Piston-to-head clearance w/ piston rock?

[pparaska]

To measure deck height, I use a magnetic base and dial indicator, and find TDC by tapping the piston down on the way up to TDC from both directions using a rubber mallet in the center of the piston (make sure to watch at the edge for the rod bearing bouncing off the crank - I tap it down just enough to take up the clearance.) Do this from both directions to about 0.020" below TDC, record degree wheel readings, split difference, move to that degree reading.

 

Then I use the bridge and dial indicator to measure the front and rear (above the pin) near the piston edge.

 

For/Aft deck height measurement (front and rear edges of piston):

 

Rear (towards flywheel end of crank):

I rock the piston about the for/aft direction (yes, the pin allows this to happen) by tapping the piston down at the rear, measuring for max reading at the front, then move the bridge to where I was just tapping (the rear) and measure the height relative to the deck next to it - that's the minimimum or lowest deck height for that position around the piston. Then I tap the opposite diametrical position on the edge of the piston (the front) to get the highest reading with the bridge, and record that.

 

Front:

Move the bridge/indicator to the diametrically opposed position at the (front) edge of the piston and record the deck height there which will be the minimum or furthest down. I know that just-tapped area is at it's lowest, so I record that as the minimum. Tap at the rear of the piston, and record the max registered on the dial indicator (highest).

 

Deck Height near valley and near block outer edge:

Repeat the above, but measure at the piston edge near the lifter valley, rocking piston both ways, and then measure near the block outer edge, rocking piston both ways.

 

Need some advice on which values to use from above method!:

O.k. So I did the above method on my 400 SB Chevy today. This engine has the Probe SRS forged pistons, and they were fitted to the minimum recommended NA piston-to-bore dimension of 0.002" (yes, that's tight for a forged piston, I know, but it's the bottom of the recommended spec that Probe has for them!) These are short pistons, for a 6" rod (1.125" compression height) and the skirts are fairly short too. What I found was A LOT of piston rock (in my opinion) for such a tight piston-to-wall clearance:

 

Typical readings: (this is cylinder #2, highest recorded heights on even bank)

Outside of block: Min: -0.002" Max: +0.023" (- is below deck, + is above)

Valley side: Min: -0.004" Max: +0.022"

Toward front: Min: +0.002" Max: +0.019"

Toward rear: Min: +0.002" Max: +0.019"

 

Yeah, the machinist went nuts decking the block, huh!?

I was really surprised I could rock the piston for aft that much too! I don't know the pin-bore clearance (I didn't assemble the shortblock), but they are the pins the pistons came with.

 

Anyway, I'm trying to figure out what gasket thickness to use. This is a 5140 I-beam steel rod (600grams each) engine, Scat Cast 9000 crank, and the pistons are fairly light (552 grams with pin). Redline will be 6500 rpm, peak power around 5800rpm (DesktopDyno2000 estimate) - the cam is pretty tame.

 

I've read about people going with high 20s to low 30s for piston-to-head clearance, and think this would be possible in this instance, considering the light parts, steel rods and lower rpm. Am I correct?

 

What concerns me is piston rock at TDC. With a deck height variation from -0.004" to 0.022" (range of 0.026", average of 0.009") at the quench side of the head near the cylinder wall, what number should be used to calculate the piston-to-head clearance?

 

If I use the conservative maximum height of +0.022" (above the deck), and shoot for a piston-to-head clearance as low as 0.030", that'd mean I'd need a 0.052" gasket. I have a Fel-Pro PN 1044 which is 0.051" thick. Would that be o.k.? Obviously, if I go less conservative in my choice of what deck height to use, I could use the average at the valley : +0.009". For that a 0.039" gasket would work (PN 1014 - have that one too), to get a 0.030" piston-to-head clearance at the quench area.

 

The other deck (odd number cylinders) has minimum deck heights at the valley side from +0.009" to +0.011", and maximums from +0.025 to +0.027". Averages go from +0.017" to +0.019" The machinist must have been further into his 6-pack at this point!

 

So, for this deck, the conservative choice of using the max number (+0.027") would mean a 0.033" piston-to-head clearance would be obtained with a +0.060" gasket. Using the largest average deck height (+0.011"), a 0.030" piston-to-head clearance would result from using a 0.041" gasket (PN 1034). Or I could just use a 0.039" gasket and have a 0.028" clearance.

 

Any advice (beyond throttling the machinist ) is appreciated!

[Tim240z]

A recent post by grumpy...maybe some help?

http://forums.hybridz.org/showthread.php?t=102632

[pparaska]

Close, but I'm looking at piston-to-head clearance.

 

That's a good post though!

[jt1]

Pete, that amount of rock from the valley to the outside is not uncommon with short skirt/high pin pistons. It's one of the disadvantages of the combo. It sounds like with the piston square in the bore you have 10 - 11 thou + deck height. It would be better if it averaged about zero, but the block may have been previously decked. I have used 35 to 40 from the average piston height many times with no bad results so far. There will always be a few thou variation unless the machine work is super close Cup style stuff, and all the variations in the components are worked out, so don't sweat that too much. I would use a ~ 50 thou thickness gasket with what you've got.

 

The fore and aft rock worries me. IMO that shouldn't be over 2 or 3 thou. It sound like the pin to rod clearances are excessive, or less likely, the pin to piston clearances. I know this isn't what you want to hear, but I would tear it down and have that checked. About a thou or a little less is desirable, and I can't see how you would get that much fore and aft rock unless the clearance is excessive. If the piston/pin/rod clearance is excessive, that baby won't run long.

 

Maybe Grumpy will look at this, I'd like to hear his thoughts.

 

John

[blueovalz]

Fascinating post Pete. I'm in agreement as to being puzzled by the fore/aft measurements. I'm going to check mine tonight when I get home as I know my port/port variance is similar to yours (but my skirts are longer with an OEM compression height, but the fore/aft measurements are very small (my pistons are pressed through the rods). Now, my pistons will slide fore/aft a bit, but does not rock in this dimension. I'll measure mine tonight and compare.

[grumpyvette]

what your looking for if Im reading the thread correctly is the MINIMUM and MAXimum QUENCH distance that will allow operation with both effective SQUISH and no contact of the rotating assembly to the cylinder heads.

Id suggest a mnimum of .035 to allow for rod stretch and piston rock and a maximum of about .044-.046 to retain at least minimum squish to get that jet of super compressed a/f mix thrown into the central combustion chamber to cool and speed combustion, limiting the potential for detonation.

but Id also point out that theres very few factory engine running that tight on piston to cylinderhead clearances as the manufacturers are far more concerned with potential contact of rotating parts than getting the squish(quench) effect maximized

run less than about .035 thousands and at high rpm levels the pistons might hit the cylinder heads, run more than about .044 thousands the QUENCH effect of forceing the fuel air mix to the center of the cylinder from the cylinders edge area looses both speed and effectiveness, remember the quench area must be so tight that virtually all the fuel/air mix is forced into the center area and none is allowed to burn untill its squirted into the burn area increaseing turbulance and burn efficiency

in theory the much better quench, combined with the shorter more compact area the flame front needs to cover and the far higher turbulance combine to allow more of the pressure to build AFTER the crank passes TDC on the end of compression and begining of the power stroke

 

its mostly an advantage in that you get a more even burn in the cylinder and less chance of detonation.

look, it takes approximately 40 thousands of a second for the flame from the ignition to cross a 4.25" bore,at low rpms and still takes about 15 milliseconds at high RPM due to the much faster movement of the compressed fuel air mix in the cylinders, lets look at what that means

if the chevy plug is located 4/5ths of the way to one side thats a time of about 32 thousands for the pressure to build as the flame travels 3.4" in the chevy but in a compact combustion chamber it could only take the cylinder flame front less than 10-20 thousands of a second to travel acrossed the combustion chamber for a complete burn at low rpms, this of course speeds up as the swirl and turbulance increase with increased engine RPMs but the ratios stay similar. this results in more useable energy WORKING on the piston AFTER IT PASSES TOP DEAD CENTER ON THE POWER STROKE. BUT MODERN WEDGE combustion chambers use increased QUENCH to speed the flame front and lower the burn time combined with a smaller combustion chambers look at this chart

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

 

keep in mind that the cylinder pressure starts, builds to a peak and drops off all before the piston moves more than about 1/2 inch away from TDC and that if your wasteing 10-20 degrees of rotation compressing the burning mix in a slow to ignite combustion chamber your wasteing engine power

http://naca.larc.nasa.gov/reports/1939/naca-tm-914/

 

http://www.me.gatech.edu/energy/ICEngines/8_CylinderCombustionProcesses.pdf

 

http://www.nedians.8m.com/Comp_IC.html

 

http://mb-soft.com/public2/engine.html

 

 

 

 

LOOK CLOSELY AT THESE PICTURES

you only have QUENCH if theres a flat area on the piston that mates to a matching flat area on the combustion chamber roof, on these pistons dual quench areas throw the compressed fuel/air mix to the center from the twin quench areas

notice, if used with this head, that only one side would have a fairly large and EFFECTIVE QUENCH area ,(the side away from the spark plug)

 

things to read

http://chevyhiperformance.com/techarticles/94138/

 

http://www.theoldone.com/archive/quench-area.htm

 

 

 

http://racehelp.com/article_racing-10.html

 

Ive generally used KB HYPEREUTECTIC or SEVERAL BRANDS of FORGED pistons (mostly from SUMMIT,JEGS,or J&E and TRW, , getting tighter than about .036 quench has generally led to indications that the pistons have come very close to contact at times, I try to stay in the .037-.045 range simply because I personally feel that getting the max quench is FAR LESS IMPORTANT that avoiding piston to head contact.BTW I generally use AFTERMARKET (H) style rods, with 7/16" rod bolts by ARP and floating pin pistons and SELDOM build engines that exceed 7000rpm , theres not much to be gained in my opinion by spinning over 7000rpm except potentialy increased valve train problems ,if thats any help, and I generally use SOLID LIFTER CAMS in a serious 355 due to thier effective opperating rpm band (4500-7000rpm,Ive noticed that STOCK chevy 3/8" rod bolts on STOCK reworked rods DO TEND to stretch more!

I built a 355 with .028 quench and 12.7:1 cpr(ON REQUEST) that had light contact and needed to use thicker gaskets, so thats MUCH TOO CLOSE

look at this

http://www.vips.co.uk/demos/mech/con_rod/vm_anim.htm

 

BTW YES BEFORE YOU ASK...cylinders to cylinder variations should be minimized but don,t get crazy if some cylinders have a thousanth or so more or less, rods and pistons do vary in dimensions, don,t get crazy over a thousandth or so varriation

 

http://www.scegaskets.com/products/procopphd.html

 

these come in .021-.080 thick head gaskets in about .010 steps(youll need to order them)

Ive used them for years with zero problems on dozens of engines WITHOUT (O)RINGS, Im currently useing them on my corvette

 

BENEFITS OF copper HEAD GASKETS 1. Conductivity. copper is the standard by which all other conductors are measured. Therefore, a copper gasket provides superior thermal conductivity and stabilizes head and block temperatures which makes tuning easier. 2. 25% coefficient of elasticity. One of the properties of copper is that it stretches before a catastrophic failure, thereby providing an extra measure of safety in case of severe detonation. 3. Strength. copper (in the form we use) has a tensile strength of approximately 32,000 psi, compare this to the 1,200 to 1800 psi tensile of most facing materials used on conventional performance head gaskets.

 

 

Yes, they can be reused several times as long as there are no signs of failure, such as carbon tracking or corosion damage if they are carefully cleaned before reuse.

I have been useing SOLID COPPER HEAD GASKETS for years with aluminum heads on iron blocks (WITHOUT (O)rings) If your surfaces are strait and true and you correctly install them they work fine, now keep in mind that you MUST run high concentrations of anti-freeze and an anode in the radiator sure does not hurt to prevent electrolosis from causeing problems but I have never yet lost a head gasket and that includes nitrous use on several engines. now they sure are not your only option but they are a good one. btw I totally clean and degrease the block deck and head surfaces then spray the head gasket wet with COPPER COAT GASKET SPRAY then install them tacky wet and torque them down in 5LB stages to factory spec http://www.jcwhitney.com/productnoitem.jhtml?CATID=5131&BQ=jcw2 RADIATOR CORROSION INHIBITOR Prevents overheated radiators caused by rust, scale and corrosion. Save money on needless flushing, repairs, anti-freeze changes, special additives! Zinc anode slips in radiator filler neck and neutralizes rust/corrosion-causing chemicals. Lasts for years. NOTE: Not for radiators with plastic tanks.

SUMMIT RACING CAN ORDER YOU A SET

 

 

many, perhaps most copper head gaskets Ive seen and used are NOT embossed they are simply dead soft copper sheets with holes in the correct locations...unlike the comon stamped steel gaskets nost guys are familiar with, and again, let me point out I spray them down on both sides fairly heavily with copper coat spray then install them between a CLEAN and degreased block and heads and torque them down in stages

 

http://members.tripod.com/torquespecs/gmfs70-88chv8.htm

 

FELPRO engineers are no-doubt totally flipping and banging their heads on their desks, but I have used nothing but copperhead gaskets (with no (O) rings) on my engines for years, if you read the above links you get more info, the reason I used copperhead gaskets(installed that both sides heavily coated ,wet with copper coat spray by the way) is that I have never seen one leak or blow from cylinder pressure even when using nitrous. Now you must clean the block and totally degrease it, before you install those copperhead gaskets, he must coat both sides of the head gasket with copper coat spray, and you must torgue the cylinder head to the correct specification in stages, on my engines I usually use 35 lbs. 45 lbs. 55 lbs. 65 lbs as the stages and then go back a second time at the 65 lbs. level, each time I follow the correct torque sequence.( use the specifications that the cylinder head manufacturer suggests, if you're using studs instead of cylinder head bolts, they will be different, on aluminum heads you will need to use washers under the cylinder head bolts head, using studs you'll need washers under the nuts) http://members.tripod.com/torquespecs/gmfs70-88chv8.htm

 

For those of you don't know the torque sequence starts in the center and spirals outward on the cylinder heads so that you're always working in a spiral pattern from the center of the head towards the outside end of the cylinder head

now I am in no way saying that copperhead gaskets are the best or only solution but there are few parts that I have ever used that have worked as flawlessly with as few problems as copperhead gaskets have worked when applied soaking wet covered with copper coat spray on a properly degreased and cleaned block have worked for me over the years, especially with a heavy dose of nitrous. And yes it goes without saying that you will have to make sure that both the cylinder heads and block or correctly machined serfaces flat, clean, degreased, and would no crud/dirt/small-objects stuck to the head gasket cylinder head or block

[pparaska]

John, Terry:

 

Yes, the for/aft rock concerns me. I'll pull one of the rod/pistons (they are pressed pin, BTW) and take it to the machine shop to have it pressed apart and have them measure the pin/pin-bore clearance.

 

Grumpy - thanks for the great post!

 

If I got to 0.035" of quench clearance, should I use the AVERAGE of the two quench area (edge of piston nearest the lifter valley) deck heights I get by rocking the piston back and forth OR use the highest measured deck height?

 

For instance, when measuring the deck height at the top of the piston's edge nearest the lifter valley, it is either -0.004" with the piston rocked down at the top (valley) or +0.022" with the piston rocked up at the top.

 

Should I use the AVERAGE (-0.004" + 0.022)/2= 0.009"

OR the HIGHEST (+0.022") when calculating the piston-to-head clearance?

 

Using the AVERAGE deck height means I need a 0.044" thick gasket. I could use the 0.051" gasket here, I suppose. The 0.041" gasket would seem to thin, giving a clearance of 0.032"? I would probably just get a 0.043" Pro Copper Gasket from SCE for that deck, and have a 0.034" piston-to-head clearance.

 

OR:

 

Using the MAX deck height (piston rocked so it is as close as it can be at the quench area) of +0.022" (above deck), then I'd have to add 0.035" to that. I could get a 0.062" Pro Copper SCE gasket for here and have a 0.040" clearance.

 

The probe pistons (the builder bought them) DON'T have the D-shaped dish, unfortunately. The raised flat rim around the piston edge is about 3/8" wide. But there's area inside of that in the quench area that is over 0.1" lower (part of the dished area). So quench is probably not possible anyway.

 

What do you think?

[grumpyvette]

" probably just get a 0.043" Pro Copper Gasket from SCE for that deck, and have a 0.034" piston-to-head clearance."

 

 

 

 

 

heres an old post that will also be useful below

 

http://www.oliver-rods.com/products/InstallInstruct.html

 

http://www.raceeng.com/Pages/Page_7sc.html

 

http://www.carrilloind.com/install.html

 

rod bolt stretch gauges are the correct way to set the bolt loads, and the only way to get really close to exactly even stress,

BUT...theres HUNDREDS OF THOUSANDS of engines built every year with a correctly used TORQUE WRENCH,setting the bolts to the manufactures specified torque settings, and using the manufactures suggested procedures on those rod bolts,if the bearings are correctly clearanced and rods resized, magnafluxed, and clearanced and are correctly machined and clearanced, Ive seen extremely few failures that were caused by overtorqueing or incorrectly torqueing the bolts so they failed., again, Id say its more usefull if your planing to run on the ragged edge of engine part strength limits (and yes I use it,on race engines(rod bolt stretch gauge) ( but mostly because I spent the money to get one)youll be suprized at how close a correctly cerified torque wrench can get in skilled hands [/b]

 

arp makes 2 different head bolts, one is rated at 170,000 and the other at 190,000 ?

 

the cheaper bolts are still a big increase in strength over the standard production O.E.M. rod bolts, I would only use the better grade bolts in an engine that might see piston speeds EXCEEDING about 4500 FPM

keep in mind that rod bolts are critical and highly stressed, but also be aware that the comon AFTERMARKET (H) style rods are available with 7/16" arp rod bolts.

[/b]

now think about this

a comon small block rod has a 3/8" rod bolt with a about .1106 sq inches of cross sectional area or about a 18,800 lb failure limit with those 170,000 lb ARP rod bolts

a 7/16" rod useing ARP 170,000 grade bolts has about a .1505 cross sectional area or aproximately a 25,600 lb failure limit with those 170,000 lb ARP rod bolts, increaseing the rod bolt size effectively increases the rod bolt strength approximately 36% now if you figure in the fact that the aftermarket rods are significantly stronger,(ON AVERAGE AT LEAST 30%)and then figure that resizeing your stock rods and adding ARP bolts could easily cost $200 PLUS the cost of AFTERMARKET RODS EQUIPED WITH THOSE ARP ROD BOLTS IS NOT A BAD DEAL

personally, I only use STOCK RODS when Im doing NEARLY STOCK ENGINE REBUILDS

once Ive decided to exceed about 4200FPM piston speeds or about 475hp in a sbc it just makes economic sence to use better rods.

[dr_hunt]

Pistons rock in the bores on the power stroke. If you look carefully at most v8 pistons, you will notice that the wrist pin is slightly offset to one side and the pistons and they have an arrow or notch indicating the direction of installation towards the front of the engine. This offset changes the rod/crank angle and minimizes the rocking in the bore during the power stroke. This was what was commonly referred to as piston slap and created problems with knock sensors in the later model engines when the engines had miles on them. the piston doesn't rock at TDC it rocks slightly after TDC when the piston is on the down stroke after firing. I'm sure that knowing the advance timing and rpm and burn rate for optimum A/F of gas/air at a known compression we could calculate the burn rate exactly. But if you think about it, it starts on the sparkplug side of the cylinder and goes across to the intake side of the cylinder. This is where you get your piston rocking in the bore. So if you are running domed pistons, it actually pushes the dome away from the head and the flat part of the piston on the intake side closer to the head. Some factors that will have an effect on the effective quench is rod growth due to stretch or heat and piston growth.

 

Some, not all, aftermarket HP pistons have centered wrist pin. If you are measureing without rings and in a dry bore, then check it with rings and oil on the cylinder walls.

 

SBC engines are a family of quench head engines, meaning that they are designed to have quench. Quench is good from several standpoints, but mainly for a better detonation free burn. To achieve proper quench, you have to have .040 or less piston to head clearance on the intake side of the head, like what grumpy's pictures show. Obviously there is a limit to the less part.

 

I wouldn't be too concerned with piston rocking in the bores, but taking a conservative approach and achieving proper piston to head clearance may lead to less problems down the road.

[pparaska]

Thanks, Grumpyvette!

 

I think you're advocating I use the AVERAGE deck height, not the maximum above-the-deck one. Cool.

 

I'll order the SCE gaskets then. 0.043" thick for the deck with that deck height. The other one has the pistons even farther above the deck, the tallest average deck height at the quench area being 0.019", so I'll have to settle for a 0.062" one (to give an 0.041" piston-to-head clearance), since the next thinner is 0.050" which would give me a 0.031" clearance. I'll call to see if they can do something custom in between - they list a 0.055" thick copper gasket Titan series, so they may be able to make the Pro series in that thickness also.

 

Thanks again!

 

Yes, if I were to do it over, I'd order H-beam rods and use a piston with a D-shaped dish. Coulda, Woulda. .

[jt1]

Somehow I missed that the deck heights varied so much from one bank to the other. That sucks to have to buy two different gaskets. I guess one way to look at it is you'll already have a set when you freshen it in the future. Pull one rod and piston out and see if you can detect any motion between the pin and piston. The pin should slide smoothly in the piston, but you should not be able to detect any movement at all perpendicular to the pin. With a pressed pin the pin should not move in any direction in the rod.

 

John

[blueovalz]

My pistons (as measured last night) have .018" rock side to side, and about 1 to 2 thou rock parallel to the pin. I wonder why yours has so much parallel with the pin

[jt1]

Pete, what comp ratio are you shooting for? Since you've got dished pistons, quench isn't a concern. Shoot for the desired CR and adequate P/H clearance with the gasket thickness.

 

John

[pparaska]

Terry, Thanks for that data. How did you get them to rock? I was using light taps with a rubber mallet at the piston edge, watching progress on the dial indicator at the opposite side of the piston, making sure that the piston didn't "unrock" away from the furthest reading.

 

John, I agree. Instead of using the copper gaskets that grumpyvette suggested, I found that Cometic has .051 and .060 (instead of .050 and .062 that SCE Pro Copper come in). So I ordered one of each from an online outfit. That'll give low .040's clearance on each deck.

 

Oh, with those gaskets, I'm at about 10.3:1. 8.3:1 DCR with the cam in at 4 degrees advanced.

 

I've yet to take the time to pull a piston but will do that this evening. I've spent the last two evenings doing cam degreeing and lobe surveys, and finding the pushrod length that will minimize the travel of the roller rocker's roller tip across the valve stem. (about .025" to .030"). I just ordered some Smith Brothers pushrods (damned expensive) in a custom length - I should have them early next week. Flatlanderacing has pretty good prices on them and just has Smith Brothers drop ship to you.

[blueovalz]

How did you get them to rock?

I followed the same method as you did just to keep the comparison as equal as possible. The parallel-pin rock for the most part did not exist. The gauge would bump .001" or so, but bounce back to normal (on both sides) but the .018" rock perpendicular to the pin was "set" with each tap.

The question I ask myself (or you) is, "are you using floating pins?" And if so, is .019" movement abnormal?

Assuming for argument's sake you had a piston pin that was .007" smaller diameter than the rod's small-end pin journal was, and this journal was 1.5" wide with a 4" piston, then that .007" (with no oil to take up any slack) would allow a rocking motion that would translate to .019" at the edge of the piston (2.00"/.75" = .0186"/.007" where the 2" is the middle of the piston, and the .75" is in the middle of the pin bore (the rocking pivot, or axis point)). This ratio pushed out to the edge of the piston allows about .019" or total rock movement.

Just trying to justify, or figure out by so much movement parallel to the pin.

[pparaska]

Terry, nice analysis, but the I have pressed pin. The play must be in the pin bore of the piston. Yes, I'm afraid to take it apart to see what's up!

[jt1]

Piston/pin/rod clearance should be on the order 0.0007 to 0.0009. If it's right, there's no detectable play in the piston assembled on the rod. The 1 or 2 thou you can get with the piston in the bore is probably moving the piston slightly in the bore, not rocking the piston on the wrist pin.

 

John

[pparaska]

I understand. I'll be pulling a piston tonight and checking the clearance, as best I can without disassembling the rod/piston. If it looks suspect, I can see the shortblock coming apart and the rods changed over to something with floating pins (they aren't very good rods anyway, according to the internet wisdom). If the clearance is out of spec, I'll be pissed.

 

I always do my own engine building. Except this time I didn't. Decks cut way too far, a distributor gear-munged front cam bearing (the cam was difficult to remove due to it), and then loose piston to pin clearance to boot!? Any money I saved on having this engine built somewhere where machine costs and labor are cheaper has already been lost due to the expensive head gaskets I've bought. I'll be going negative if the rods need to be worked/changed. This is the LAST time I have anyone build me an engine!

 

Damn, I may call off my Z convention trip due to this.

[pparaska]

No worries on the pin-to-piston clearance. I pulled one of the pistons that rocked quite a bit for/aft and the pin is tight - I can't feel any play in the vertical direction, it slides smoothly, so that's probably in the less than .001" range. Certainly no more than that!

 

The piston diameter is quite a bit smaller at the top and between the 1st and 2nd ring than at the pin CL and at the bottom of the SHORT (around 2") skirt. I can't remember the numbers, but it was enough to make it difficult to install the piston, since the diameter goes down so quick going from below the oil ring to the top of the piston that you need to tighten the ring compressor to get it to not catch the top ring on the deck. Minimum difference between the piston and bore measured at 0.003". The piston is largest at the bottom of the skirt. This seems like an uncommon piston shape, and I remember the builder commenting that way as well.

 

The piston pin is RIGHT below the 1/8" oil ring. The ring package is pretty tight and high. The pin height is 1.125", and all the rings are just above that. Below the oil ring, just next to the piston pin, there's only about 1/4" height of piston that can touch the wall, and it measures a good bit less in diameter (can't recall the numbers) than perpendicular to the pin at that height. But perpendicular to the pin just at that height (just below the oil ring), the dimeter is about .003" less than at the pin CL, just ~0.4" below. In other words the piston is a good bit smaller than the largest piston diameter in the entire area that it can touch the wall if measured in the front/aft direction (along the pin.). Factor in the 6" rod and .0025" bearing clearance, and the piston rocking isn't hard to imagine. The rod side clearance isn't excessive, 0.004".

 

Surprising this all is to me, since I'm used to dealing with 327 pistons with a 5.7" rod, which is A LOT longer piston. The short piston, of an unconventional shape, is the cause of the rock.

 

Terry, any idea what the the height of the piston in you engine is in the for/aft direction (parallel to the pin)? I'm betting it's a good bit taller than these pistons and maybe the piston shape is more conventional (and tighter to the wall in this area).

 

I'm through worrying about the for/aft rock. I am wondering if it'll knock at idle when cold though, but mostly due to the rock in the other direction, where there's a forcing function involved.

[blueovalz]

Pete,

My pistons are of a convensional design with the OEM compression height and typically longer skirts than your piston has. I did get off the phone with a guy I trust that works a great deal with stroker motors. I explained the situation and he felt the rocking motion parallel to the pin was normal if you've got the typical stroker type of piston (short skirts with short compression height). "You've got nothing to worry about" was his comment.