Decking head/ compression

[alexg89]

I can remember reading somewhere on here a while back about how someone was gonna deck their head.

Someone else had chimed in about how much was being decked and that the change in CC's would bump up compression..

Is that right?

and if so is there a formula on how to figure that out??

[datzenmike]

Decking the head reduces the combustion chamber size. As the swept cylinder volume is now squeezed into a smaller space the compression goes up.

 

Tell me what you have and I can work it out for you. I assume a KA24DE? So I need to confirm the dish volume of the piston and the combustion chamber size in cc's. I have them but unconfirmed.  

 

To a lesser extent, sometimes it's possible to deck the block as well to do basically the same thing.  

[alexg89]

Its a single cam Ka24e . My head should be done this week which will be ported and decked .020

 

 

[alexg89]

also the truck pistons  have no dish to them they are flat 

[datzenmike]

Actually they are but only 2.8cc just barely noticeable.

 

I know this engine slightly. Based on a 65cc combustion chamber (correct me if this is wrong) and a crushed gasket thickness of 1.2mm I get an 8.647 compression.

 

Shaving 0.020" is 0.5mm from the head reduces the combustion chamber by 3.1cc and will raise the compression to around 8.96

 

 

 

The KA piston stops short of the deck by 0.45mm and there is still a 1.2mm gasket between the deck and the head so if compression is what you're after you could deck the block at least 0.45 or 0.5mm. this would remove another 3.1cc from the combustion chamber and the compression would be 9.30. 

[alexg89]

13 hours ago, datzenmike said:

Actually they are but only 2.8cc just barely noticeable.

 

I know this engine slightly. Based on a 65cc combustion chamber (correct me if this is wrong) and a crushed gasket thickness of 1.2mm I get an 8.647 compression.

 

Shaving 0.020" is 0.5mm from the head reduces the combustion chamber by 3.1cc and will raise the compression to around 8.96

 

 

 

The KA piston stops short of the deck by 0.45mm and there is still a 1.2mm gasket between the deck and the head so if compression is what you're after you could deck the block at least 0.45 or 0.5mm. this would remove another 3.1cc from the combustion chamber and the compression would be 9.30. 

This is just the answer i was looking for. now for issue number 2. 

If i go say .45 or even .40 to be safe the slack in the timing chain, will that affect timing?

[datzenmike]

The KA doesn't have provision for chain adjustment like the earlier L and Z series with the 3 hole sprockets. They even went to a single chain and sprockets (This tells me that the L and Z timing chain was over engineered and they reduced it to save money) but I bet the earlier ones would fit. A KA forum might be able to tell you how to adjust the cam timing or what to buy.

[alexg89]

4 minutes ago, datzenmike said:

The KA doesn't have provision for chain adjustment like the earlier L and Z series with the 3 hole sprockets. They even went to a single chain and sprockets (This tells me that the L and Z timing chain was over engineered and they reduced it to save money) but I bet the earlier ones would fit. A KA forum might be able to tell you how to adjust the cam timing or what to buy.

There is an adjustable cam gear that i can purchase if its going to be an issue .. I was just not sure if taking that much off the head is going to run the slack out on the hydraulic tensioner.

The KA forums are dead. Noone is using them other then the Datsun guys and even alot of them are using the Dual cams.

 

Think ill just stick with the .020 and maybe try to find some high compression pistons. Which is going to be another PITA.

The aftermarket on these motors is just not good  

[datzenmike]

If it helps the first 3 months of the KA24E production on the S13 240sx used a domed piston (or perhaps a true flattop?) and 9 to one compression. I don't know where you would find these or even if Nissan still has them in an over size. I bet if you ordered them after market you would get the 2.8cc dished ones. What compression are you after???

The point of the tensioner is to absorb slack, the problem is the shortening of the chain on the tension (driver's) side. This shortening of the chain retards the cam opening and closing events, very slightly. Forth thou. isn't much.

[alexg89]

I was not aware of that Datzenmike. Good to know about the s13

 

As for a goal i was looking for about 9.5 if i can could.. i know once i get anywhere after that there becomes detention problems from what i was told

Edited February 4, 2020 by alexg89

[alexg89]

Came across this.. i knew it was to good to be true 

http://datnissparts.com/x-flat-top-pistons-early-240sx-s13-ka24e-size-std-89-0mm-set-of-6-for-big-bore-l6-12010-40f10/

[datzenmike]

Those 89mm still have the small dish. I'll check the P.t No. when I get home. 12010-40F10

[alexg89]

3 minutes ago, datzenmike said:

Those 89mm still have the small dish. I'll check the P.t No. when I get home. 12010-40F10

 

It looks more like a dome then a dish .. but yah please check.. either way they are not available lol 

[datzenmike]

 

The eye sees what it wants to see. Maybe it is!

[Stoffregen Motorsports]

Yes, they were actually domed about .020"

[alexg89]

i came across other people talking and they had mentioned this

 

Assuming the engine will never see boost, I'd go with 4032 forged pistons as light as possible with a 1.0mm ring pack and low (10-12 lb) oil ring tension. I'd also have the cylinders done with a shallower, more fine hone to improve sealing with the lower tension rings. This will also favor a lower viscosity oil without increased blow-by or oil consumption. It'll also take more bobweight off the crank during balancing which will make the engine more rev happy and take torsional stress off the rod and main bearings.

 

Does that sound like something i should invest in or  is that a waste of time 

[datzenmike]

 

These are the 07 '88 to 10 '88 high compression pistons. Unfortunately they are stock bore. It would be best to have an over size (40F11 89.5mm or 40F12 90mm) in a freshly over bored block. This is not the time for a hone and ring job.

 

Mine are for a Z22 block bored and honed to fit stock 89mm KA24E pistons 

[datzenmike]

7 hours ago, alexg89 said:

i came across other people talking and they had mentioned this

 

Assuming the engine will never see boost, I'd go with 4032 forged pistons as light as possible with a 1.0mm ring pack and low (10-12 lb) oil ring tension. I'd also have the cylinders done with a shallower, more fine hone to improve sealing with the lower tension rings. This will also favor a lower viscosity oil without increased blow-by or oil consumption. It'll also take more bobweight off the crank during balancing which will make the engine more rev happy and take torsional stress off the rod and main bearings.

 

Does that sound like something i should invest in or is that a waste of time 

 

Forged pistons are over kill if all you want is an engine with a little more compression and go. They ain't cheap, nor the rings, nor the crank mods and balancing.

[alexg89]

9 hours ago, datzenmike said:

These are the 07 '88 to 10 '88 high compression pistons. Unfortunately they are stock bore. It would be best to have an over size (40F11 89.5mm or 40F12 90mm) in a freshly over bored block. This is not the time for a hone and ring job.

 

Mine are for a Z22 block bored and honed to fit stock 89mm KA24E pistons 

 

Im going to do a compression test before i tear the motor down and if the compression is down or varied ill probably go through it .. if not, at this point trying to find a set of pistons thatll will never be found ill just pop the head on and go .. 

 

Id love to find another spare motor and do it correctly and have the motor gone through completely, but once again budget doesnt allow right now.

 

You say yours are for a z22 but fit stock ka pistons.. What do you have? im curious

[datzenmike]

Boring more than 2mm is not without risks. I would have done an L20B but that's 86mm out to 89mm. It's do-able but I had a chance for a Z22 block and they are 87mm so less to take out. I haven't finished it yet, maybe never will, just a project. Z22 block bored to 89mm, decked 1mm, KA24DE pistons from an S13 so fully floating piston pins. I'm using L20B rods that I honed out so they 'float'. Main oil gallery plugs removed and tapped and plugged. The head is also from a KA24E that was milled 1mm. The rear drain back hole closed off. Front pump boss drilled out to fit an L series mechanical pump. Timing cover is a Z22 with Z or L series gear and I expect around 9.3 compression and a 2,288cc displacement.

[Stoffregen Motorsports]

This all falls into the "custom engine building" category and using a machine shop that is familiar with Datsun engines is always preferred. Case in point - the Z22 block can be bored to 89mm and even larger, but the block was designed with steam ports between the 1-2 and 3-4 cylinders. Boring can cut into these steam port holes, so you do what's called a "spread bore" to clear these hazards.

 

On the topic of custom pistons and rings - the only real reason for forged pistons in a naturally aspirated street engine is physical shape and size. Sometimes you simply cannot find the right pin height or dome shape. Modern pistons are more than capable of handling the horsepower potential of a street engine. Another reason for modern pistons is the rings. As you guys mentioned, the modern rings do a much better job at sealing while also being stronger and creating less drag.

Edited February 5, 2020 by Stoffregen Motorsports

[alexg89]

On 2/5/2020 at 11:56 AM, Stoffregen Motorsports said:

 Modern pistons are more than capable of handling the horsepower potential of a street engine. Another reason for modern pistons is the rings. As you guys mentioned, the modern rings do a much better job at sealing while also being stronger and creating less drag.

 

 

The issue im having is finding anything at all. Any of the ka24DE pistons that will work will all lower the compression . ive not been able to find anything for the Single cam which creates a real issue unless im just gonna call it a day and stick the stock stuff back in. .If thats the case ill have to rockauto some pistons and hope for the best

[alexg89]

On 2/4/2020 at 10:12 AM, datzenmike said:

Those 89mm still have the small dish. I'll check the P.t No. when I get home. 12010-40F10

 

Did you ever find anything out on this??

[Stoffregen Motorsports]

1 hour ago, alexg89 said:

 

 

The issue im having is finding anything at all. Any of the ka24DE pistons that will work will all lower the compression . ive not been able to find anything for the Single cam which creates a real issue unless im just gonna call it a day and stick the stock stuff back in. .If thats the case ill have to rockauto some pistons and hope for the best

Ok, then you need custom pistons.

[G-Duax]

First, forged pistons are not over kill. as they can be insanely lightened, and not break.

 

Another way to use an El-Cheapo OEM style dished piston is to find a longer rod.

You can just cut the dome down, to make it into a flat top.

This is a lot more of a challenge, as you have to find a rod with the same lower rod bearing dimensions, or smaller diameter, and wider so they can be bored larger, and narrowed to what you need.

A lot of cost unless you own a machine shop, then it is just a lot of work.

That is why, custom pistons make so much sense.

 

If you ever get a chance, look through old hot rod magazines, and books from the '50s & '60s.

The amount of work people went through, like stretching rods, and welding up piston domes.

This was way before you could enter an order with a speed shop like Summit, and order H-beam rods, or off the shelf high compression pistons.

 

What is your rod length, and pin height ?