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RB26 Crank and pistons into RB25 NEO. What about VCT?

Finn_GT-T

By Finn_GT-T,
in Engines & Forced Induction

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Finn_GT-T Apprentice

Finn_GT-T

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Hi guys,

i am doing a rebuild of an RB25det NEO engine. I have the chance to get an RB26 long nose crank for a good price. My idea now is to put the crank into my rb25 block an buy forget rb26 rods and pistons to get a cheap stroker and solve the short nose problem of the rb25. The crank should fit into the block but what about the combination of the pistons and the neo head? Will there be an issue with colliding pistons and valves when using the vct of the neo block? Will the compression rate be ok? Any experiences with that combination?

GTSBoy Grand Master

GTSBoy

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Neo rods are 26 rods anyway. So any aftermarket 26 rod is same same also.

The trouble with the Neo is that because they used the 26 rods, they had to have a piston with a 2mm (or maybe it's only 1.5mm) pin height difference so they don't poke out of the block. Those Neo pistons have a different dome (to vanilla 25s and 26s) to work with the ~10cc smaller combustion chamber, to yield approx the same compression ratio as all the previous engines. But you can't use the Neo pistons with the 26 crank.....because then the 2mm stroke difference brings back the deck height problem. So you end up needing to use 26 pistons. But....the 26 pistons are suited to a 10cc larger combustion chamber, resulting in sky high compression. This would commit you to E85, large cams and careful tuning. I haven't done the calculation to work out what the compression would be (well, not in the last 15 years anyway) so can't really tell you how high it would be. I just remember it was "quite high", something north of 11:1 I reckon. The last time I thought about it was pre-E85 in the world, so maybe it's more realistic now. It's not very realistic on 98 though.

But, this seems like a complete waste of time. 100cc is an undetectable increase in capacity (4% !!). You would do better to spend money on turbo or headwork. The oil pump drive can be dealt with the on the 25 crank as easily as on any other RB.

Finn_GT-T Apprentice

Finn_GT-T

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I was hoping it was a common mod because it is a cheap (but small) stroker for the 25.  I've already found a few threads on this subject on UK Skyline forums. They say it works, but don't mention what pistons they used. In the end, as you said, the main problem would be the pin height of the pistons (if using 25 pistons) or the compression ratio (if using 26 pistons).

The compression ratio could be lowered if I use a thicker head gasket. The thickest one I found is 1.8 mm thick. Wiseco offers pistons that lower the compression ratio to 8.25 compression for the rb26.
If I have the head and block machined, I may lose 0.1 to 0.2 mm that would also slightly increase the cr.


So maybe the cr problem could be solved. I just wonder if the pistons and valves do not collide. Especially if I use the VTC of the neo head. I don't get the problem solved if I enlarge the combustion chambers in the head a little. Does anyone have experiences with this?

You are right, the increase in capacity would be low but with the right pistons it would be an easy modification because i am rebuilding my engine anyway with new rods and pistons.

My current setup is a gtx3076 gen2 on the stock neo with with 1.2 mm metal head gasket and i want to keep the turbo because I installed that setup just 1,5 years ago. I am just aiming for a bit more low end torque.

r32-25t Grand Master

r32-25t

Posted
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I’ve seen it done and it works, the rb26 has a lower compression ratio then the rb25 and may not be as high as you think.
 

After measuring everything up, if the compression ends up to high you can machine the dome on the piston down to bring the compression down to what you require 

Finn_GT-T Apprentice

Finn_GT-T

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I think I would feel a little bit bad about machining down a shiny new set of forged pistons :(  I would also worry about the stability without the dome... maybe that's a too high price for the little increase.

Do you know which pistons they used in the engines you saw running?

r32-25t Grand Master

r32-25t

Posted
Posted

People have been machining pistons since the beginning of engines, that’s how they lower compression or make room for the valves when using cams with huge lift, it’s even how they’re made in the first place 

  • 2 weeks later...
Finn_GT-T Apprentice

Finn_GT-T

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On 1/30/2022 at 3:53 AM, GTSBoy said:

The trouble with the Neo is that because they used the 26 rods, they had to have a piston with a 2mm (or maybe it's only 1.5mm) pin height difference so they don't poke out of the block.

I've just had another silly idea :) 

Maybe I could use conrods from the tomei rb28 stroker kit. These are 3 mm shorter than the std rods. Does anyone know the exact pin heigth difference between the rb25 and rb26 pistons?

Finn_GT-T Apprentice

Finn_GT-T

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Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

CefiroA31 Apprentice

CefiroA31

Posted
Posted
On 2/14/2022 at 6:57 PM, Finn_GT-T said:

Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

RB26 crank, rods and pistons with machined down domes modeled after RB25DET NEO lower smaller piston domes to lower compression about 9.0:1 = RB26DET NEO.

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BK Grand Master

BK

Posted
Posted
On 14/02/2022 at 9:27 PM, Finn_GT-T said:

Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

Actually, RB26 standard rod length is 121.5mm. HKS or Tomei RB26 2.8 stroker rods are 119mm as their strokers use stock pin height RB26 pistons.

Conversely, the RB26 Nitto 2.8 setup uses 121.5mm rods with lower pin height pistons.

Finn_GT-T Apprentice

Finn_GT-T

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Posted (edited)

Thank you for the infos. I would need 120 mm rods if I wanted to use the rb25 pistons with the rb26 crank.

Machining rb26 pistons to suit the rb25 neo head is not that easy. the rb25 neo pistons do not have a flat surface but a little dome and the pockets for in and ex valves. To find someone with the right cnc skills would be too much effort for a 61 ccm higher displacement.

These are the cp pistons for the neo, that I are already have here:

image.thumb.png.d24762743c51cfbad56790d0c0c9bb29.png

Edited by Finn_GT-T

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