Thread: Piston to head clearance

Can anyone tell me what is the minimum piston to head clearance that I can safely run on an SR20 with stock crank, stock rods and stock cast pistons. Intended RPM ceiling is 8500 rpm.

Also if there may be a clearance difference between the 4 CW and 8 CW cranks and the different N/A stock rods, DE, DE-RR, VE

Further to this can someone specify if certain aftermarket parts (rods or pistons) or even coatings can help reduce this even further.

Any help will be greatly appreciated. Even data on other non nissan engines with similar stroke and bore/piston weight.

piston to head clearance can be very tight but it really depends highly on the piston to wall clearance because when the motor is cold, the piston will want to rock in the bore at TDC when changing directions. If you run a headgasket that is atleast 1.0mm thickness then you are more than safe with pistons that are even with the deck height. With a 1.2mm head gasket, I have run pistons as far as .015" out of the bore ( protruding above the deck) safely but this was on a highly spec'd out motor and I wont not recommend this to anyone because you can run into problems very fast.

Just wondering, but were you wondering more about piston to valve clearance and not piston to head clearance?

-Ted

thanks for that reply.

it is piston to head clearance I am looking at, trying to find ways to increase knock resistance.

what headgaskets comercially available are 1.0 mm? the stock ones are 1.2mm correct?

Can you expand on the concept of the "highly spec'd" motor because if I have to go there I will.

Does nissan menion in an FSM a minimum clearance or indirectly a maximum the block can be decked? or minimum distance from crank bore to block surface that kind of thing?

You can deck the head as much as you want and the piston to head clearance will not change. Its when you start to really deck the block that it changes because as you deck the block the piston comes out of the hole. Nissan specify's that the piston be atleast .1mm in the hole i believe ( dont quote me on that 100% though).

If you run flat top pistons with valve reliefs that is your best bet. I believe what Ted is talking about in a "specd out motor" is that you have gone through and set all your clearances and tolorences in the motor and you know what is happening. The easiest way to roughly measure piston to head clearance is to put the piston at true TDC and then rock the piston in its bore all the way to one side then measure how far above the block the highest part of the piston is protruding. If you have domed pistons do not measure the dome part, just the part of the piston that will be covering the quench area. On stock Cast pistons with std piston to wall clearance of .0008" the piston will not even come out of the block above the deck. The more piston to wall clearance, the more the piston will rock and protrude. As the engine warms up and the piston expands, the piston will setttle more "squared up" in the bore.

Another option for decreasing quench area for knock resistance is quench welding and shaping. I now do combustion chamber welding and reshape the quench pads and add a lot of material to the combustion chamber and allows for a much faster burn rate as much more of the A/F mixture will be centered towards the spark plug. In addition to that, when done right you can gain a considerable amount of low lift flow on both the intake and exhaust. On the last K20 head that I did, we got about 15-25cfm increase from .050" lift to .250" lift just from welding and shaping. This helps guide the airflow around the valves. On the exhaust side of that head, it now flows more than the intake up to about .200".

On a stock head, this will remove about 4-5cc of chamber volume. On a stock motor with stock pistons, this can raise the compresion ratio by over 1pt but it will now have better knock resistance than before.

PM me if you are interested in prices. I can post pictures later if you wish of the head i'm currently working on.

It was deck the block I meant to write, usually its "shave or mill the head". My target head is actualy a VE, so I see u mentioned K20, have you done this kind of work on a VE head as well? I have been thinking of exactly what u are speaking about plus some other ideas based on some stuff I read from the honda camp "endyn" or somebody like that. Also there was an article on this or some forum about (nissan)piston shapes(both domes and skirts/pin bosses) and of course the information and ideas are kind of there but how to get it into a live fire breathing SR is the problem. Those endyn people also mention piston shape.

Why flat top pistons? This is my preference as well more from a gut feeling than anything else really. I wanted totaly flat not even valve reliefs but I don't think I can get away with that and the VE head and any good cam.

I don't live in the US so I was just focussing on what I could do safely here with the basic machine shop work. get the clearance down to the minimum for my setup. Forged rods and lighter pistons may reduce this even more. Maybe run the engine a few degrees cooler with a different thermostat for a bit more knock resistance.

my intention is a tight piston to bore fit and I was thinking if the engine is warmed up first and the piston squares up before reving to redline it would help too.

I was thinking that at high RPM the rod would stretch a bit, the piston would have expanded some as well when warmed up but so would the block. Dynamic warm clearance would be different to static cold clearance and my thinking is that whatever nissan specified was to accomodate for this plus a safety margin for production tolerances.

The reason why I love flat top pistons over domed, even when raising compression, is because of flame travel. When you can get an fast even burn across the piston you can get a very good, strong burn which results in more BMEP. If it takes too long to burn A/F charge, the piston begins to move away from TDC and can actually move away from the flame front on a very high piston speed or long stroke short rod motor. Also, with a flat top piston, you can achieve a very light piston because you do not have to brace the piston around all the valve reliefs and then through the middle with the solid dome. You can then actually machine a nice strong billet piston in the end which has great weight savings as well as a more equal heat expansion rate. Now you can run a tighter piston to wall clearance and be safer than before.

You will need valve reliefs if you want to run N1's or even a larger cam in the future if they become available.

My feelings about the flat top pistons were mainly centered around flame travel, I didn't think about all those other benifits.

Looking at honda pistons used in HC motors I was begining to think my gut feeling was gas, since the piston used "looked" similar to the 16ve although they tended to come to a sort of peak rather than a small plateau.

Thanks for your input snickers.

The reason the honda pistons are very peaky for high compression motors because of the valve angles are much steeper. However, because of these higher valve angles, they can gain a much higher compression ratio. This allows them to have a smaller closed combustion chamber for faster burn, but the super high domed pistons can negate that.

Welding the chamber to close it up for a better quench area is always the better way to go, but it is very very time consuming or expensive if you are not the one doing the work. It is very easy to get $5,000-$7,000 wrapped up in a cylinder head.

Piston to head clearance itself should not be a huge issue. If you can keep about say .030" then you should be ok.

-Ted

Thanks mrslappy for the explanation about the hondas and for a number I can work with, 30 thou sounds good.