Going nuts - Piston height - deck

[Bryan]

Down the rabbit hole. Was looking at piston prices and ran across another issue.   Correct compression height.   

Mahle Piston Info 49-59 Dodge 218

Piston Length (in): 3.660
Compression Distance (in): 1.978

 

SEALED POWER 37P Specifications
Bore (IN)	3.2500
Compression Distance	1.980

 

EGGE -> Compression height is 2.000   Egge Piston Set with Pins – Egge Machine Company

Silvolite 1280 pistons - Compression height  1.980   https://www.summitracing.com/parts/uem-1280-030

 

That's a .02 difference.    Thought maybe it was hype on EGGE's part so I decided to measure my old pistons.  Couldn't measure the center of the wrist pin so I measured the outside diameter and divided in 1/2.  My wrist pin was .8595", close to others .8593.    1/2 is .42975.  1/2 of .8593 is .42965.    Then I measured from the outer part of the pin to the top of the piston.  Was getting 2.4295 to 2.4305.   So 2.4295 - .42965 = 1.99985.   2.4295 - .42975 = 1.99975    Always very close to 2.0, NOT 1.980 or 1.978.   

 

Reckon if EGGE's numbers are correct, you shouldn't shave/deck your block too much.    I read a post on another site (I think) saying that some piston makers made the piston's compression height shorter figuring that people might be having their block decked.

 

Could someone measure their original pistons and see if I got it correct?  And anybody with new pistons not installed yet, measure them?

[FarmerJon]

I believe I have heard of this issue with rebuilder grade pistons for all sorts of engines. 

I wonder if this is why several members have seen 217"s with pistons well below deck, and others have seen them with them near flush.

[Bryan]

7 minutes ago, FarmerJon said:

I believe I have heard of this issue with rebuilder grade pistons for all sorts of engines. 

I wonder if this is why several members have seen 217"s with pistons well below deck, and others have seen them with them near flush.

That's what worries me.  You get a .02 lower piston, your block's flat so you don't mill it, you mill the head .040 and actually only get a .02 improvement.

[allbizz49]

If you bring your pistons and block/head to a good machinist and tell them what you want, you should be able to get what you want. Honestly, any fresh build, especially these flatheads with pathetic compression, should get both the deck and head milled. If you're spending the dough to rebuild one of these engines, why not spend a few more to true it all up?

[Bryan]

2 hours ago, allbizz49 said:

If you bring your pistons and block/head to a good machinist and tell them what you want, you should be able to get what you want. Honestly, any fresh build, especially these flatheads with pathetic compression, should get both the deck and head milled. If you're spending the dough to rebuild one of these engines, why not spend a few more to true it all up?

Normally yes, but I measured my block and a .0008 feeler gauge would just fit under the straight edge at a few points.  If I had decided NOT to have it decked, and not known about the pistons, would be a problem.  Trying to confirm that some pistons are .02 lower than others (possibly EGGE).  They are listing $329 a set.  If I buy a $240 piston set with lower compression height I'll have the block decked minimum .02.   But first I need others to confirm what I'm saying is correct.

[Bryan]

So, anyone have a few new uninstalled pistons they can measure?

[Bryan]

Anybody out there? This is Lawrence, Kansas..

[Sniper]

I can't help.  Sorry.

[James_Douglas]

You should hunt down two or three different head gaskets. You would be very surprised at gasket thickness differences. A lot more than the differences between pistons you are noting.

 

Deck the block. Mill the head. CC the head. Calculate the compression ratio using the gasket you are going to use. Then go looking for pistons. Not the other way around.

 

If need be, get some forged pistons and specify the height. Also, depending on the casting...ask manufacturer, you can sometimes take a little off the top of a flat headed piston without any real issue.

 

James

[Bryan]

6 hours ago, James_Douglas said:

You should hunt down two or three different head gaskets. You would be very surprised at gasket thickness differences. A lot more than the differences between pistons you are noting.

 

Deck the block. Mill the head. CC the head. Calculate the compression ratio using the gasket you are going to use. Then go looking for pistons. Not the other way around.

 

If need be, get some forged pistons and specify the height. Also, depending on the casting...ask manufacturer, you can sometimes take a little off the top of a flat headed piston without any real issue.

 

James

On another post I noted that the top of my block is flat to .0008". Not kidding.  Could have it decked .02, but first trying to figure out if what I'm seeing is correct, that some pistons come with a compression height .02 lower than stock.  Just came from my shop and measured the pistons that came out of the first block (the 48 Dodge).  The wrist pin measured .8595" (again).   From bottom of pin to top of piston measured 2.4350".   The piston out of the 53 block previously measured 2.4295-305".  Doing the math gives .8595/2 = 0.42975" Subtract that from 2.4350 and it gives 2.00525. So I "assume" the stock compression height must be 2.000".    I also did something a little strange, I put the piston back in the block with the top ring on it.  It stops consistently on the ridge with the piston sticking out .05-.06 above the deck.  Wonder if this what it's actually doing when the engine is running?   For me personally (not anybody else) forged pistons is a little too much.  On gaskets I bet if I search or ask on the  forum I'll find the different thicknesses.   I would think knowing the 3 different available compression heights out there (1.980, 1.978 and 2.00) I would have the machine shop tell me with my crank what the deck height is before decking the block.  Main goal is not to have the piston hitting the flat part of the head, which doesn't change height no matter how much the head is milled.   I might CC the head initially, but I can't go back and forth to the shop trying for an exact compression ratio. Seen what you have done, and it's great details, and would be nice to do. But I'm planning to just have the head milled a safe amount one time, some say max .060, some max .080 but no more than that (probably .060).  Don't want to be dealing with warped heads and leaking gaskets.   I will start researching the gaskets. 

[Bryan]

So, looking through various posts:  Sniper on Dec 2021 -> Best brand gasket, listed info, not measured was nominal .080, compressed .052 =/- .002

Kencombs' Milled head post  -> .040 compressed.   Don Coatney -> .072" measured from composite/copper gasket.  

Thebeebe5 -> .075" Felpro, good quench is .035-.040".  

Plymouthy Adams ->  compressed gasket at .043"

[Sniper]

Unless we know the brand and part number of the head gasket measured the numbers are functionally useless. 

 

That is why I try to give a specific part number when talking about specific stuff like this.  In my posting, Best only lists one gasket, so I didn't feel the need to post the number.  Maybe I should have.

[James_Douglas]

I have three blocks sitting in the corner and two in cars. ALL had different piston deck heights when I took them apart. Some as much as a 1/4 inch difference.

 

My advise is to stop thinking about what is "THE" piston deck height. It is a fools errand as there were many deck heights as my blocks are examples of it.

 

Have the block decked even if it is only slight so you get a good gasket "bite". Put the crankshaft in and measure the deck height. Then find the head you want to use and CC it. Then take the make and part number gasket you want to use and buy two. Put one on the block and stick the head on and torque it down. Take the head off and measure the compressed gasket thickness. Toss the gasket.

 

Now you have all the numbers to do the math on final compression and head clearance you need.

 

Go hunting for pistons that will achieve that goal. There are a lot of NOS pistons running around that have different pin locations. Some have thicker heads on the pistons that may be able to be cut a little. I read someplace that someone found a Chevy six piston that can work in the 25 inch flatheads, but I have not first hand knowledge on that.

 

At the end of the day the incremental cost if you have to go the route of a forged piston to spec will be the same if you force the issue and end up having to take the thing apart again for a blown gasket or a piston that wacks the head one day when you are on the engine hard.

 

James

[Bryan]

1 hour ago, Sniper said:

Unless we know the brand and part number of the head gasket measured the numbers are functionally useless. 

 

That is why I try to give a specific part number when talking about specific stuff like this.  In my posting, Best only lists one gasket, so I didn't feel the need to post the number.  Maybe I should have.

It's okay..most people didn't but I wasn't trying to get an exact list.  The numbers give you a range of .072-.080 new and .040 to .052 compressed.  If I took worst case compressed (for not banging the piston on the head) I'd use .035..  

[Bryan]

First thing I've been mentioning is compression height, not deck height. I'm not fixated on deck height.  Also not fixated on achieving an exact compression ratio.   I have 2 heads and that is all I'm going to use, one or the other.  I'll use some of your methods and maybe sacrifice a gasket, will mill the head a safe amount.  

 

What are all these different NOS and other brand (CAST) pistons?   I've seen Silvolite 1280, Sealed Power 37P, EGGE and VPW NOS pistons (their normal ones they don't say what they are).  If you mean other Chevy, Ford, Toyota, etc pistons I'm just not doing that.   If you've seen my posts you know about the level of complexity I'm comfortable with.  Might be great for you and others, and I've done that when I was in my late 20s with motorcycles, but not now.

[Bryan]

Just for info.

 

Edited March 28, 2022 by Bryan
larger picture

[Sniper]

Nice, I might have to see what I have and if that will get me to zero deck height.  I know I will at least need one piston in my 230, broken parts of the skirt are in the oil pan, lol.  If I have to bore it out, may as well optimize.

[Los_Control]

I seem to be posting UTG videos a lot lately. I say do your own research, do not believe everything you hear on the internet.

 

I also never cared about deck height of pistons.

 

I felt this was a honest explanation of why someone would care.

Probably some good tips here.

 

[Sniper]

What he's saying is to just live with what you got by juggling parts to get it as close as you can.

 

Or just have the block squared up.  I think rather than spend who knows how much time measuring and sorting parts to find the best compromise.  I don't have a rule book to follow or hundreds of stock parts to sort thru, so I'll just get the machining done right and build on a good foundation.

 

But I only build engines rarely, so I build them to last, I'm lazy I don't want to build it over and over.

[Bryan]

It's good to know that manufacturing tolerances are that much off. I didn't know it.  But now knowing the differences in compression height between 3 brands of pistons helps. If you have a flat block top, don't deck it and go with the 2.000 CH pistons. If the top is warped, have it decked and go with the .02 lower ones. Imagine how pissed I'd be if I didn't know this, didn't deck the block, got the .02 lower pistons, had the head milled .04 and wonder why it didn't change much.  For practical purposes as you say, we don't have a bunch of parts to try out..just the 6 pistons in the order.   

[Bryan]

Another thing I'm noticing is both pistons from the new & old blocks had a spacer on the top land, to get a 3/32" width.  I'll definitely keep an eye on that too.

[kencombs]

11 hours ago, Bryan said:

Another thing I'm noticing is both pistons from the new & old blocks had a spacer on the top land, to get a 3/32" width.  I'll definitely keep an eye on that too.

About those spacers, here is one of the vintage tools used to prep used pistons for them:

https://www.ebay.com/itm/294253849303

Not mine, no interest, just a too I would enjou haning on the wall.  Or, if working on a really rare engine where parts are unobtainable, one could save a piston.

[Bryan]

Got my NOS pistons from VPW today.  ELKO.  Look a little rough but you can't feel anything on the sides, just discolored but very smooth. Tops are slightly crusty but will polish them up.   .030 diameter with a compression height of 2.00".   Came with clips and clean piston pins.   Didn't think they would, so already ordered pins from somewhere else for $25. Oh well.

Shipping was about $30.  Still happy with them. Other alternative to get pistons with a 2.00 compression height was EGGE at $329 not incl postage.

[Sniper]

But hey. Egge gives you a kiss, right?

 

Nice job on those pistons.  Now you get to weight match them :evil grin:

 

lol

[Loren]

0.002?

I hope we all know the width of an average human hair is 0.004.

As a rule of thumb once a head gasket is torqued down I have found them to be roughly 0.055 thick, or enough to loose the difference of 0.002 in.

 

What is of note is when you get a crankshaft ground, the machinists have this nasty habit of trying to clean up the bearing surface with the least amount of material removal. A 0.010 divot in the journal becomes a 0.020 undersize crank journal if ground on center. So they use a little trick whereby they kick the centerline over to grind only the side that has the divot. The result is less stock removal and all journals the same size BUT, and you knew there is always a but, you could end up with a crankshaft with 6 different strokes!

Rarely are the effected cylinders given longer strokes. The machinists don’t care because most mechanics do not have the means to accurately measure the difference.

Besides the pistons rock in the bore at both ends of the stroke making it even harder to measure.

Since there isn’t much you can do to change machinist behavior, this is one of those things I’ve stopped worrying about. Besides if the piston clears the head with no gasket, it will certainly clear with a gasket.

Something one should worry about are the valves.

You absolutely should check to see if they clear after milling the head or getting new valves or seats or a reground cam.

I hope I didn’t spoil your day.