dhirocz

iTrader: (2)

Just finished blueprinting my motor. 350 Vortec, just polished the crank, put in new ARP waveloc rod bolts, ARP main studs, new freeze plugs, glazebreak cylinders (no ridge), clean all gasket surfaces, hot tank block, etc.all clearances are OK. I think.

I have some D-cups std. bore 350 pistons for my motor. decent shape, plan on using these. I know clearance is supposed to be more with forged pistons (first engine with them). Looking it up, I found recommended specs between .006 and .0075". I have measurements of the bores averaging 4.001 to 4.0015". The pistons (TRW forged) measure 3.994" at the wrist pin centerline. that would give me an average clearance of .008-.0085". Should this worry me? I have heard of people running these things loose, but that loose? The skirts fit OK in the bore but the tops of the pistons are very loose, I'm assuming the rings make up for this. Any reccomendations on clearance? I dont want a motor that sounds like a can of marbles. I think I'll be OK, but I want this one to last a little while. Also, about rod clearance (GM tech, help out a fellow tech), they are typically .018-.026". Mine measure .010, .012 MAX. I know the newer 350 have high idle oil pressure, I'm just wondering if this should be a concern. I wouldn't think so, they all mike out the same. Just double checking, an 18 year old can only build so many engines and judge clearances so much up to this point. Thanks guys

RB83L69

If your skirts measure 3.994" and your bores are 4.001-4.0015", then you have a bore clearance of .007-.0075". This is fine. Frankly, anything up to .010" will work OK, it just may make a little more noise when cold. If the clearance is too low, they run the risk of seizing to the bore under long-term heavy loads. I'd rather have mine loose than tight actually. That has nothing to do with the rings being "loose". The part of the piston where the rings go is usually about 1/16" smaller in diameter than the skirt part.

Rod side clearance is typically about .012-.015 in a stock set of rods and crank, so yours are in the typical sort of range. That isn't a really critical measurement; a good description of what you need would be "some" but you don't want "too much". Real specific, huh? As long as they have room to move without so much that the piston pins hit the bores, they're OK.

Newer 350s don't have any different oil pressure from any other 350s, that's determined by bearing clearances and the oil pump you use. My preference is a regular M55 pump, with the IS-55E drive shaft (metal sleeve instead of plastic), with the Mr Gasket #26 spring installed in the pump.

The 2 most important things are your bearing clearances in the mains and rods, and the way the cam bearings are installed. You should ideally have .001-.0015" in the mains and the rods, no more than .0025" and no less than .0005". I prefer mine as close to .001" as I can conveniently get them. The cam bearings should be in the block such that the oil holes are at about 4:30-5:00 when viewed from the front. If they are up at the top, they will create a significant oil leak since the cam is always being mashed to the bottom of its bore by the lifters; but if the holes are exactly at the bottom, the cam bearings may wipe out due to oil starvation.

I like to drill a tiny hole in each of the 2 plugs to the sides of the front cam bearing (not the one directly above it) to oil the timing chain. A #55 to #60 drill bit is about right.

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"So many Mustangs, so little time..."
ICON Motorsports

AJ_92RS

RB83L69,

Just a couple quickies.

1. Don't forged pistons expand under heat more than cast & hypereutectic slugs? and approx. how much? So will this decrease that slightly large clearance that he has now?

2. For lack of a better term, what is your logic on running bearing clearances that tight? Not intended as an argument, but I've read, heard, and seen that .0020-.0025" is a general rule of thumb for most engines. I guess it's around the same as the molecular structure dimensions of fossil oil (although not synthetics). And if you are running higher rpm, say around 6500-8000rpm, then .0028-.0032" is supposed to be optimum due to the oval shape that the bearings form, and pressure increases on the oil, at rpm levels in that range due to inertia and stress on the bearings.

Again, not for argument, just knowledge. Just cause I've always heard, seen, and read it, doesn't make it right. Kinda like 65 yrs. ago, everyone thought that if you went over 100 mph, that the forces on your body would kill you.

JoelOl75

The pistons already come with the 'looseness' ground into them. As stated you need more clearence with forged, but at standard bore (4") they should have proper clearence (+/- .0001") right out of he box.

Hypereutectics generally run with .001" to .0025" of clearence although this is for NA street cars. Speed Pro ups this figure even to .0055" for supercharged nitro drag cars (Hypereutectics in these things??? They must have alot of faith in their product!)

Grand Prix

I wouldn't run a set of pistons on the street with 0.0075" clearance. If the pistons require that much get a set made for street use. They are going to beat on the cylinder walls when they are cold and make quite a bit of noise. Piston ring life will be shortend by the abuse seen when the engine is cold. You are ok as far as meeting the recommended piston clearances, so long as the bores are straight and clean. For a performance engine 0.001"-0.0015" rod clearance can cause the crank to turn blue from heat and damage the bearing/crank surface. I have a crank right now that was set up for 0.001" clearance. Even with a mild(er) 400HP the bearings were running hot and losing lube. That is what happens when someone builds an engine "real tight". I don't like to go above 0.0025" even if the engine is running high RPM. The reason is that the rod is going to expand some at high RPM. If you already have 0.0035" and it expands you could go as high as 0.005" in operation. Just wayyyy too much for a small block. For a performance engine 0.0015"-.002" is generally best on the rods. 0.0025" is fine, but don't go much over that. On the mains 0.002"-.0025" is preferred. The oil is part of what keeps the bearings cool. If the clearances are too tight you can fry the crank and bearings under power. On a stock engine this isn't as much of a concern, RPM is generally low and power output is low. So with stock bearings 0.001" to even 0.0005" could be work.

A point on the hypereutectics. This material is kind of funny. They expand almost nil to a point. After that they expand drastrically. I have no scientific explanation to offer. Worse case I ever saw of it was a drag car punched 0.030" over running stock bore pistons. It made a horrible noise till it heated up, and then quieted right up. Under the heat of a supercharger and nitromethane they are probably lucky to get away with only 0.0055". I am not 100% convinced they run hypereutectics. After knowing a few pro racers you begin to find they run whatever makes them win. Then they dress up the parts to look like their sponsors to keep the advertisement guys happy. Who has torn down an engine after a run to check the piston material? Would anyone other than the Keith Black advertisement department care? Given the brittleness of the metal and the shock loads seen in a nitromethane car it is not a likely piston choice.

RB83L69

I prefer to use tight clearances on the bearings in street motors so that they last longer. The clearance will open up some after a short period of running, from the bearing getting seated into its bore. This usually amounts to a good .0005" in almost any engine, sometimes more. They'll do this even with no visible wear. A racing engine does require more clearance, for a number of reasons; for one, they don't usually run even 1000 miles, so longevity of that sort isn't an issue; and they don't have a "break-in" period unless they're run on a dyno before installation, so they have to be built already broken in as much as possible. I liked to use .002" or so when I was building motors for round-track racers. For another, the crank heats up more than the rods or the block when it's heavily loaded, so clearance goes away, so you have to start out with a little extra. This motor doesn't appear to be such a thing though.

I've never had any experience that I would classify as "losing lube" in any motors I've built (not that I've built very many, probably not more than 50 or so by myself, and maybe twice that many in conjunction with other experienced builders) except when something elsewhere in the motor failed, resulting in low oil pressure. This also allows the use of thinner oil, which has lower internal friction ("viscosity" and "internal friction" being the same thing), which will give better gas mileage.

I've also done re-ring jobs on motors that were deliberately honed out to .004" or .005" beyond their nominal bore, with pistons designed for .005" clearance, and had them work fine in spite of the .010" of clearance. Of course they sounded like stuff was going to fly out the side of the block when they were cold, but they raced just fine. Taper in the bore is a whole lot more of a problem than the absolute amount of cleaarance. As I stated before, I'd rather have them too loose, than to set them up tight, and have them grow to a diameter greater than the bore when they heat up, which will make them seize to the bore. IMHO .007" is about right for forged pistons, because they do in fact expand more than most cast ones do. BTW the same thing applies to ring end gaps: sure it would be great to have as little gap as possible; but I'd a whole lot rather have a slight amount of blowby when they're cold, than to have them grow large enough that their ends butt when they're hot. Instant destruction.

Everybody has opinions about how this should be done, especially people who have never done one or who have done very few and haven't had the opportunity to tear back down their own build after it reached the end of its useful life. There's alot of myth and unscientific guesswork and "I heard" kind of lore out there, as well as people who will tell you all about how a motor who should be built that can't get one to last 10,000 miles, let alone 100,000 or more. I've seen the insides of a couple of my motors after more than 100,000 miles, and some others after something else broke in a race; I have yet to see any evidence of improper clearances.

Like GP said, oil is the coolant for all those moving parts, as well as being the "lubricant". The goal of the oil system is to guarantee that the metal parts never touch each other; the oil is supposed to be a film in between them that has enough resistance to the pressure applied by the parts to keep from being forced out from between them.

Oil molecules have diameters in the sub-micron range. If they were .002", you'd be able to see a little pile of them just like you can see the grains of grit on 500 sandpaper (500 per inch = .002" each), and they sure wouldn't go through an oil filter medium designed to trap particles in the tens of microns (.000010") range.

All in all, dh, your numbers so far sound OK to me. I'd have no problem with building it.

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"So many Mustangs, so little time..."
ICON Motorsports

Damon

You're at .007-.0075 clearance, not .0085, assuming you typed in the numbers in the post correctly. So you're right there.

Yes, forged pistons expand more than cast or hypercowabunga pistons. That's why the extra clearance required when using them. They ALL rattle when cold due to the larger clearance required. When warm, they're quiet as a church mouse, jsut like cast ones are. That's just the nature of forged pistons.

DO NOT MEASURE ANY CLERANCES AT THE TOP OF THE PISTON! It's a MEANINGLESS measurement. ALL pistons are slightly barrel shaped at the top. That's becuase the top of the piston takes all the heat of combustion, runs hotter than the wrist pin or skirt area and, therefore, expands more. Once they're heated up, trust me, they're a LOT tighter at the top, too.



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dhirocz

iTrader: (2)

Cool guys, this helps out alot! Thanks!