Ok, my question of the day:
Put my new 96mm pistons and cylinders on and measured deck height, .033 all the way around. Let's say .032 to be conservative.

I will not be running head gaskets.
The calculator gives me
8.8 CR at .033
8.6 CR at .043
8.4 CR at .053


The pistons have a valve relief (pretty large) cut. I am shooting for 8.4 CR.

Heads are in the shop so I can't check the CC yet.

Everything I read says .04 - .06 for deck height. With the valve relief do I need to worry about going lower? If I had dishes would you measure from the edge of the piston or the dish?

Not sure I understand how you can establish the CR with the gaps you are showing if you don’t have the head volume . Also make sure you consider the valve pocket in the pistons as volume.

The 0.040” gap is not necessarily for interference between piston top and valve but piston top and head face.

Hopefully my comments can help answer your question…

You have to measure the head volume to get compression ratio accurately

O.040 deck height is considered the minimum deck height for t-4 engines. Sure you can go lower or shim the bottoms of the cylinders to increase or decrease the CR(compression ratio)

However the area between the bottom of the head and the top of the piston will determine the squish area. Too tight, you will restrict the flame-front travel and really hurt performance.

some reference on squish:
https://www.thesamba.com/vw/forum/viewtopic.php?p=8865130

Thanks Finegan, that's what I am looking for. I am just modeling several scenarios until the heads come back, you guys are really obsessed with CR. I know the head will be between 64 and 56, so for the moment I am using 60, as I get more information I will revisit that. My concern is slapping the piston against something. Now I understand the where and why of the .04-.06. It isn't the valves...

Very interesting read. Reminds me of why I rarely look at thesamba unless I have to. Firehose.

So, it sounds to me sitting somewhere around 1.5 to 2.5mm is good. It looks like if I put in a .02 shim that will keep me around 8.4 CR as long as my CC isn't too big. That would give me a 1.45 mm deck height. Really seems like a ton of factors go into the 'optimal' deck height. Has Jake Raby ever said anything about it? Since it is a stock 2.0 head/cam and a really common 2056 improvement. might there actually be a 'best practice' for that combination?

For what it's worth, our 2056 had pistons with valve reliefs that cc'd at 3 CCs per the manufacturer's specifications and real world measurements.

Also, I'd still check the valve to piston clearance as you wouldn't want them to hit. There are ways to end up with inadequate clearance, like the pistons are upside down or what's the actual valve lift, etc. Mock them up and measure a few times, use some play dough. Then order the spacers and re-measure when you get them.

That's what I was thinking. I want to check the valve geometry so I will be putting it together to test that with full intent to tear down again. That valve relief is what is going to make for issues I think.

A tip on piston to valve clearance.

The closest relationship is not TDC for the pistons to valve, but about 10 degrees before top center(piston chases the exhaust valve as it closes) And about 10 degrees after Top Center (valve chases the piston as it opens. I think I got that right...

EZ to test

Place play doe /clay in the piston relief and tighten head. Rotate the assembly minimum of 2 complete turns to have the valve compress the play doe.
Remove the heads and measure the thinnest location on the play doe and that will be your piston to valve clearance.

0.080 exhaust
0.120 intake (I will check this in my notes) Memory only for now

Normal engines require .080 on intakes and .100-.120 on exhaust.
VW air-cooled might be backwards...

Quench (piston to cylinder head dimension is the single most important measurment in a modern engine that uses pump gas.

I want .035 minimum for a cast piston and .045 for forged.

I would not change quench to solve a valve interference issue. I'd have the valve reliefs "fly cut" if required.

Heads are back. They measured 55 CC but I am going to do my own to double check. I also want to measure the valve relief in the piston.Click to view attachment

With stock valves and what cam are you using? The piston to valve clearance is worthwhile to check but stock components have LOTS of clearance. Unless something was not assembled correctly like the timing of the cam. Plus you said the new pistons have a generous valve notch already. The largest valve notch I was able to put together in my pistons was 0.050 inch based on the crown of the pistons from KB. Yes i did measure and called Keith Black tech support

Yeah. All stock but the piston/sleave combo. I got 2ccs on the valve relief, and 60 on the head. Not super confident on the head, lots of side spill etc. I am think between 58 and 60.

Did a real thorough job on the deck height today. Measured all the sleeves and tried to do some matching. I don't think it mattered much, it doesn't look like my case has ever been cut.

Bolted down nice and snug I got
.028
.031
.023
.028

So I guess I am pretty much stuck with a .03 shim. When I get ready to put it back together I will see if swapping cylinder/piston combo on the .031 with the .023 makes any difference. Looks like around 8.3 for compression ratio, which is what AA said for the combo.

Couldn't stop thinking about it, so I swapped the cylinder/pistons and measured again... No difference on the .023. Argh. If they were all .023 I wouldn't care. Maybe it is just measuring error.

Yes moving the cylinders around may help. Those deck numbers have some variation 023 to 031= 0.008 inch
Remember to measure as close to the center of the piston to keep the rock as small as possible (flat top)top of stroke
or measure on the highest edge ( dish) top of stroke and rock them all the same way to get most consistent measurement.
or

Average rock= highest-lowest/2 and use that deck measurement

I bet that is contributing. I didn't really consider the rocking piston.

here is how I did it..
https://www.youtube.com/watch?v=Wj3R2Zh-HNo

As far as rock , I would think if measured across the piston pins (center) right to left would not rock

but top and bottom would rock?

Are you mocked up with out rings or assembled?

yes look like 8.0 to 8.2 will work nice on pump/premium 93

I have the rings in but there is still quite a bit of free play. I have been generally measuring across the center of the pin, but I have not been paying attention. I bet it will make a difference if I pay better attention to that. I can use a micrometer to compare heights top to bottom as well.

Case hasn't been decked?

In a perfect world, send to the shop and get it machined. That way, you won't have any regrets (or hopefully leaks) later. But, then how many of us live in a perfect world??

Assuming the shop gets it right, easier said than done

What are the specs on your case now? very possible she's just fine

Everything seems nice and level on the case, no sag. I am going to remeasure again today being careful about the piston being level in the cylinder and measuring across the pin position.

Ok, redid them all using 4 bolts instead of two, extra careful to tighten equally and a little at a time. Made sure all measurements were taken at the same place along the midline of the pin. .028 all the way around. Much better. Surprised there was so little variation among cylinders.

Woohoo..great Job! Just need the 0.030 shims for 8.1 cr
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That looks about right. Maybe while I wait for the shims to arrive I will try cc'ing the heads again.

.028 deck height on all four would make me want a base shim of .010-.014 with hypereutectic pistons (Keith Black).

With forged pistons I'd want a base shim of .015-.025 depending on the base material.
4032 would get .015-.018
2618 would get .020-.025

I'm a believer in NOT using anything between the cylinder and the cylinder head.

I prefer the hyper pistons OVER the forged pistons for a street car.

Want to scare yourself? Check deck height with a forged piston with no rings on it at 90 degrees from the pin bore.
Then rock the piston on the pin bore.
You will see a total range of .020 change in deck height between low and high.
Forged pistons can rock in the bore SO MUCH more than you expect...

Thank God that variation doesn't actually occur during operation.
There is so much resistance generated by the quench areas on the head that the pistons tend to stay flat during operation.

If they didn't, we'd need double the quench clearance that we use now, and the quench action would be greatly diminished.

I do have one question. Why do you need shims to check the combustion chamber CCs?
Figured it out! poor reading skills on my part...

How are you checking this now?

I've always used a plexiglass plate (round plate at the correct diameter for VW air-cooled stuff) with a couple of holes drilled in it and either a cattle syringe (60 cc) or a burette to measure volume.

The reason for two holes is it is a shitload easier to clear the bubbles with a vent hole.

I use Vaseline to form an airtight seal to the cylinder head.

These are hyper pistons. I am going with the .058 on the deck, flat tops with a relief, most people I have been reading (Wilson as well) say .04 at bare minimum. I don't want to take any chances and somewhere around 8.3 is good for me.

I was using veg oil and a piece of Plexi (two holes) but it was real thin Plexi and the Vaseline didn't seal well due to uneven pressure yada yada yada.

Decided to go about it differently. Play doh.
Shot a little oil spray into the chamber first. Worked pretty well. I think I am getting about 58 or 59. So pretty darn close to 60. Definitely not more.
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I believe you are leaving 5-10 hp on the table with a quench distance that high.

.040 is what I use for customer engines where I'm being a bit more conservative.

Cast hyper from a reputable company on decent H-beams can often work at significantly lower distances.

One builder I know runs hyper-eutectic "claimer" SBCs at .029 without trouble.

Yes, I know this is not a T4 and is probably more dimensionally stable than a VW boxer but .029 is too tight in my book.

The tighter the quench the more detonation resistant the engine becomes.
It can run fuel with less anti-knock resistance and commonly requires 5-7 degrees less total timing to make MBT happen.
Compression increases slightly.
The piston crown and combustion chamber stay cleaner as well.
The only drawback IMO is you have to be far more careful during assembly and really nail the deck height measurements.
It's less forgiving for trash running through the engine as well but if you have foreign objects entering your intake track you deserve what happens...

The "quench" argument is a moot point if you're running a fuel that has extremely high detonation resistance. For a methanol or nitromethane application this distance is far less important.

For folks running pump gas it is the single most important "blue-print" distance to get right IMO.

This measurement for me is "the smallest clearance possible that has the ability to turn at 110% redline without the piston crown touching the head."

If this ends up placing the piston crown too close to the opening/closing valves then the valve pocket in the crown needs to be deepened/modified.

It is possible to build a 10.5-1 flat top Datsun Z motor that can run regular with less chance of detonation than the stock 8.3-1 motor has.
It also requires a much larger duration cam profile (has to with the compression) and makes 40% more power than the stocker.

I believe the quench phenomena starts about .065" and only gets better right up to the point of contact.
At which point it all comes apart.

I've built multiple engines (not T4s) with a quench of .035-.040 and had no failures.
I'm also beyond anal regarding engine clearances...

One thing to remember. Changing deck height on a T4 is EASY!!!
Doing it on almost anything else requires adjusting rod length, offset grinding the rod journals, decking the block: significant machine shop processes and if you go too low...
It sometimes requires a new/different block.

Being able to place shims between the cylinders and the block is a very welcome change for me.
Gives me options I've never had before...

You are tempting me to go with .02 but since I am not touching the bottom end I suspect I should be pretty conservative right?

"Leaving horsepower on the table" nobody wants this.

I mean, if I go with .02 that puts me at .048, that's almost .05

..

.048 wouldn't bother me with forged but...

If I had it out, down as far as you have gone, it would go all the way.
I'd know exactly how much bearing clearances I had, and the basic condition of all the lower end parts.
I would NOT put a stock camshaft, stock rods, or stock compression height pistons back in.
I'd do a bunch of little things that you are not doing.
I'd change quench, rod length, compression height, cam profile, pushrods, bore size.
If the crank requires grinding, I'll have it offset ground. (I think it's good to go as is)
I did a bit of weight reduction measurements a month back and found 3 1/2 lbs. of reciprocating weight to dispose of (without going to exotic parts). That's a BUNCH.
I'd drill extra holes for rear main seal drain back.
I'd try to find a reverse lip rear main seal.
I'd smooth the combustion chamber and port profile the head (mild porting).
I'd have the heads heavily fly cut.
I'd lap the cylinders into the heads.
I'd ceramic coat the piston crown and combustion chamber. I might use a heat dispersing coating on the outside of the cylinders and heads. (Still researching this)
I'd do all of the oil system mods.

I purchased a nice core 2.0 from 914sixer and am currently entertaining re-using the stock 2.0 crank with the above mods. I may re-use the Mahle cylinders and have them bored out to 96mm. It's a actually more expensive than buying new cylinders but I've seen way too many complaints on the forum regarding the quality of currently available new parts.
You, yourself, wanted to re-use pushrod tubes and I agree with you.

Jake Raby killed a new flywheel for me just recently. His machining .300 material of a brand-new German flywheel was an eye opener for me.
His advice- find used real VW stuff from the 70s. I have four...

What I'd prefer to do is find a good 78mm crank but the costs associated with the stroker build quickly escalate to greater than the price of a used conversion engine.

I haven't built a performance engine with greater than .042 quench in 20 years.
All my stuff is streetcar (and crappy fuel) based and quench is paramount to me.

If you're shooting for 8.3-1 you are fairly detonation resistant just because of low compression and it is far less important.

A water pumper below 9.5-1 is an appliance motor to me (not worth going hard) but this is not normal in the air-cooled world.

It appears that the T4s most of you are running have bigger cams and lower compression (a full point) than water cooled stuff.

So, I'm a builder and have to rip it all the way down. I cannot resist the urge...
You should do you, if you want conservative, stay conservative. It's cheaper.

I'd have no issues running your combo down to .040 but I wouldn't actually do it.
I'd do more!

I'd expect a proper 2055 build, with the above mods to generate 125 HP at the flywheel using normal supporting pieces (dual 40s or stock FI / 123 distributor / premium fuel / conservative tune).

My two liter will have 9.5-1 (or higher) and a cam profile nearing 300 degrees of duration.
I will run LESS than .040 quench on this motor but it will be blue-printed, balanced and assembled by an OCD pin-headed bitch.
With a modern sequential COP FI system and E/85 fuel I want to hit 125 at the wheels which is getting close to doubling the stock power

My first 914 was a stock 1.7l with a big bore kit in it (1911?). It was assembled by me when I was 16/17. I didn't know what I was doing back then but it still was great fun and was one of the early builds that started my automotive career.
I didn't even know about quench back then...

All good points, but I exceeded my budget on this (time and money) long ago. In a couple years I will do it again and get the bottom end.

I think I will add .02s to my order and make a decision at install. I will start with the .02 and measure it up and see how I feel about it.

Engine is going back together now. Ended up with a .02 shim that put me at .043 for my deck height. A little tighter than I wanted but the 8.6ish compression is right around what I want. Checked the valve clearance on the pistons with some clay, no issues there. Exhaust didn't even touch the clay and .960 depth of clay on the intake. Tomorrow I will check the valve geometry and it's just engine tin and reinstall....

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Exciting stuff

Go man go

I’ve built three for mine and those three days when your hard work roars back to life, are never hard to recall for me. Unlike last week...
Even your anticipation is killing me. LOL Slow and steady wins this race, have fun. Maybe check the end play on the crank while the flywheel is off