Gtr Crank & Rods + Rb25 Pistons

[zebra]

Hey guys,

Quick question.

Thinking about putting a new engine together as a part time project,

I have a set of 8.5:1 Comp forged rb25 pistons and im thinging of using a GTR crank and rods in a RB25 block,

I know the longer stroke will increase the comp ratio but by how much (assume standard thickness head gasket)

I think I worked it out to be 8.8:1 but not sure if I worked it out properly.

Cheers

Andrew

[The Dan]

That's a 2mm longer stroke. You'd be looking at a half point right there, depending on gasket thickness and squish area modifications.

[zebra]

Awesome thanks mate...I just didnt want it to go over 9:1

[The Dan]

Yeah but you are going to have the problem of the piston touching the cylinder head.

The pin height is different between RB26 and RB25 so the RB25 piston is going to stick out the top of the bore

[zebra]

damn

oh well. no cheap stroker for me then

[Sydneykid]

2mm extra stroke = 1mm extra piston rise. So they might be OK depending on where the rings are located in the RB25 pistons and how thick their piston crown is. Machining 1 mm of the piston crown is no big deal for most forged pistons.

Cheers

Gary

[zebra]

just looked at the ACL webpage, the comp height for their RB25 piston is 31.3mm vs 30.3mm for the RB26

which means that my Slugs will be 2mm higher in the bore with a 26 crank

[Adriano]

No, they will be 1mm higher

[zebra]

Oh I see, I confused myself lol

I have 6mm between the top ring and the base of the piston crown (not the domed part)

I should have measured it before i pulled the engine down

[The Dan]

Oh I see, I confused myself lol

I have 6mm between the top ring and the base of the piston crown (not the domed part)

I should have measured it before i pulled the engine down

Good luck with that one. I forsee a long future of huge problems for you with this. I hope it all works out but personally I would advise against it considering you can easily get the correct piston for the job.

By taking 1mm off the piston you will be altering the design of the crown. You can have it machined to be identical but by the time you've done it and stuffed around for a couple of weeks, you would easily be half way to a new set. You also reduce the amount of material on the top of the piston which is not such a good idea. It is possible to do it and most manufacturers leave some room to move but it's better if you don't, if you don't have to.

When it is all done and finished, you will be bitterly disappointed with the result as it won't give you 2/5ths of f**k all benefit.

Your cheap stroker will turn into an expensive waste of time.

[zebra]

Good luck with that one. I forsee a long future of huge problems for you with this. I hope it all works out but personally I would advise against it considering you can easily get the correct piston for the job.

By taking 1mm off the piston you will be altering the design of the crown. You can have it machined to be identical but by the time you've done it and stuffed around for a couple of weeks, you would easily be half way to a new set. You also reduce the amount of material on the top of the piston which is not such a good idea. It is possible to do it and most manufacturers leave some room to move but it's better if you don't, if you don't have to.

When it is all done and finished, you will be bitterly disappointed with the result as it won't give you 2/5ths of f**k all benefit.

Your cheap stroker will turn into an expensive waste of time.

Yeah thats what I figured, I was only really looking at it due to the fact that R33 rb26 cranks already have the large oil pump drive and buying a 2nd hand GTR crank would've been around the same cost as having a crank collar fitted to a 25 crank.

cheers for the advise though

[The Dan]

CNC 4340 Crank collar supplied and fitted is $240

[zebra]

Prob with that is...im closer to canberra and cbf sending a crank to QLD and back

[The Dan]

Prob with that is...im closer to canberra and cbf sending a crank to QLD and back

lol, I see your dilema.

Have you considered getting it done by someone in Canberra? That's a normal price. The collar can be shipped to you for little over $100 and your engineer will have no problems fitting it. ~$100-$140

[zebra]

yeah I was gonna get Pro- Engines to do it, along with a few other things