zero decking

[lyonsy]

Drift, you know about measuring squish by crushing a piece of solder or by claying, right?

no i have not i have done a lot of engine work but not race engine work so if there is anything any of you can enlighten me on it whould be much appreciated.
reason iam doing my engine's now is iam sick of blowing engines up so i know its done right as i have been iffy on occasion
basicly i cant do any worse then my last one that last 10 laps and i have 5 conrods i whouldent use in anther race engine as thats what was left of it everything else was total ducked

 

[XPC66]

If you're going full house you are going to be operating in the 0.025 squish band area. That will allow carbon buildup, rod stretch and with a bit of luck the pistons just kissing the head. Anything over 40 thous you might as well just install open chamber heads or hope that they are pent (or hemi) enough to accelerate and swirl the flame front into the 30+ m/s range.

If you are blowing engines there is probably another cause like not running at 5% CO, wrong valve spring rate, too much slap, incorrect rod/stroke ratio, too much side wall loading, incorrect blow down angle, insufficient lubrication, etc.

 

[lyonsy]

the first one i beleave was caused by high speed detation broke the ringland on no5 between no1 + 2 ring. that one expired much to earlyer for my liking only done 150laps or so.
second is one of 3 things it pumped a lifter up causing all kinds of grief
droped the head of the valve off not at the weld point allmost like it had contact with the valve cutter
or the kline (which i dislike and told not to go in again but did due to this problem)
tighten up on the valve holding open
unfortenly it was impossible to tell as there was simply to much damage done to the head and stem of the valve to trace.

 

[Stubby]

Stub, those figure for the rocking of forged pistons are COLD, correct? Since forged pistons grow more than the high-silicon cast pistons, they all ought to have roughly the same amount of rock when HOT, right??

Exactly. That is why I started taking notes and comparing. Every combonation is different.

How can a piston slap 0.010, when there is only 0.005 bore clearance and a set of rings in the way?

Even if there was a 0.010 side play a 4" piston would only be about 1.25" long to get the topland rotation of 0.032 above deck from zero. Somewhere in that 1.25" you'd have to get a gudgeon pin, a compression ring, a scraper and an oil ring. Then there is the problem with ringlands snapping with that much movement. Maybe my maths is wrong.

As for the amount of rock. You must remember, the skirt of the piston is tapered. Big at the bottom and smallest at the top. Then add the fact that the head of the piston is smaller than the skirt diameter. You can't figure the rock with math, you can only measure it. This goes with anything on an engine. DON"T USE MATH FOR THE NUMBER, MEASURE IT.

The rings and ring lands are engineered so that the ring never bottoms in the ring groove. If the ring is bottoming in the groove, you have another problem.


I am not offering numbers for any setup. I am offering my observations. Observations made in pursuit of the ragged edge and checkered flag. I will get fairly aggressive with my stuff, but I only reach for ragged edge when classes call for it. I can promise you, if the SCCA tech guys have .001" error in their indicator when checking stroke or deck height, we would get DQ'ed. If you squeeze it more than the next guy, you will get a little more than the next guy.
[/quote]

 

[Lazy JW]

The pistons in my 1981 300 stand .005" above the block. Gasket was advertized at .042" compressed thickness. I never measured it, sure runs nice It NEVER sees more than 3500 rpm, usually 3000 tops.
Joe

 

[XPC66]

How can a piston slap 0.010, when there is only 0.005 bore clearance and a set of rings in the way?

Even if there was a 0.010 side play a 4" piston would only be about 1.25" long to get the topland rotation of 0.032 above deck from zero. Somewhere in that 1.25" you'd have to get a gudgeon pin, a compression ring, a scraper and an oil ring. Then there is the problem with ringlands snapping with that much movement. Maybe my maths is wrong.

As for the amount of rock. You must remember, the skirt of the piston is tapered. Big at the bottom and smallest at the top. Then add the fact that the head of the piston is smaller than the skirt diameter. You can't figure the rock with math, you can only measure it. This goes with anything on an engine. DON"T USE MATH FOR THE NUMBER, MEASURE IT.

The rings and ring lands are engineered so that the ring never bottoms in the ring groove. If the ring is bottoming in the groove, you have another problem.

Hmm well I think you'll find the rings do stop the piston from binding with the cylinder walls as does the thrust of the gudgeon pin. In the "rock" scenario you are talking about the piston toplands would be in contact with the wall and bigger problems than pistons knocking the head would prevail (siezure for one). The ring groove depth is not the inhibiting factor, but the ring groove gap clearance and ring gap itself. The torsional twist locks the rings from disappearing into the grooves, maintaining ring contact with the wall. The slippers also provide further protection from rollover and quiter operation.

Insofar as the piston tapering, yes there are extremely minor diameter variances, but the there is no set rule where the max diameter occurs. Many are measured at the skirt, but some are measured midway at the pin and others above the pin.

And yes empirically testing is always desirable, but the maths need to be right in the first place. Personally I've never seen a piston able to rollover enough to raise the topland 32 thous but I'm easily swayed if you can show me a pic.

 

[Stubby]

OK, I reread my post and I don't see where you came up with the piston rocking .032. If I have mislead you, I am sorry. There seems to be a comunication problem. I could be the problem, but I need you to point it out.

 

[Anonymous]

Now wait a minute, XPC; Are you saying your MEASURED squish clearance (cold) is .025"??? The only engines I can imagine getting away with that clearance would be pretty small and have well-supported cranks. And your idea that any clearance over .040" is ineffective is your idea alone. Racemotors have big MEASURED squish clearances so that their RACING clearances are effective. All of those solid-looking parts prove to be dismayingly flexible at racing rpms and loadings.

Drift, here's how to measure squish on an assembled engine. Find a spool of big diameter solder. Pinch the end with a pliers so that the flux doesn't get squeezed out. Figure out where the squish area(s) are in your engine. Starting with #1 cylinder, bar the crankshaft so that the piston is anywhere but top-center. Unroll several inches of solder from the spool and put a curve in it. Poke the solder in #1 spark plug hole and feel for it touching the cylinder wall in the squish area. Have a helper bar the engine over until it comes up to top-center and past it, so it flattens the solder. Pull the solder out of the sparkplug hole and measure it with your calipers or mike. Thats the squish dimension for #1 cylinder, and it should be fairly close for all cylinders, but check them all anyway. Once you know the dims of your engine, this is also a good quick check for a bent con-rod. You might then squirt some WD-40 or whatever in the cylinder in case some flux did get out of the solder. NOW, given what Stubby tells us, you might want to do all this while the engine is still somewhat warm . . . .

 

[XPC66]

OK, I reread my post and I don't see where you came up with the piston rocking .032. If I have mislead you, I am sorry. There seems to be a comunication problem. I could be the problem, but I need you to point it out.

I can tell you're a good sort, but if anyone's going to take the blame for misreading threads it me damnit! It's nice to actually come across someone who actually takes the time to test these things out and post his observations.

 

[XPC66]

Now wait a minute, XPC; Are you saying your MEASURED squish clearance (cold) is .025"??? The only engines I can imagine getting away with that clearance would be pretty small and have well-supported cranks. And your idea that any clearance over .040" is ineffective is your idea alone. Racemotors have big MEASURED squish clearances so that their RACING clearances are effective. All of those solid-looking parts prove to be dismayingly flexible at racing rpms and loadings.

. . . .

Squish height of 25 thous is not a rarity in race engines... I'll let you find a calibration engineer in a race engine development organisation to confirm this. I'll repeat that the most desirable condition is the piston just about kissing the quench pad.

The idea of over 40 thous clearance between piston top and quench pad is not my idea alone. It is well documented and if you take the time to work out the exit velocity of the compressed gas you will come to realise how rapidly the flow rate drops (ie by the square) in relation to pressure. I'm interested to know what closed chamber race engines run "big MEASURED squish " = interpreted as much greater than 40 thous? They must being running fairly advanced timing and suffering some det issues. Of course if they have sloppy pistons and high co-eff alloy rods then maybe 55 thous is for them, but they aren't fair dinkum race people.

I've built and run turbo engines at 10,500 rpm with only 25 thous and they haven't been a problem. Does that mean my work is invalid?

 

[Stubby]

The way I was taught to measure deck clearence.
Zero the indicator on the deck.
Put the indicator on the piston at A
Rock the piston side to side.
Half way between the high and low measurement is the deck height.
If you put the indicator at B, it should match your measurment.
The theory is, the piston expands and becomes stable at temp. (less rock)
IF THE DECK IS SQUARE!

Some pistons have the clearence measured at D and some at C. A forged piston with .005 measured at D will rock way more than a hyper piston that has .0005 to .001 at E. Obviously a piston with a 4.5" dia will rock more than one with 3.6".

Yet with these differences, ZERO is still the target number.

That is where I started hunting a better method. The deck may not be perfectly square. The difference in cast/hyper/forged/high silicone/low silicone/amount of taper, amount of expansion. Too many variables for zero deck to be the magic number. This doesn't account for aluminum block with steel rods where the deck clearence actualy grows when the block heats up.

If you start recording the closest measurement and compare your notes, you will see some amazingly close numbers. In a perfect world, all of the blocks would have a perfectly square deck, the crank would have exactly the same stroke on all throws, all of the rods would be perfect center to center. This is rare, even with todays tech.

So, in the end, you should measure closely and work off of the tightest measurement.
Zero deck will net you whatever the compressed thicknes of the gasket is.
If you have a steel shim gasket, don't go to zero. It will probably crash.
.035 to .040 total deck clearence is a good place to start.

I post this sumary to make sure I don't lead someone down a path that will lead to heartbreak. If you chose to hunt the ragged edge, do so cautiously. You wouldn't want to find a dead end with a piston traveling at speed.

XPC66
If you take a piston with rings installed and push the ring all the way into the groove, you will see that the ring will not stop the piston from rocking and actualy touching. It will offer some dampening like a shock absorber. I thank you for this insight. This is something I had never considered. I would suspect that it would be pretty effective at dampening on the power stroke under pressure. 8)

 

[XPC66]

I can understand how you are concerned that boy racers will take the 40 thous as gospel, without regard to machining and blue printing every thing correctly.

Someone touched on using soft core solder to test. My preference is to test prior to putting all the trimmings on the engine. Four pieces located east, west, north and south on all the pistons and the head cramped down to spec; the engine rotated once and the heads lifted for micrometer readings on the solder.

Of course if the bigend and mains clearances are sloppy that clearance will be transferred to the squish on the exhaust stroke.

 

[Stubby]

I am sure everyone has their own method of measurement. I can't knock anybodies method because I choose to measure before installing the head and trust the gasket manufacturer.

That is why I went a little more indepth with my method of measurement and the variables. Smitty has been doing this for a long time, and I wouldn't hesitate to have him build be an engine. Believe me, there are very few people who I would trust to build anything for me.

 

[lyonsy]

thanks for all the info very much appricated on every account.
so ill start at 40" then play with head gaskets chasing the edge
and for check's
i do stubby rock test then
Seattle Smitty soilder check
any other checks for it?
or varients of doing checks
also this should probly be made a sticky i think lots of good info for firt timers

 

[Anonymous]

You are too kind, Gary!! Most of my hands-on squish-setting experience is with racing 2-strokes. I'm getting my auto info from reading the opinions of more experienced car-guys than me. XPC, I'm always interested in new info, esp. if it puts the established wisdom in question. I'm with you in thinking that an engine wants the tightest squish it can get short of ever touching. The people calling for wide squishbands generally offer some kind of table of clearances relating mostly to the rpm range of the engine, whether you are using aluminum rods, etc.. Now, maybe they are including a fudge-factor to save the less meticulous engine builders, and maybe they are even trying to avoid angry letters and lawsuits. There are several parties on line offering squish info for cars, bikes, and sleds.

I posted my method of claying a while back, but don't know where it is. Maybe one of you will describe how to do this for Drift, and me, too.

 

[Lazy JW]

....
I've built and run turbo engines at 10,500 rpm with only 25 thous and they haven't been a problem. Does that mean my work is invalid?

Nope. But it does mean that you have yet to find the limit

In my business (sawmills) I have earned the reputation of finding the limits and then ALWAYS running on the ragged edge. Years ago I took Inspector Callahan's words to heart.
Joe

 

[XPC66]

....
I've built and run turbo engines at 10,500 rpm with only 25 thous and they haven't been a problem. Does that mean my work is invalid?

Nope. But it does mean that you have yet to find the limit

In my business (sawmills) I have earned the reputation of finding the limits and then ALWAYS running on the ragged edge. Years ago I took Inspector Callahan's words to heart.
Joe

I must admit there is that devil inside me that gets a certain amount of satisfaction seeing an engine getting lunched when it's exceeded it's design intent.

 

[Stubby]

Smitty
I owe you thanks. Not long ago, I was wondering how much deck clearence I have in the 68 engine. I know it has been rebuilt and had a late model head on it at one point, and I know it has a composition gasket on it. I have no idea if it has been decked.
I had resigned to waiting till I could pull the head and measure it. I sat here and debated over the best way to check the deck clearence and completely overlooked the fact that you told me how to check it. I feel I have a good system for checking while in the machine shop. A system that is acurate and I am comfortable with.

Now I have a system that will allow me to get a good idea if the block has been milled. I can also figure approximate compression ratio. 8) That just proves that an old dog can learn some new tricks. Provided he opens his mind and closes his mouth long enough.

On the clay method of checking clearences, I have seen some inconsistant results.
First, use only modeling clay. Crayola brand for kids is OK. Not playdough!
Second, make sure it's not dried out. A few drops of oil will help it out.
The inconsistancies I speak of are a different measured amount from an indicator. I have and will use it for piston to valve clearence. However, if it is close, I will mount up an indicator and check it closer. Some times it will spring back a little. I think it is mostly when it is dried out a little, or if you turn it thru too fast, but it can spring back and give a false reading.
You can get good results with clay, you just need to be aware of it's shortcomings.

I use it at the plant where I work. I can measure clearence inside a mold, while the machine is running. 8) I just have to let it cycle at least three to four times for good results.

I have to go now. I have to check the deck on the 68 engine.

 

[Anonymous]

Wonders never cease! I was leery of offering the solder-squish system yet again for fear that everybody knows it and is tired of hearing about it. It's funny how we can go for years and miss out on some fairly routine thing along the way. I just hope I haven't got XPC so annoyed that he won't tell us more (and further MY education).

 

[Stubby]

I closed my mind to the solder because I couldn't see the point in measuring the deck clearence after assembly. I mean, while it is apart, you have all those indicators handy, you can measure it exactly.

I was at work and the solder method just floated thru my head, searching for something to connect with. Then it hit me like a ton of bricks. I have thought more about it. I used to modify jet ski heads for a friend. I have also seen some strange pistons in two stroke motors, some on blind cylinders that you load from the bottom. I have also seen some aviation cylinders that are blind cylinders. The solder method would be about the only practicle way to check them. :idea:

Thanks for the insight. We all have things to share and learn, I just got caught up in sharing and forgot to learn.