Best way to achieve piston to head clearance?

[HizAndHerz]

I need some info I haven't found by searching and I'm trying to get my stroker block put together this week.

 

EDIT: I managed to get myself totally confused by reading too many posts and articles with conficting data. I think I am on the right track with the following:

 

I'm going for ~0.8mm piston to head clearance (0.031") with a 1mm head gasket. Deck height with KA24E pistons shaved flat (0.5mm) is 0.13mm. With a 1mm head gasket, the clearance should be 0.87mm (0.0343").

 

Questions:

1. Does this amount of clearance reasonable? Does the clearance need to be smaller to optimize quench?
   
2. After assembling the engine and measuring the actual deck height, what is the best way to adjust piston clearance? Getting the block milled or using a thicker or thinner head gasket?
   
3. What are my head gasket options? Do Kameari and Tomei still make gaskets in 0.1mm increments? I believe that Kameari also make a 0.8mm gasket. This is a N/A street car so a $250 gasket is overkill, IMO.
   
4. Who sells HKS headgaskets?

I realize that these questions could be answered by the machine shop, but I don't have much experience in dealing with them. Better to go in a little more educated, yes?

 

BTW, the build is for a N/A street car:

• F54 bored to 89mm
  
• V07 crank with L24 rods w/9mm bolts
  
• KA24E cast pistons shaved to 33.5mm (no dish)
  
• Unshaved P90 head (mild port and polish)
  
• .450/280/280 cam (for now)
  
• 6-2-1 headers to 2.5" exhaust
  
• JSK rail with 330cc/min injectors
  
• Ported intake w/60mm TB

Thanks in advance for your expertise!

[JMortensen]

.13mm = .005" So you have pistons sticking up .005 out of the deck. If you want the piston to deck clearance to be .024, that leaves you only .029" for the headgasket. So you need a super thin gasket, not a thicker one. A 1mm gasket is .040, so that would give you .045" clearance. If you look you might find a thread about HKS 2mm gaskets where the idea of peeling off layers was talked about. I had been under the impression that all of the layers were the same thickness but apparently they are not. I can't recall how thin the thin ones were, but I seem to think that there were 5 layers, so they'd have to be thinner than .040".

[e_racer1999]

i think that the thicker head gasket comes into play if the block is milled MORE than the .25mm (i'm assuming that milling the block is necessary)

[HizAndHerz]

.13mm = .005" So you have pistons sticking up .005 out of the deck.

I knew I was going to make a big boo-boo with all those numbers in that post.

 

Actually, I'm going for 0.8mm piston to head clearance (0.031"). With a 1mm head gasket, that would require -0.13mm deck height. I edited my post to reflect that.

[JMortensen]

You still have down in the original post that you need a thicker headgasket.

[HizAndHerz]

You still have down in the original post that you need a thicker headgasket.

Wow, right. I need a thinner headgasket. I deal with numbers everyday as a EE, you'd think I could wrap my head around this.

 

Here's the data:

[1 fast z]

You can NOT have any less than .022" Piston to head clearnce or they WILL tap.

 

 

Trust me, this is one area I Have experemented ALOT in.

[BRAAP]

Sorry to for being slightly off topic here, but do any of those L-series engine calculators take into account that the OE Flat tops pop up out of the cylinder bore .020”-.025”? I always just use my own spread sheet for compression ratio calculations, I have never used those L-series calculators, but have heard them mentioned quite a bit.

 

I was just curious.

Paul

[JMortensen]

Sorry to for being slightly off topic here, but do any of those L-series engine calculators take into account that the OE Flat tops pop up out of the cylinder bore .020â€-.025â€? I always just use my own spread sheet for compression ratio calculations, I have never used those L-series calculators, but have heard them mentioned quite a bit.

Yes, the lengine.exe one does.

[HizAndHerz]

First, thanks for the help so for. My brain has tuned to mush over this. Now I am assuming that the optimum piston to head clearance is 0.030" to 0.040". That works out perfectly with milled KA24E pistons with 33.5mm pin height and a 1mm head gasket. Please check my data above and tell me if this is all reasonable.

 

Paul, from what I've seen, there's at least 3 L28 engine calculators out there. All of them seem to use data from "How To Modify..." which I think contains errors. The programs aren't bad, just the data. I've seen the stock L28 rod length specified as 130.2, 130.35 and 130.4 in different sources. I modified one of the online calculators with the data from here: http://zhome.com/ZCMnL/tech/128combo.html

 

I'm going to go through the calculations by hand and see if it matches my modified code.

 

 

 

 

EDIT: I can't come up with 0.020" deck height calcualting it by hand or with the online calculator. Can anyone spot erroneous data? By hand, here's what I calculate:

Deck Height = Pin Height + Rod Length + Stroke/2 - Block Height

0.0031" = 0.08mm = 38.1 + 130.35 + (79/2) - 207.87

My version of the online calculator comes up with the same result for a flattops + Maxima N47 + 1mm gasket engine:

 

[JMortensen]

L28 rod is spec'd at 130.2 in the How to Modify book. Courtesy Nissan shows 130.5 and also 5.13" 5.13" is 130.302mm, so ??? I never realized this was an issue before...

 

A 1mm gasket is .040" thick, so if you used one you'd have .035" clearance due to your .005" pop up height. Sounds like you're right in the ballpark to me.

[Tony D]

In the VW world, assembling the engine in the case (block) to be used and physically measuring the piston deck height is how the cut on the pistons is determined.

 

if you know compressed gasket thickness, I would assemble the whole thing with UNCUT pistons, do a physical measurement of the deck height of the pistons, and go from there.

 

You can specify the end-to-end blueprint dimension when you have your rods rechecked and honed. Whatever dimension you specify is the number you should use for your calculations. But I wouldn't cut the pistons until I trial-assembled the bottom end and physically measured the deck height.

[HizAndHerz]

You can NOT have any less than .022" Piston to head clearance or they WILL tap.

... the OE Flat tops pop up out of the cylinder bore .020â€-.025â€...

 

If those are both accurate, that would imply that you can't use a 1mm (0.039") head gasket with OE flattop pistons, no?

 

0.039" - 0.25" = 0.014" clearance

 

I read somewhere that flattops + 1mm gasket + Maxima N47 was a good setup.

[HizAndHerz]

Ahah! Here it is:

 

With the OE L-28 flat tops on OE L-28 rods, the pistons pop up OUT of the cylinder bore at TDC approx .021”-.025” and with a 1mm gasket this offers pretty much “perfect” squish. The pistons come so close to the head deck surface that if the pistons has its oversize stamped in the right area, that stamping will ghost imprint on the head deck surface, yet the pistons do NOT actually contact the head.

[HizAndHerz]

But I wouldn't cut the pistons until I trial-assembled the bottom end and physically measured the deck height.

That's good because my pistons have not actually been shaved completely flat (the full 0.5mm). With any luck there will be enough to play with when the engine is assembled. If they're too short already, I guess I can have the block milled.

[HizAndHerz]

One more quote:

... QUENCH is the key [to controlling detonation], and you need NO MORE THAN .025" clearence from piston to head to make that quench efficiant...

 

So, more than 0.025" clearance and quench is not efficient, but less than 0.022" results in clashing pistons. Very interresting. I gotta ask: are those tollerances really obtainable? How might engine wear affect all of this?

 

Your knowledge is appreciated!

[Tony D]

Generally connecting rod bearing wear is on the top bearing, letting the clearance increase as the engine wears.

 

That is why things go boom when rod bolts start stretching....

[1 fast z]

Anything over 25 and you loosing, exponentially from there. 22-25 thou is perfect. Anything less than 22 is DANGEROUS. Pistons from the factory protrude out .016". Ive measured TONS of L series blocks, and never found a number more than this.

[BRAAP]

First off, this post is NOT meant to discredit 1 fast Z in any manner. I hold Bryans work in high regard and would have no issues running one his cylinder heads or engines in my own personal Z car. At his young age, he has amassed a great deal of actual hands on experience building, machining and taking Datsun power plants to extremes that the rest of us only bench race about. Some feel he is abrasive, and yeah he can be sometimes, the point is, his work speaks for itself. He KNOWS what he is doing and talking about in regards to engine building, machining, and extracting power from the L-series.

 

With that said and the fact that I’ve haven’t measured any OE flat top piston "pop up" at any less than .019”, it would seem that L-28’s in Arizona just aren’t as “pop-up” endowed as Oregon L-28’s. (Sorry, I couldn't resist that one).

 

In regards to piston pop up, (negative deck height), there are many factors that can and will affect piston deck height. Nissan machining consistency throughout the years was quite good, though machining tolerances such as crank pin offset, crank main line to deck height tolerances, etc can all affect deck height. Align honing a block, resizing the connecting rod big ends, offset grinding the crankpins, staggering bearings for fine tuning the bearing clearances, etc will also alter the piston deck height to some degree.

 

In all the flat top L-28 builds I have measured, I think .019” is the shortest I’ve measured though it may have been shorter, I just don’t recall. The norm I have seen seems to be between .020” and .025.” Pictured below is one such flat top L-28 currently in our shop that we degreed in the cam and now are machining valve relief’s in the piston tops. It is an F-54 block punched +1mm, L-28 rods, OE +1mm flat top pistons, Rusch Motorsports built MN-47 head with a Rebello .520” cam, triple Webers, FelPro head gasket. Rods have been resized with ARP bolts, Clevite bearings used throughout. Piston pop up measured “along” the wrist pin, on all 6 cylinders, at the front and rear of the piston, this engine measures a negative piston deck height of .0225”-.0230”. (Twenty two and a half thou to twenty three thou.)

 

So with a .023” negative deck height, and a .045” compressed head gasket, piston to head deck clearance for this particular flat top L-28 and MN-47 combo is .022” I have ran this combo on more than a few engines and it will leave the imprint of the pistons oversize on the squish portion of the head, but there is NO evidence of actual contact.

 

As Bryan eluded to, when you are building your engine, you either want squish, and all of it, or if you can’t acquire squish, you need more than .100” clearance between the top of the pistons and the chamber of the head. Between .050” and 100” is a nasty detonation zone. Between 050” and the .022” clearance that Bryan is talking about and the anti detonation tendencies of squish are realized and the benefits of added static compression can be realized.

 

 

FWIW, I found and down loaded the “Lengine” calculator produced by the “Sydney Z car club” and found a discrepancy in the Felpro head gasket volume. It is listed as being only 5.7cc. (Maybe a different Felpro head gasket than we get over here?), The FelPro Print-O-seal head gasket, whether it was just recently torqued up and disassembled, or ran for several years and dissembled, I’ve measured them at 044”-.045” compressed thickness, fire ring ID is a perfectly round exact 3.5” or 88.9 mm which equates to a 7.0 cc volume.

 

 

[HizAndHerz]

Paul,

Thank you, thank you for sharing your insights!

 

Sorry to for being slightly off topic here, but do any of those L-series engine calculators take into account that the OE Flat tops pop up out of the cylinder bore .020”-.025”?

 

LENGINE.EXE reports a deck height, or as you say negative deck height, of 0.030mm or 0.0012". That's way off from the measurements you guys have taken. I went out to the garage and measured a F54 with flat pistons and came up with 0.021" but that was with a straight edge and feeler gauges.

 

One thing is clear to me, the engine calculators can get you in the ballpark, but that's it. Also, there are different specs for the L-series engines floating around out there.