Pistons protrude from block - thicker gasket?

[rick240zed]

I have an F54/P90 combo with ITM .040 oversize flattops that is just about ready to get put together.

 

The fellow doing my machine work has let me know that my pistons protrude from the block deck by .025 and he feels i should use a thicker gasket than the Felpro one I have sourced.

 

Any suggestions on an appropriate gasket?

 

Any other solutions?

 

Thanks

[malibud]

Are they cast ? May have to take them out and machine them down ? 

[rick240zed]

Yes, they are cast - but getting a 1.5mm head gasket would be much cheaper.

TEP has 1mm and 2mm MLS in stock.

It looks like there is a TOMEI 1.5mm; just wondering if there are others?

[Tony D]

I had 2mm pop out with my pistons (0.080") no ill effects. These are not zero-deck-height engines in all applications. Clearance to the head is what matters, with those 2mm pops and using a standard fel-pro gasket clay check of the dome to CC showed 0.040"+ clearance on the E88 head I used.

 

The ONLY way to know absolutely for sure is to assemble it with a used head gasket and clay the domes. 

[rick240zed]

The E88 and P90 headshave totally different shaped combustion chambers.

 

In an E88 the piston can stick above the deck (and probably the gasket) with no ill effects; on the P90 this can't happen.

[Leon]

I'm going off of memory, my build thread should have my numbers but my L28/shaved P90 with flat-tops popped out the deck something like ~.023". I used a Nissan gasket. Like Tony said, clay-check your clearances. It's the only way to know. I came up with something like .025" of clearance. I did a lot of reading and researching to make sure this was OK. There are some really great posts in the archive by BRAAP and Brian Blake aka "1 fast z" on the topic. Haven't had an issue so far...

[NewZed]

I saved the following thread a while ago because it had some interesting considerations and numbers.  Quench and piston rocking,  I've also seen that you either want to be in the "quench" area or completely away from it.  If you're not in it but too close you can actually create a detonation zone.  No facts to back it up, just a consideration.  Some people seem to think that the Datsun "quench" heads weren't doing any quenching at all.

 

http://forums.hybridz.org/topic/106806-long-rod-short-piston-combo-l28-tech-questions/?do=findComment&comment=999867

 

Numbers-wise, .025" = 0.635mm.  So with a 1.25 mm HG (after installation), you'd have 0.615mm or .024" "quench" zone.  So your mechanic might be right, depending on which part of the discussion in the thread I linked you look at.  BRAAP suggests that .014" quench is "perfect" (Post #16), but two others stop at .028" and .025".  

[Xnke]

I've had stock, nissan-assembled engines with pistons 0.22" above the deck. They had run 100K+ miles at that point and my daily driver has pistons 0.022l above the deck as well. This was with E, N, and P prefix cylinder heads, never an issue. All heads run with fel-pro gaskets, or Ishino gaskets.

[rick240zed]

I saved the following thread a while ago because it had some interesting considerations and numbers.  Quench and piston rocking,  I've also seen that you either want to be in the "quench" area or completely away from it.  If you're not in it but too close you can actually create a detonation zone.  No facts to back it up, just a consideration.  Some people seem to think that the Datsun "quench" heads weren't doing any quenching at all.

 

http://forums.hybridz.org/topic/106806-long-rod-short-piston-combo-l28-tech-questions/?do=findComment&comment=999867

 

Numbers-wise, .025" = 0.635mm.  So with a 1.25 mm HG (after installation), you'd have 0.615mm or .024" "quench" zone.  So your mechanic might be right, depending on which part of the discussion in the thread I linked you look at.  BRAAP suggests that .014" quench is "perfect" (Post #16), but two others stop at .028" and .025".  

 

So according to Braap (if i read correctly), then with .025 protruding and a Fel-Pro (1mm compressed, I believe) then it should be perfect.

 

So which gaskets are 1.25mm compressed (for safety, and slightly over .020 quench)?

Edited September 16, 2014 by rick240zed

[Tony D]

The E88 and P90 headshave totally different shaped combustion chambers.

 

In an E88 the piston can stick above the deck (and probably the gasket) with no ill effects; on the P90 this can't happen.

 

You COMPLETELY missed the point. Please Read Leon's Response, it is the correct thing to take away from what was written.

 

And the "which E88" comes into that discussion as well... it's moot, the head is irrelevant.

Edited September 16, 2014 by Tony D

[Tony D]

I've had stock, nissan-assembled engines with pistons 0.22" above the deck. They had run 100K+ miles at that point and my daily driver has pistons 0.022l above the deck as well. This was with E, N, and P prefix cylinder heads, never an issue. All heads run with fel-pro gaskets, or Ishino gaskets.

EXACTLY! They are NOT 'zero deck height' engines, 0.020" range is not anything to be concerned about...and changing gaskets without an actual PHYSICAL CLEARANCE CHECK is foolish.

 

Assemble the engine, determine what you need, DO NOT GUESS!

[rick240zed]

EXACTLY! They are NOT 'zero deck height' engines, 0.020" range is not anything to be concerned about...and changing gaskets without an actual PHYSICAL CLEARANCE CHECK is foolish.

 

Assemble the engine, determine what you need, DO NOT GUESS!

 

I understand how measuring is the only way to know exactly what clearances will be; however, I am a bit confused on a number of things.

 

If I understand correctly, a head gasket should only get used once - and all instructions for measuring clearances say to use a used gasket in the process. So any measurements taken would only be valid in the field for a new head gasket that compressed to exactly the same thickness as the one you used in your measurements.

 

If it is valid to say that using a new version of the same gasket as the one used in measurements will give the correct results; then why wouldn't it bevalid to say "if your pistons protude .025 from deck then you will need a gasket of xxx thickness - and your options at that thickness are zzz or yyy"?

 

I am not the first person to use a P() on a block with flat tops - I am assuming not the first whose pistons protuded .025 (I have in this thread learned that all engines protude in the .020 range), I am just looking for experience in selecting a gasket.

 

Also, the fellow who built my engine has built many engines, but not a lot of Datsuns. His concern is mostly from stretch at higher rpms (if I understood him correctly) which of course is something a static measurement can not tell.

 

I am not trying to be argumentative, I am just trying to get my engine in my car - which I had hoped would happen in June, and is ready to get assembled today - assuming I have a head gasket I can safely use.

 

I remain confused - sorry.

 

 

Also, I can understand how clay measurements may be more necessary to determine piston-to-valve to deal with head shaving and various lift cams. I have an extremely modest cam (Don Potter mild street, designed for stock springs) so I am really only concerned with piston-to-head clearance - which should be able to be determined by measuring height above block and knowing head gasket thickness. Please correct me if i am wrong.

Edited September 16, 2014 by rick240zed

[z-ya]

I believe the stock deck height of an L28 is 0.030", so as long as you are less than that, you can use a stock head gasket.

[Tony D]

Give my your data on compressibility repetition of various gaskets, and try that angle.

 

They say to use a previously compressed gasket as it's a known quantity. If you change to a different gasket, the 'thickness' uncompressed is what compressed again? Nobody knows.

 

The only way to KNOW thickness is to do a physical CHECK.

 

This is a problem with engineers, they calculate. They recalculate. They BUILD it and wonder where that knocking sound comes from...

 

This is one of those things where you measure to PROJECT your needs, but ASSEMBLY and CLAY CHECK is the ONLY way to KNOW what you have.

 

If you do this for valves in a dynamic state the results are similar. Same for the piston to head clearance... the way to know is to physically measure.

 

Anything else is just a guess.

[rick240zed]

Give my your data on compressibility repetition of various gaskets, and try that angle.

 

They say to use a previously compressed gasket as it's a known quantity. If you change to a different gasket, the 'thickness' uncompressed is what compressed again? Nobody knows.

 

The only way to KNOW thickness is to do a physical CHECK.

 

This is a problem with engineers, they calculate. They recalculate. They BUILD it and wonder where that knocking sound comes from...

 

This is one of those things where you measure to PROJECT your needs, but ASSEMBLY and CLAY CHECK is the ONLY way to KNOW what you have.

 

If you do this for valves in a dynamic state the results are similar. Same for the piston to head clearance... the way to know is to physically measure.

 

Anything else is just a guess.

 

I still am confused by part of the process.

 

In the end - when you do the final assembly of the engine, do you use a new head gasket?

 

If so, how do you know the compressed height of that new gasket will be the same as the used one you had for your measurements?

 

If not, how many times can you re-use a head gasket? I thought it was advisable to use a new one every time.

[Tony D]

In the end - when you do the final assembly of the engine, do you use a new head gasket?

Yes, you use a new head gasket when assembling the engine the final time. The used gasket is only for setup and clearance checking. Nobody has said to re-use a gasket for anything other than a consistent known quantity to set clearances.

If so, how do you know the compressed height of that new gasket will be the same as the used one you had for your measurements?

You don't, that is why 'CLEARANCES' exist in a range. You can be sure the repeatability of crush on a given gasket will be within a 3-Sigma Limit and that if you used a compressed Fel-Pro, you can be sure that within a thousandth or so, the gasket will crush to that same area. Using a FelPro to check a Ishino--no use whatsoever. You may have to waste a gasket if you want to gauge the height correctly. Welcome to Proper Engine Assembly 101. The clamping forces are calculable, the engineering is sound...you can take 100 new gaskets, crush them down, measure the results and do your own research and come up with your  own numbers to use...but given that this process has been used since before I started doing this, it's a sound and accepted way of doing it, your comprehension or agreement with it notwithstanding. 

The key is the VARIATION from gasket to gasket when compressed is 0.001" MAYBE. The variation of an uncompressed gasket is quite big as you aren't measuring the fire-ring when clamped. I know shops that keep a variation of gaskets to use for this setup, the compressibility and setting/gasket creep phenomenon is too much to enter into here, suffice to say once it's compressed, the change of compression is nil. Afterwards you can clamp and clamp and as long as the clamping forces do not exceed the original forces used, the gasket will not compress any further.

If not, how many times can you re-use a head gasket? I thought it was advisable to use a new one every time.

Nobody here has EVER said to re-use a head gasket. I think you are not comprehending that the used gasket is for SETUP AND CLEARANCE CHECKING and not for running the engine. Some gaskets  are designed for re-use, you are not at that level where you are using one of them. Using a new gasket is mandatory for competent engine builders...again, we can get into the engineering discussion of engineering a jointed gasket and compressibility but it's not necessary... the clamping forces are present on the fire ring to seal combusion, but universally the issue arises of coolant leakage from the fiber compressed portion of the gasket when re-used. You can clamp the gasket all you want, but it will NOT compress further unless the fire ring embeds into the head....to allow further compression of the fiber portion and seal it upon re-use.  The fire-ring is the limiting factor in combustion chamber clearance checking, and it will only go so far from new. The fiber outer section is another animal altogether, and it generally spent after first removal---it's condition doesn't matter in combustion chamber clearance checking, only the fire ring. It crushes to a known distance and physically stops due to the pressures exerted, and will not measurably change unless clamping forces are greatly increased. 

 

This is why you use the USED gasket to set your clearances, not a new one. You can waste a new gasket to check them (compress it, then throw it away) if you want to, but most guys just keep the old FelPro sitting on the wall to aid in clearance checking. There will be no measurable difference between clearances arrived at using a used gasket, as opposed to wasting a new gasket. You just save the cost of the gasket.

[rick240zed]

 

This is why you use the USED gasket to set your clearances, not a new one. You can waste a new gasket to check them (compress it, then throw it away) if you want to, but most guys just keep the old FelPro sitting on the wall to aid in clearance checking. There will be no measurable difference between clearances arrived at using a used gasket, as opposed to wasting a new gasket. You just save the cost of the gasket.

 

 

Thanks, that all makes sense; and confirms what I thought, that the thickness of a compressed gasket can be counted on before installation.

 

It also confirms what I thought, that use of clay is most relevant for determining piston to valve clearances.

 

For piston to head clearances, you can measure the amount of pop-up and you can measure the thickness of a compressed gasket; unless I'm still missing something, then the difference of the two numbers is the measured piston to head clearance (and should provide the exact same number as claying the chamber).

 

It would seem that a listing of expected compressed gasket thicknesses for all common gaskets would be a useful resource. Has anyone ever put one together?

 

I envy you and others who have multiple old gaskets lying around, and a wealth of knowledge (I hope to gain from). My project is the first time I have ever been so involved in an engine for my car, and is likely to be not only the first but the last engine I build (I have owned my car for 34 years with a couple of used engines and a couple of rebuilds done by others; I am trusting this one will last me another twenty years of cross country pleasure driving with maybe a head job in 10 years).

 

So can you tell me what the compressed thickness of a Felpro 8799 is? I have heard it is about 1mm, but is that .040 +- .001 or .042 +- .001? I have no idea.

 

Do Cometic MLS gaskets change in size when compressed, or are they the same height as advertised (I read this in another place, but again i have no real world experience or knowledge)?

 

Thanks for your continued patience.

Edited September 17, 2014 by rick240zed

[Leon]

As a double-check, I stuck a dial indicator on the block and bolted a plate onto the head to measure against (I think I got the idea from Lazeum?). After clearance checking and upon final assembly, I put the head on the block without a gasket and zero'd the indicator. I then installed the new gasket, torqued the head, and recorded the dial indicator measurement. That measurement indicates total crushed gasket thickness. If you got a good measurement of piston deck height, you can then easily calculate piston-to-head clearance.

[Tony D]

"It also confirms what I thought, that use of clay is most relevant for determining piston to valve clearances." 

 

How do you get that out of anything I said? The combustion chamber is not necessarily flat... the clay determines the exact clearance when installed. On pop-dome pistons and not simple flat tops, this is as good as you will get as most of the time these parts are hand-fitted to the combustion chamber in some places. The clay will show highs and lows.

 

I don't have gaskets laying around, if I need one, there is a reason you trial assemble engines. As I said, "you will waste a gasket" ... Most people starting out and properly advised will have a gasket they install with anti-seize liberally applied, and run it through several torque/relax/retorque cycles and then wipe it all down and go to town with their new "checking gasket"... nice an reusable.

[clarkspeed]

Everything Tony said, and he said a lot.

 

I will add the NISMO gasket just happens to be 0.6mm which is .023", which just happens to be lower clearance limit recommended by 1fastz and others on this forum. I can also say from experience if you spin a rod bearing with this clearance the piston will hit something.