rb20/25 combo

[Mase]

An oppurtunity has come up to get an rb25 block with crank, pistons etc all still there. Im throwing up two ideas, trying to put the rb25 bottom end in instead of the rb20 bottom end or using rb25 internals in the rb20 block. I've done a few searches but information is very slim. Can anyone add some light to the situation??

Cheers

Jayson

[meggala]

you need custom pistons if your going to use the rb 25 crank and rods and probably custom head gasket.

pistons can be 82 mm instead of 79 mm

cheers

meggala

this guy did one I think but has gone back to the 2.2

http://forums.skylinesdownunder.co.nz/show...&threadid=30030

[Mase]

that's a real pain, Any ideas about putting an rb25 block and internals in replacement for the rb20 block? With the 2.2 L did he just bore it out and put bigger pistons in??

Thanks mate

[Roy]

that's a real pain, Any ideas about putting an rb25 block and internals in replacement for the rb20 block? With the 2.2 L did he just bore it out and put bigger pistons in??

Thanks mate

You wont be able to bolt the RB20 head onto the RB25 block without causing yourself a bunch of heartache. If you can get the RB25 block at the right price, you may consider getting an RB25 head and just going down that path.

Yeh, the RB22 is just bored out RB20 block.

[Mase]

Actually i know where i may be able to get an rb25 head cheap but then there is still a whole heap of bolts on i'll need i guess. Think i'll just forget about it.

[Mase]

What about boring the rb20 block out to fit the rb25 pistons? This should work shouldn't it as the rb25 pistons are larger than the rb20's and i use the rb25 crank rods??

Cheers

Jayson

[Roy]

Problem with boring the RB20 block from 78mm to the RB25 bore size of 86mm is you will run out of wall thickness and compromise water jackets etc. There just isnt enough meat in the block to do so.

From 78mm to 82mm, some jap kits go to 83mm pistons. So sorry, no go.

[Mase]

lol, damn!!! Alright then. Was worth a try anyways!!

Thanks for the info roy

[Skyrine-Dave]

get some 4AGZE pistons jayson.

i read somewhere that people use those pistons with the rb25 crank and rods...

Dave

[Sydneykid]

Hi Mase, here's the go................

Use the RB25 crank and rods and set of standard 4AGZE pistons, that gives you 2.3 litres. The last set of used 4AGZE pistons I bought cost $100 for the set of 4, so $150 for 6. New set of rings was $20 per piston (AGE's are as common as). Balance all the up & down and round & round bits while you have them out.

Sell the RB25 block and pistons, you'll get most of your money back.

Stick a set of standard GTR camshafts in it (they are cheap), adjustable pulleys and with the right turbo it will fly. I have seen 260 rwkw out of this combo, with a good spread of power across the rev range. Unlike a standard RB20, which at 260 rwkw is very peaky.

Hope that helps

[Roy]

So the RB25/26 rods small end and 4aGZE pistons rod bushing/piston pin match up ok and also the piston height works out ok.

For every time i have read they bolt straight in i have read or heard that the piston pin needs attention or the piston requires machining of the head to ensure correct height???????

If thats the case id rather get forgies, sure the casting process of the 4AGZE offers improved strength over most other cast pistons, and i have also read that the 4AGZE (supercharged vs the 4AGE NA) pistons also have a form of ceramic/ treatment std, in which case any maching of will only further weaken the piston??????????

ALL SPECULATION AND THINGS I HAVE HEARD. If i had the bankroll i would love to go and find out for general info as much as my own sake.

[DVNT32]

Any idea what the approximate cost of assembling the engine would be? I went for a ride in a 2.4L gtst and it would convince anyone this conversion is worthwhile

[Roy]

Depends on how far you want to go, how much DIY etc etc.

But for a reputable workshop to do the whole hog, you would be looking at $15k. Thats:

- Remove RB20

- New quality bearings/studs

- Purchase of 2nd hand RB26 rods and crank

- Forged pistons to suit your desired compression

- Increase the daim of the combustion chamber to suit 83mm bore

- Re-co of head, (Perhaps some new valves, valve springs and cams, GTR like Sydneykid suggested)

- The engine is being completely stripped so new gaskets

- NDT of block, crank, rods

- General machining/decking

- Balancing bottom end

- Re-assembly of engine

- Install new engine

Consider that many workshops charge $70+ an hour and you can see why it gets so expensive. Do much of the running around, do the nuts and bolts stuff yourself and you can save thousands...best to do your own numbers after talking to the people your comfortable using to do the work.

Your doing well to have it done for less then 6K

[Sydneykid]

Hi Roy, I'll try and answer a few of your questions...

^ 4AGZE pistons require no machining to fit

^ I believe they are high pressure cast

^ The crown is ceramic coated

^ The skirts are coated with oil retention compound

^ The 4AGZE gudgeon pins fit the little end of the RB25/26 conrods, with very minor machining of the bronze bushes. This is normal stuff.

^ Ceramic coatings is applied to the crown as a heat barrier, it has no effect on the strength of the piston

^ All of the block machining and balancing similar to what you would have to do to make a 2.3 litre RB20 cost me less than $500.

^ Lots of RB30 (Commondoor) parts fit, made by Nissan sold cheap by 'Olden. Think gaskets, seals, water pump, oil pump, bearings etc

So if you can do the engine R&R and the assembly, it will cost less than $5K if you have to buy everything. If you have some of the stuff it will be even cheaper.

But Roy is right, if you have to pay someone to do it, you might as well do an RB31 because it will cost the same. For example a set of 82 mm forged pistons will cost the same as a set of 87.5 mm forged pistons. Balancing an RB30 crank costs the same as balancing an RB25 crank, etc etc.

So as much as I would like to build a 2.4 litre RB20, bottom line is I can build an RB31 for about the same. That's why I haven't done one......yet...

Hope that helps some more

[BOoStEr]

Can you just use the rb25 crank and rods to stroke an rb20 without changing the pistons?

[meggala]

they are pretty awsome but I would do the 2.2 if going that way less hassele and you can buy the piston off the shelf from tomei . 2k or so for the pistons also remember you need a special head gasket as teh stock one wont fit tomei have these as well

cheers

meggala

[666DAN]

Hmmmmmmm *Eyes up twincam corolla in the driveway* LOL

[BOoStEr]

Why is it a hassle to stroke an rb20 with rb25 crank and rods? Should cost next to nothing unless there are skirt clearance problems etc.

[Roy]

Thanks Sydneykid, most helpful as always.

Just trying to be realistic with costs, as i suspect too much 'buck-fitty" stuff is posted here, and hate to see someone start down this path thinking it was a cheap option.

The dollars soon add up, main reason i am hesitant of building such an engine is that i would want to do ancillaries as well, so things like GTR oil/water pumps, better valve springs, bigger cams, to ensure that the thing can cope with sustained 8,000rpm track work. Which always makes like you said the RB31 option better, especially considering the flow benefits of the RB25/26 heads and ability to get away with lower rev limit.

Then i sit back and sure the power i have may be a bit peaky at the moment, but its still fun and its not like i do any competitive stuff with my car, my track days are purely "therapy" sessions so as long as im having fun which my car ....that said if i had the surplus coin i would do it all in a heartbeat.

[Mase]

rb25 block $250 for me

I have an rb20det

pistons 4AGZE say $200

maching 20-30 per cylinder. worse case $180

new rings $120

Bearings $100??

blueprinting roughly $200

New headgasket (metal) $200

All labour is done by myself and a mate. My mate has done the rb30/25 to a mate's r33.

The other stuff like cams etc can be done at a later date right?

This is worse case scenario.

Then, i can sell rb20 crank and rods, good rb25 block and rb25 and rb20 pistons.

Thankyou very much for your help sydney kid, straightened up a few things.