from my dad, so I cannot be 100% accurate). TEARDOWN was due to two busted pistons, lost 110#'s of compression on Cylinders 1 and 5 (had 60#'s) all others had 170PSI. This is believed to have happened due to poor quality fuel and misadjusted timing. No cylinder wall or block damage..... got very lucky.
5.7L 350 block .040 bore
906 vortec heads with guide plates and screw in studs (Heads possibly shaved/valve job etc.)
.480 lift cam (I know this is sitting at the maximum lift without head work) I believe it is made by comp cams.
stock 350 crank
heads had the stock 80# vortec springs, but I am opting to upgrade to 105# springs this go around
Block is believed to be 0 decked, with the piston at TDC it is even with the block
Holeshot intake
Holley 650 mech. secondary carb
Flat top pistons with cast Mahle rings
Assault Racing 65K distributor
1.5 roller rockers


Here are a few questions I have
1. Machine shop recommends I run dish pistons (or I will certainly be tearing the motor down again).
2. What gasket set should I look into using (originally used the KS 2666 set from felpro)
3. Ball park, what CR am I running with flat tops vs dished pistons, and am I going to see a significant power decrease in doing this?

 

Those heads don't use any different gaskets from any other SBC heads. Intake gaskets of course are specific to them though, therefore the set is as well.

The FelPro set is fine. Look it up for a 98 Silverado 5.7 or something similar.

Compression will be in the high 9s to low 10s with flat-tops, those heads in stock condition, and the typical .040" or so gasket. Hard to say anything closer without knowing the piston deck clearance (almost certainly greater than stock) and any work done to the heads.

 

Thank you sir, I know the block is just a old style 5.7 block (the truck is a 1970 c10, but I'm not sure if it's the original block from it).

Will I be suffering much going to a dish piston vs a flat top per the shops recommendation?
Intake is a single edelbrock super victor
Transmission is a TH400 with manual valve body and a 3000RPM stall

 

Your compression ratio and cam are a hand in hand affair. Do you have any idea of the cam specs?

 

I believe (and I cannot verify until the machine shop pulls it out) it is a comp cams 280H Jegs link: https://www.jegs.com/i/COMP-Cams/249/12-212-2/10002/-1

 

Taking what you specs you given, a flat top piston at zero deck and a .040" gasket will be around 10.4:1. That cam is 280 advertised, 230 @ .050" with an intake valve closing at 66 ABDC.
That would put your dynamic/effective compression ratio around 8.2:1. My past experiences have demonstrated that this is difficult to manage on pump gas on the street. Especially if there are no modern fuel and timing controls in play. Getting your CR to no more than 10:1 will make things easier to tune and help to keep you out of spark knock. That half a point drop might be about 2% in the difference of total power output all else being the same.

 

Or, 9.5:1 if it has flat-tops w/ 6cc valve reliefs and .045" of deck clearance, which is probably underestimating it, and calling the heads 62cc.

So there you have it... an upper-ish limit of 10.4, and a lower-ish of 9.5. Chances are, it's somewhere in there. More likely toward the lower end of that range.

Not the most exciting cam in the world, in 2020.

Personally I'd go with the flat-tops.

 

The 10.4 was based on the OPs comment. But yeah, without hard measurements, it's all a guess. I can agree on the flat top piston if the CR is in line with what's manageable.

 

Stock deck clearance is .025", +.010" or so, -.000". "Rebuilder" pistons, which are the majority of them out there though by no means all of course, add .020" to that. And even THAT is, assuming that the crank still has the correct stroke, and wasn't ground "on the wear", which might knock anywhere from .004" to .015" off of the stroke, AND add a further .002" to .008" to the deck clearance.

Since the machinist doesn't even have the new pistons, the block isn't zero-decked, by the very definition of the term. If it had dishes in it before, the odds of it having been zero-decked to those are somewhere between infinitesimal and quantum-mechanical... IOW, about as close to zero as you can get while still allowing for the possibility that it might be true. NOBODY zero-decks a block to dishes; it makes no sense at all to do that. And further, I doubt any competent machinist in his right mind would spend the time effort and money on zero-decking some random block, and then would leave those stock trash valve springs on the heads. Consequently I don't share the OP's "belief", and chose to ignore that comment altogether. I'd welcome being proven wrong though, for his sake. And of course, it might have been decked some, but even if so, who knows whether it was "true" (with the decks perpendicular to the crank axis in the one plane and parallel to it in the other, and perpendicular to each other). Meaning, the compression could EASILY vary by a quarter point or maybe more, from the cyl with the least to the one with the most.

And I COMPLETELY agree, without ACTUAL PHYSICAL REAL TEST EQUIPMENT measurements, it's all a guess. Having long experience with 70s blocks and real-world "rebuilds", I tend to guess pessimistically.

 

It's all guesses on my end, and I didn't exactly want to start trying to do anything with it without it being checked out. It was machined 4 or 5 years ago at a local machine shop, the owner happened to pass away not long after, and it's now at a different shop/acquaintance of the previous builder.

I know the pistons that I pulled from the block were flat top .040 pistons with 4VR cut into them - I cannot tell you the brand, CC's anything like that as I didn't purchase the parts, assemble it, or anything inbetween. My dad did all of that and once he had the issue with the two cylinders he decided he didn't want to mess with it and passed it my way. when the piston was at the top of the cylinder it was even with the block..... just my assumption that it was decked, but no measurements were taken, if it was decked - it would have been decked to the flat tops that are now pulled out (all parts, bolts, etc. are sitting at the shop with the machinist as he said it all needed inspected after the piston failure.)

I hope to hear back from the machinist within the next 2 weeks, and asked him to let me know if he finds any issues, abnormalities, or pluses? that he may run across in his investigation of the block.

 

Case in point which about the 70's vintage blocks. My 2nd go around into racing started with a zero mile 350 that was plucked from a high school shop class demo. Into that went new hyperutectic flat top piston (5cc). The build sheet from the shop measured the decks at a range of .030" -.036" down. I confirmed this with a dial indicator and bridge micrometer. That would be inline with the values mentioned. Combining that with a .026" gasket and the Vortec heads (which I'll call 64 cc) brough the CR to 10:1. Piston to head clearance was little excessive at .060" but it ran very well with a range of cams.
Your shop will be able to tell what the current deck height is.

 

The 4VR pistons usually have about 6cc more chamber volume than 'totally flat", counting the VRs and also the usual chamfer around the outside edge.

At a certain level, if you're just going to buy stuff to fix what's broke and put the same heads and the same cam back together, it hardly matters what the compression is; you're not doing anything any differently because of what you learn, it just is what it is when you're through. If you don't DO anything with the number, that is, make a decision differently in some way because of what it is, it's pointless to even worry about it. But if you assemble the heads to the block, you can at least check things like the deck clearance and the stroke, and make a reasonable estimate. Use a straightedge and feeler gauges to measure deck clearance, and be sure to check at least the 4 corner cylinders. For the stroke, use a caliper.

I can't see your parts, and have no "knowledge" of any of what you've got, of course; but in my life experience, DAMN FEW blocks are ever zero-decked. It's tedious and expensive. About the best you usually get is "cut" some random amount, like .010" or whatever. But even assuming that that's "straight" in the sense that it took a uniform .010" off of the deck all the way across, that still leaves the factory's original completely crappy tolerances in effect. I've seen untouched (factory) blocks with over .010" of difference in the deck between the front and rear cyls, and a like amount between the left bank and right bank. It almost never gets better during a "rebuild".

And once a crank has been ground, all bets are off as to whether it has the "nominal" stroke, or whether all 4 journals even have the same stroke; it hardly ever does, first because it probably didn't from the factory but was almost certainly less than "nominal" to begin with, and second because the journals wear at TDC, and if they grind the crank "on the wear" (by far the most common approach... there's less chance of scrapping it that way) it not only leaves the factory's inaccuracy in it, but also adds that amount to the deck clearance and subtracts TWICE that much from the stroke. Again, like accurately decking a block, it's possible to "index grind" a crank, and yours might have had that level of detail applied to it; but it isn't cheeeep, and consequently is VERY RARELY done to a street build.

So I'm just kind of going off of what I've become used to finding whenever I've worked on a "rebuilt" motor. In the vast majority of them, only just enough work to get them back to operational, was all that was done. I seriously doubt that yours is any different. It's just too much the way the world works.

I'm sure you've heard of or seen engines that run circles around others that are built out of the exact same parts; and no doubt have seen ads, such as for Edelbrock engine packages for example, that claim "435 HP" or whatever, but your buddy that bought the kit and slapped it in his as-is motor didn't even get 335 out of his. It's not that the ad is necessarily a "lie", but rather, that what they DON'T tell you is sometimes as important as the bill of materials (the stuff they're selling) that they DO tell you. Attention to detail is what makes the difference. No doubt Edelbrock took great care building their test mule; selected a block with no core shift, carefully measured everything and machined it as perfectly as they could get it, trued the decks up so that they're perfectly parallel and perpendicular to the crank as described above, bored the cylinders so that they're parallel to each other and the 2 sides are EXACTLY 90° apart and are all EXACTLY perpendicular to the crank and pointed EXACTLY at the crank centerline and EXACTLY at the center of the journals, index-ground the crank so that it had EXACTLY the correct stroke and all the journals were EXACTLY 90° apart, bored and honed it with a deck plate, and so forth; but every one of those details that The Other Guy DIDN'T do, cost power. It's the same deal here. If you "assume" stuff that isn't true, you'll be just like the guy with the "435 HP" kit that can't seem to figure out why he got 100 HP less than that out of his supposedly "the same" motor.

 

It's really hard to see .020-.025, did you notice any stampings on the decks? Usually there are a few letters stamped here and there and easily definable. Any SBC block I've seen 0 decked, the light stampings with the id from the assy plant etc on the pad are not visible. That's kind of a first clue.

If you are zero decked and running flattop pistons, with 62cc heads, your machine shop did give you good advice for a street engine and using dished pistons.

The other thing is it's possible, due to cost, the build with the flat tops were with Hype pistons, they are super sensitive to ring gap setting being spot on. Hypers come in several different pin heights, rebuilder pistons usually have the pin height .020 or higher on the piston.

Now if the block was not zero decked, then using a dished piston with a higher pin location will put the piston way down in the hole, so you won't want to be using a rebuilder piston. The cost then go up to a better piston to put you where you should be, CR wise.

You be wise to check the deck height or have the shop check it, as well as the current pin position on the pistons being used to check the deck height.