Rb20 with rb25 internals

[fikemoo]

Hello guys just a quick question I have looked through the various topics that already exist and I couldn't seem to find a straight answer. I have a rb30 which my rb25 head will be placed on to. So I am now left with a rb25 bottom end.

 

I've just put a rb20 in a bmw e36 so as I have the parts laying around I thought about putting the rb25 internals in the rb20 I know the pistons are way to big so the question is what pistons can you use. Can a combo of rb25 crank rods and rb20 pistons be used as I'd rather not bore the rb20 block

 

Thanks for any help/advice mike

[Dobz]

Not to sure of main and big end journal sizes would allow the 25 crank in the 20. If you could fit 25 crank with 20 rods and pistons it could work but you would only gain maybe 150-200cc. Piston pin height from 20 and 25 would need to be the same and squish pads would need to be recessed to accommodate the extra piston protrusion. Then piston to valve clearance could become and issue. If you have bits lying around I'd say go and start measuring and trial fitting to see if it's possible. Time and effort wise it is better to just get an rb25 and there will be a lot of people replying the same. If you like a mechanical challenge and don't mind spending the time then go for it.

[fikemoo]

Hi thanks for getting back to me that's the information I've been trying to gather. As far from what I've seen all three engines share the same rod length so the differences have to be in the pin height so in theory 20 pistons with a 25 crank won't work unless somebody can clarify and share any info they have

[polzin]

Pretty sure its 4agz pistions what will work but think with 26 crank have a look at my link

rb24build

[fikemoo]

Hi thanks for your reply you are correct if you want to make a rb24 and you can make a rb23 using the rb25 crank using the 4agz pistons. I did my own measuring and calculations regarding using the rb20 pistons with the rb25 rods and pistons and due to the difference in pin height between pistons it would require a custom piston and it would make a rb21