installed piston height

[SteveK]

What is the maximum amount that a flat top piston can protrude above the deck at TDC

After installing my pistons I am seeing a positive height above the deck of between 0.008 and 0.010 of an inch (8 and 10 thou). I am using a fel-pro gasket that, uncompressed is 50 thou and compressed should be 40 thou.

Does 40 thou - 10 thou (30 thou) = trouble or will I still have enough to give adequate piston to head clearance and tight enough to give a good squish area.

If this is a problem would I be better for safeties sake to us a slightly thicker head gasket ??

Once again Phred if you’re around I would really appreciate your input

Ta

[SteveK]

Anyone any ideas

[Bambikiller240]

I have no numbers, but on my stock L24 engines, Original and rebuilt (.5mm overbore), there IS a slight protrusion of the flat top pistons above the deck of the block. I'm speakin about L24 engines here only. I know nothing about L26 or L28 in this regard.

You might consider sending Phred a PM as he doesn't spend as much time here as many of us.

[Ed]

What is the maximum amount that a flat top piston can protrude above the deck at TDC

After installing my pistons I am seeing a positive height above the deck of between 0.008 and 0.010 of an inch (8 and 10 thou). I am using a fel-pro gasket that, uncompressed is 50 thou and compressed should be 40 thou.

Does 40 thou - 10 thou (30 thou) = trouble or will I still have enough to give adequate piston to head clearance and tight enough to give a good squish area.

If this is a problem would I be better for safeties sake to us a slightly thicker head gasket ??

Once again Phred if you’re around I would really appreciate your input

Ta

I think all this is really going to depend on the condition of your head. How much has it been shaved? Or has the combustion chamber been modified? You will really need to measure the combustion chamber cc's, provided along with your bore/stroke and the head gasket cc's and the 0.08 positive deck height. You can then find out exactly what your compression will be.

[SteveK]

Thanks guys, its not really the compression ratio that concerns me (the chambers are cc'd at 40) its the potential for the piston to hit the underside of the head which is far more worrying

[MikeW]

Thanks guys, its not really the compression ratio that concerns me (the chambers are cc'd at 40) its the potential for the piston to hit the underside of the head which is far more worrying

I think the thickness of the head gasket is all that really matters here and as you've observed it more than covers the slight protrusion. When the piston is at the top the valves are both completely closed. The valves are the only part attached to the head that ever protrude beneath the bottom of the head. Therefore the valves are the only part that descend beneath the top of the head gasket.

[SteveK]

Mike, the diameter of a 2.8 piston is 86 mm, the diameter of the combustion chamber is less than this at the sides, with thermal expansion, directional change lalala there is an area under the head that could be impacted (squish area ramp) any impact at these speeds is disasterous.

The valves do not worry me as I have machined cutouts on the piston heads

[MikeW]

Mike, the diameter of a 2.8 piston is 86 mm, the diameter of the combustion chamber is less than this at the sides, with thermal expansion, directional change lalala there is an area under the head that could be impacted (squish area ramp) any impact at these speeds is disasterous.

The valves do not worry me as I have machined cutouts on the piston heads

I understand your concern and I'll defer to someone like Phred who knows far more about this than I ever will. I wasn't thinking about thermal expansion of the piston causing the gap to become even smaller. I'll be interested to hear more about this myself. I've attached a crude diagram that I just made that I think describes what you're running in to.

[SteveK]

Cheers Mike, I am led to believe that there are quiet a few dynamics that can affect things.

Plus I have read that even if the gap were safe it is unwise to have it too small as the squish area becomes inefficient -- 40/50 thou being optimum

I have sourced differant pistons now with the correct pin height (one was a little iffy on the bottom ring groove--- shame cos with the skimmed dished pistons, I was using the longer L24 rods that gave me a small torque gain, now I have to go back to the 130.2 L28 rods

Its all fun --- only thing left is to get the multiple throttle bodies mapped and hey presto

[ZSaint]

I think the stock Nissan headgasket is thicker then the Felpro gasket. In terms of cc's... The Felpro gasket has 5.7 cc's while the Stock gasket has 7.1 cc's. The stock headgasket will provide you with additional clearance. I believe they say you need .040-.050 clearance on your piston to head clearance. Everything stretches a bit at 7,000+!

[Phred]

ZSaint should know this as he has smacked more than one piston into a head.

What it gets down to is known as Piston to Head Clearance. Basically, you just do whatever is required to keep those two important parts from hitting each other. The magic number varies a bit depending on the type of engine, rods, and RPM. For Z engines, (that will not go past 7000) the magic number is about .037. I have snuck up on that number a little at a time. At .037, the piston will just be so close as to not even let carbon build up, but not cause damage. I did one at .032, and it left a mark on the head. So to be safe consider .042 a minimum. Nissan gaskets will compress to about .045. which is to say, if you have a plus .003 deck height, you're ok. Depending on what head you have, the edge of the piston can whack the head in different areas. When pre-assembling, I always put a little childrens modeling clay in the suspect area, bolt the head down, and run it through one revolution. Then carefully remove the head and cut sections of the clay out with a razor blade. You will be able to see excactly where the danger points are. Then I'll either touch up the close areas with a die grinder, or if I need more, take the pistons to the mill, and take a bit off the domes in a more accurate manor.

Phred

[ZSaint]

I think that a piston "rocks" back and forth once it gets broken in. If it remained plum in the bore, all would be well. I remember the first 'race' engine I built was a 2.0L BMW. The 12.5 Venolia pistons came with a huge dome. I had to modify the head to get the domes to fit. I also remember the pistons hitting the head! After that, I let Phred and Hatch do all of my stuff. Pay 'em up front rather than pay for another set of new pistons! I did a lot of 'trial and error' engineering. It ended up costing me a lot of time and money. It was a nice way to learn engine building, however. I also remember building an oil pan baffle for my Tiger. It was a thing of beauty! I forgot about the crank throws... It made a heck of a noise when I started it. Oh well, I learned the hard way. Things were good in the 70's!