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Discussion Starter #1
I know some of you out there have these... I would like to know what needs machining on the crank (rb26) to go in the rb25 please and also I got a bit lost in the grading of bearings in a thread I found, if the crank has never been ground before can I just buy standard size acl race bearings and fit them or is it not that straight forward?

thanks in advance for your advice.
 

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I know some of you out there have these... I would like to know what needs machining on the crank (rb26) to go in the rb25 please and also I got a bit lost in the grading of bearings in a thread I found, if the crank has never been ground before can I just buy standard size acl race bearings and fit them or is it not that straight forward?

thanks in advance for your advice.
I don't know 100% my self but I have an RB25DET engine & an R33 GTR crank sitting here waiting to be built up when I work it out my self.

To the best of my knowledge

The crank rods & pistons all fit straight in with no special modification or grinding. The GTR crank has a different throw the rods are the same length & size as GTS ones all be it a bit stronger & the GTR pistons are different height.

From the manual specs the only thing I can see that is different is the GTR main bearings run a bit slacker maybe because the GTR has more oil pressure or flow haven't worked that out yet but I will be using an uprated GTR pump with mine anyway when I start to build it.

You must match the bearings correctly as there are 3/4 different standards between the RB25 & 26.

I think you should also use the GTR front crank damper & again I haven't worked out what clutch & flywheel will be needed so cant help there yet but I hope my push GTS one will fit/work.

The only other things I am aware of atm that may cause a problem is the blocks deck height with the GTR guts & the possibility of the valves hitting the pistons both will have to be checked properly as the compression ration will be unknown as I would expect the RB25 head to have a different clearance volume than the RB26 head but again I dont know for sure.

If no one can help us out we are just going to have to work it all out together build one & see what happens :D

Regards

Craig.
 

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GTR crank fits 25 block with no modifications.

You need to size bearings exactly as per a RB25 crank. They both have same dimension grades. That means no you can't just buy a handful of ACL race bearings, you need to order the right size ones. Yuo determine this by reading the numbers of the crank and the block and use the table in the manual. Or preferably plastigage to get the exact size.

Rods? Both GTR and GTS rods are exactly the same length and bores, meaning you can use RB25 rods on 26 crank if you wanted to. RB26 rods are slightly beefier (heavier).

Pistons? You must use RB26 pistons with an RB26 crank. They are 1mm lower to accomodate the extra 1mm throw. So unless you want sky high CR and pistons protruding into the head more, use the 26 pistons. Although fitting used pistons should be discouraged, as they are sized according to the block. Just as crank and rod bearings are graded, so too are the Nissan pistons. Although worn pistons probably don't meet the size anymore in any case.

Block height? Identical between RB25 and 26.

Flywheel? Same mounting pattern, and interchangeable. So no issues there.
 

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You need to size bearings exactly as per a RB25 crank. They both have same dimension grades.
NISFAN at the risk of sounding ignorant & rude the journals, bearing thickness & clearances are different between RB25 & 26 certainly on the R33 anyway.

Have you done this mod & are speaking from experience ?

The oil squirter's have a different outlet hole too on the GTR probably for greater crown cooling which is probably where the oil pressure is lost at high rpm between the RB25 & 26.
 

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Aaah yes, my bad.

The manufacturing sizes are exactly the same, but RB26 has 4 graduations for block and crank instead of the RB25's 3. (RB26 is more accurately adjusted) So going off the stamps, you would use the RB26 table, and call the RB25's blocks 2 a 3, and RB25's 1 would be a 1, and 0 would be 0. Then plastigage them.

Conrod grades are the same on both cranks.
 

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can you use a second hand gtr crank or do you have to use a new one? just wondering as id like to use a gtr crank in mine with steel rods and gtr pistons (are gtr pistons ok or am i best using low comp gtr pistons as im after 600bhp?)also if i was doing this combination what size steel head gasket is best?? and what would the finish compression ratio be?
 

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Discussion Starter #8
cool ok thanks, at the expense of sounding a bit lame can someone explain how a plastigauge works?
 

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can you use a second hand gtr crank or do you have to use a new one? just wondering as id like to use a gtr crank in mine with steel rods and gtr pistons (are gtr pistons ok or am i best using low comp gtr pistons as im after 600bhp?)also if i was doing this combination what size steel head gasket is best?? and what would the finish compression ratio be?
Used is fine & its what I am using the rest you will have to work out yourself to suit your power needs.
 

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Plastigauge is a deformable strip that you lay across the bearing surface. You then bolt the crank up to full torque, then disassemble again. You measure the width the plastigauge has spread to, and this gives an ultra accurate measurement of clearance.

A used GTR crank is ok, as long as it is in good condition.
 

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Discussion Starter #11
I was under the impresion the standard gtr pistons failed before the standard gtr rods so I was going to go for gtr crank and rods then forged pistons...
 

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87mm forgies and RB26 crank takes you to 2630cc.:bigthumb:

Yes, fit bearing in every journal then plastigauge the lot together.
 
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