RB25 or RB26 pistons?

[Darius]

I have an RB25DET and am planning on using the RB25 crank with RB26 rods. The rod bearings are a separate issue that we will leave alone for now. The rods are the same length, but the RB26 piston pin is slightly further up in the piston effectively lowering the piston ~2mm. If I were to use the RB26 piston, my compression ratio would drop significantly from 9:1 stock to something a little less than 8.5:1 (maybe 8:1 at the lowest). This is assuming the RB25 and RB26 share the same/similar combustion chamber head volume. If I stuck with the stock RB25 pistons, my compression ratio would remain 9:1.

Should I use the RB26 pistons and lower my compression ratio that low or should I keep it at 9:1 and run the RB25 pistons?

Is there any difference in strength because they are both cast and look nearly identical?

How critical is it to engine design that the piston does not reach the top of the block deck at the top of its stroke? Does this produce unwanted lateral forces on the cylinder walls at the top of the block?

[98birdls1]

I've never seen anything on the difference in strenght from the 26 pistons to the 25's. That would be nice to know.

You could always take more material off the block and use a thinner HG to raise the compression up some if you use the 26 pistons. You machine shop will know how to do the calculations to come out with the right number.

OR, just buy some forged 25 pistons. Can get a nice set for 750-800. Good insurance.

Luke

[Darius]

I already had the head resurfaced (as you know) so some material was taken off there, but they did not tell me exactly how much. I'm planning (maybe) on using the Cometic head gasket which is 0.2mm thicker than stock. I'm also having the block decked, but don't know how much will be taken off there either. I should find out next week because I'm last in line right now, but no hurry. With the material being removed and combustion chamber shrinking, I'm a bit concerned that the RB25 pistons may be a little tall and create higher compression than 9:1. I'm seriously considering going with the RB26 pistons just to keep the compression ratio in check and not have to worry about pin compatability and all that stuff. Unless someone can talk me out of it, I'm going to throw the RB26 pistons and rods in there as is.

[GTR Shop]

Prob you're going to have is that vi deckign head and thin gasket, you have a messed up quench.Run Rb25DET pistons at - deck, 1.5mm head gasket and you'll end u p ~8.5:1PERFECT!

[Darius]

Wouldn't decking the head increase compression? and wouldn't decking the block increase it even more? I highly doubt using the 9:1 RB25 pistons with this machining and head gasket will result in an 8.5:1 CR. Probably more like 9.3-9.5:1. That's not exactly what one looks for in a turbo motor.

[Carl H]

also remember that the rb26 pistons have the wristpins in a slightly differnt spot to accomdate the stroke from the crank...

[98birdls1]

Thats why he said use the 1.5mm headgasket instead of the stock size.

If the shop takes of .15mm off the head and .15 off the block (guesstimates) than using a HG .3mm bigger than stock would retain the stock compression with stock pistons.

My plan is to use a thicker HG to negate the material being taken off the head and block and to lower the compression just a tad. Like I said, your machine shop will know how much to take off, and with what thickness HG will give you your desired result. (As long as they have a copy of the service manual).

Looks like things are going along good for ya Matt. Remind me I still need to paypal you some money for the oil restrictor.

Luke

[Darius]

The Cometic gasket is reportedly 1.3mm (0.051") and isn't the stocker 1.0mm? I thought it was thicker than that when I took it out, but that was a long time ago.

I hesitate to use the RB25 pistons as they are taller and the block and head decks are slightly shorter than they were stock. I'm assuming the RB26 pistons on the RB25 crank will lower my compression, but less than if I had not had any machining done. I'm going to fathom a guess and say it will be somewhere around 8.3-8.5:1. I'll have to install both and take pictures of the difference.

Luke, my paypal is [email protected]. If anybody else feels sorry for me, please feel free to donate to the cause

[98birdls1]

Money sent Matt. I threw in an extra penny for the RB cause.

Find out how much the machine shop took off your head, and then see how much they take off your block. It shouldn't be much. No enough to warrent the 26 pistons I don't believe.

Decking the head and block doesn't effect compression THAT much. Have the shop remove .03mm off the block to make it straight and run 25 pistons with a thicker HG. HG is going to have a bigger effect on CR than the milling will.

I think you'd have to remove alot of material to make up for the drop in compression with the 26 pistons. Can't go to thin on the HG.

Luke

[GTR Shop]

Stock 1.1mm 9:11.5mm 8.5:1

If you take 0.15mm of the head, thats approx 1cc.. will raise on a 1.5mm gasket 0.1 comp.

Just cc the head and piston, don't forget about gasket thickness, then work out manually, check with a computer prog and if comp is right, 1-1.5mm quench, then you're golden

I do this all day long

[Darius]

Cool, thanks for the input guys and the penny Luke. I'll throw that thing in the mail when it comes. Shouldn't take more than a couple weeks. The guy who machined my block is on vacation this week so I won't find out how much he took off until next week at the earliest. The guy who milled my head was kind of a dumba$$, so he never told me how much he took off and I highly doubt he'd remember as it was about 6 months ago. Are there kits you can buy to measure the volume of the combustion chamber to approximate compression ratio?

[eh?]

Search the web, there are tips on how to diy. Basically you need plexiglass with a couple holes drilled though it and a large syringe. You place the plexiglass over the combustion chamber and add a colored liquid through the holes with the syringe. Whatever amount you used to fill that combustion chamber to the top = head cc.

[Darius]

That's what I've been picturing in my mind and have seen pics of. Now I just have to track down my local heroin dealer for a big syringe.

[Darius]

Ok two nights ago I did the plexiglass thing with oil and got a syringe from the pharmacist (who was overly enthusiastic to give it to me for some reason). And I found that I could only get ~61.0 and ~61.7cc in the head space (two runs on the same chamber). I have found some other info online that guys have done this to stock RB25 heads and found ~66cc. From the volume calculation I am trying to back calculate the depth removed from the head. Assuming an 86mm bore, which is generous, I am getting a depth removed of 0.74-0.86mm. If you subtract the squish areas, that depth increases slightly. That seems like a lot to just flatten the head.

What do you guys think?

[eh?]

stock heads vary 62-64 cc. I had 62cc.

[Darius]

Ok that makes more sense. Comparing to your number, he only took off .05-.17mm. Maybe .2 mm at most. I was a little worried there, but I'll take measurements from the other chambers as well to compare results.