Rb25DET crankshaft from Rb26

[Dawid20]

Hello !

I'm building an engine for my Skyline R33.

I want to use the RB26 shaft in the RB26 block.

Configuration:
RB25 S2 engine block
RB25DET NEO cylinder head
Mahle Motorsports RB25DET S2 87.00mm forged pistons
Forged H-Beam connecting rod 119.5mm
Crankshaft RB26DETT

Will this configuration be ok? I'm still thinking about converting the pistons from S2 to NEO pistons using a milling machine. I'm worried about the compression ratio. I already have all the parts except the connecting rods.

[GTSBoy]

10 hours ago, Dawid20 said:

I'm worried about the compression ratio

You should be. You're either going to need Neo pistons. This is because the Neo chambers are ~10cc smaller than vanilla 25 chambers, which is about 15% less. Quite a bump in compression.

[r32-25t]

You’ll need a 26 piston due to the pin being in a slightly higher position to compensate for the increased stroke, you can then machine the crown to take the compression to where you want it to be 

[Dawid20]

I want to use pistons from RB25 but use forged 119.5mm connecting rods and additionally adapt the pistons to the NEO head.

Edited January 2, 2024 by Dawid20

[CefiroA31]

3 hours ago, Dawid20 said:

I want to use pistons from RB25 but use forged 119.5mm connecting rods and additionally adapt the pistons to the NEO head.

Your pistons will be 1mm below block deck which means less ideal squish/quench so may be easier to knocking.

Stock 25det : 121.5mm rod length + 31.5mm piston height + 35.85mm half stroke = 188.85mm.

Your combo : 119.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 187.85mm.

Sell your pistons and buy rb26 pistons, mill down its dome to get proper CR for rb25det neo head, that way you have proper piston to deck clearance/squish and uses common rb25/26 121.5mm rods which also have better rod/stroke ratio than less available shorter 119.5mm rods.

If have to use rb25 piston with 121.5mm rod, pistons will be 1mm above block deck :

121.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 189.85mm

Can use thicker head gaskets to compensate pistons above block deck, cometic has rb25det 87mm head gasket thickness of 1.8mm, 1.9mm, 2mm, 2.5mm, that will work fine with pistons 1mm above block deck :

https://www.cometic.com/applications/automotive/nissan/rb-series/rb25det--152ci25l-i6

[silviaz]

I heard rb25 neo engines use a rb26 crank, is this true? I can't find any info on it and the part numbers are different. Makes me wonder for the neo engine since there are less parts, what do you do if the crank is completely stuffed.

[Duncan]

19 minutes ago, silviaz said:

I heard rb25 neo engines use a rb26 crank, is this true? I can't find any info on it and the part numbers are different. Makes me wonder for the neo engine since there are less parts, what do you do if the crank is completely stuffed.

They use rb26 rods, not an rb26 crank. The throw of the crank determines the capacity of the engine (rb25 vs rb26) as the bore diameter is the same

[GTSBoy]

Here's some previous threads from this very site.

This one didn't resolve very clearly.

 

This one shows the compression situation

One where I piped up with the usual comment, and @r32-25t with his ditto, and some other useful tidbits.

 

If it were me doing it (and I might, if I ever have to build my 25, because I have access to a 26 crank if I want it, despite me previously saying that it's hardly worth the effort), I'd just call up Nitto and say, "Hey, here's my plan, what piston can you magic up for me?", and leave it to them to make a nice forgey with the required combo of pin height and crown volume. They probably use the same blanks for all their RB pistons anyway, and just carve out what needs to be carved out as required.