Z24 piston valve relief cuts

[Smoking Joe]

I am rebuilding my 85 z24 engine. Replacing pistons rings bearings etc. my question is regarding the valve reliefs that wound have been on the original stock pistons.  The pistons I removed and the new pistons I purchased have 4 valve reliefs as if it was for the later 4 valve double overhead cam engines.  Did the original z24 dual spark plug engine pistons have 2 or 4 valve reliefs cut into the pistons. 
if these pistons are intended for a later version of the z24 engine am I losing or gaining compression based on piston design.
Pictures are of removed as well as

new piston for reference   Thanks for anyone’s quick responces 

Edited May 15 by Smoking Joe
Spelling

[Rustbin]

Anything that adds more volume to the combustion chamber drops compression, but relative to the size of the dish in the pistons it looks minor. 

[datzenmike]

All Z series pistons came with 4 valve reliefs. You are losing nothing. 

 

............................... Z24........................................ Z22...................................... Z20E............................................ L20B

[Smoking Joe]

Thanks Mike just curious if Nissan used pistons for a 4 valve cylinder in engines with 2valves per cylinder.  The other question would have been if my original pistons weren’t the same. Would have been a question of valve to piston interference since the valve reliefs were not matched to the valve locations. 

[Smoking Joe]

Would love to have a set of the ones that are 2nd from right. Those would be the higher compression pistons for certain. 

[Smoking Joe]

16 hours ago, Rustbin said:

Anything that adds more volume to the combustion chamber drops compression, but relative to the size of the dish in the pistons it looks minor. 

The CC volume of the two pistons are the same I am loosing nothing in compression from what was in the engine. My question was are the ones that were in the engine what would have been used when the engine was originally assembled by Nissan. Or has someone rebuilt the engine in the past and potentially used the wrong pistons 

[datzenmike]

10 minutes ago, Smoking Joe said:

Would love to have a set of the ones that are 2nd from right. Those would be the higher compression pistons for certain. 

 

As noted those are Z20 pistons. Neither the diameter 85mm matches the 89mm Z24 pistons, nor the piston pin height as the same but the pin height. If you want a higher compression piston get a set of Nissan KA24E pistons which are the same diameter and pin height. The dish on top is barely 2cc vs. the Z24's 15cc. A Z24 engine with KA pistons has a compression of about 9.61. 

 

[datzenmike]

25 minutes ago, Smoking Joe said:

The CC volume of the two pistons are the same I am loosing nothing in compression from what was in the engine. My question was are the ones that were in the engine what would have been used when the engine was originally assembled by Nissan. Or has someone rebuilt the engine in the past and potentially used the wrong pistons 

 

 

 

16 hours ago, datzenmike said:

All Z series pistons came with 4 valve reliefs. You are losing nothing. 

 

............................... Z24........................................ Z22...................................... Z20E............................................ L20B

 

33 minutes ago, Smoking Joe said:

Thanks Mike just curious if Nissan used pistons for a 4 valve cylinder in engines with 2valves per cylinder.  The other question would have been if my original pistons weren’t the same. Would have been a question of valve to piston interference since the valve reliefs were not matched to the valve locations. 

 

No Nissan engine used 4 valve pistons in engines with two valves. If you ordered Z24 pistons that's what you get. 

 

There are 4 reliefs cut into the piston tops because the exhaust and intake valve positions in the head are different in 1/3 and 2/4 below. With 4 cuts the piston would work in all cylinders.

 

 

[datzenmike]

If rebuilding it why replace with standard diameter pistons???? Why not just use the old ones? If you think it may have been rebuilt did you measure the bore. It might have been over bored 0.5mm. Standard bore would now be much too loose.

 

 

 

[Smoking Joe]

My bore is good as you probably know the Nissan block is cast from some very hard cast iron material. A slight hone in a professional honing machine/Tank cleared up any cylinder scaring etc.  so my stock bore is still well within spec. The package deal of rings bearings gaskets etc included pistons and pins. So I decided may as well use them.  Now  I’m considering buying a set of KA24E pistons to gain a slight compression ration increase.  What in your opinion is the HP gain with that slight compression increase 

[Smoking Joe]

Looking at KA24E pistons I see they have C-Clip piston pin retainers. Assuming these still

press fit the pin into the rod same as the Z24 pistons. Or would I need a set of KA24 rods as well 

[datzenmike]

Only the '89-'90 240sx had the KA24E engine and it had the fully floating piston pins with the bushed rod small ends. The clips are needed to keep the floating pin from rubbing the cylinder walls.

 

The '86-'96? D-21 Hardbody also had the KA24E engine but with pins that were pressed into the rods.

 

Z24 and KA24E rods are the same length but if a Z24 I would stick with the Z24 rods. The KA rods seem thinner/lighter. Check the rod small end width fit if installed on KA pistons. Make sure the rod bottom end can center itself on the crankshaft.

 

 

[Smoking Joe]

So should I try to find KA pistons for the 86-96 D21 engine that did not require the retaining clips and your thoughts on the increase of power from the increased compression ratio.  Also the new Z24 pistons I have, have less clearance on the small end of the rod to allow the rod centering on the crankshaft. 
 

last question could I call you for a brief conversation to maybe get more clarity from you regarding exactly what I have going on. Or if

you prefer call me at +1-828-817-0412.  If

you want to call me Please text first so that I know it’s you calling me. I’ll gladly pay you for your time thanks 

[datzenmike]

Compression increase also increases the engine's efficiency but you now have to contend with pre ignition. Now you may have to turn the timing back and/or go to higher octane gas and perhaps a colder plugs. More can be extracted from the fuel and air mix if it's compressed harder. On a 100 hp engine going from say 8.5 to 9.5 (on a good day that is) might get you 3-6% more power. That 3-6 hp is all at the top end and equates to just a few hp at driving speeds. 

 

If you include some porting (another couple of hp) manifold gasket match, bigger carb or dual carbs, (5 hp) a cam and header (5-10 more hp) these all add up but compression by itself isn't that much. 

[Smoking Joe]

About what I thought the increase would be and truly not worth the extra hassle and time for that small amount of increase.  I will

just put it back with original style

pistons

amd

enjoy it like it is.  Thanks again 

[Smoking Joe]

 Now that I have everything ready to reassemble my engine cylinders professionally honed' freshened up z24 head' new z 24 pistons pressed onto my z24 rods'  all gaskets etc Now I find a good KA24E head lol  

 

Cant decide to order a new set of KA pistons or just assemble with the Z pistons.  Need to get it back going and onto other projects. So I considered just using the z24 pistons regardless of which head I wind up using. Obviously for me to use the KA head I have to configure a carb onto the Fuel injection intake as well as facing the valves and seats  machining the head surface to insure flatness etc.

 

One last question of opinions should i continue with the z24 distributor or use a KA single plug distributor when using the KA head

[datzenmike]

1 hour ago, Smoking Joe said:

 Now that I have everything ready to reassemble my engine cylinders professionally honed' freshened up z24 head' new z 24 pistons pressed onto my z24 rods'  all gaskets etc Now I find a good KA24E head lol  

 

Cant decide to order a new set of KA pistons or just assemble with the Z pistons.  Need to get it back going and onto other projects. So I considered just using the z24 pistons regardless of which head I wind up using. Obviously for me to use the KA head I have to configure a carb onto the Fuel injection intake as well as facing the valves and seats  machining the head surface to insure flatness etc.

 

One last question of opinions should i continue with the z24 distributor or use a KA single plug distributor when using the KA head

 

Woah whoa whoa. You cannot use a KA head on an L or Z series block if that's your intent. For one there is an oil drainback hole on the rear of the KA head that would line up with a coolant hole in the block below it.

[Smoking Joe]

Well have I misunderstood to a major extreme. I thought this conversion had been performed

many times on a z24 block. I have read that you have to block off an oil drain back passage in the head but beyond that it’s a simple swap.  As

long as you have the KA intake and

exhaust manifolds. 

[datzenmike]

Long as you are aware of the drain back hole. 

 

AND...

 

The KA-E combustion chamber is also roughly 10cc larger so with a Z24 piston's 15cc dish, that's about 7.6 compression. Better run KA24E 2cc pistons for about 8.6 compression.