Rb25det Rebuild Issue - Help

[zebra]

If you look at the arias AP331154A RB25 pistons with a 12.5cc dome, you get 9.1:1

Note Arias call these an 8.1:1 comp piston, but check the comp height, these will likely sit 0.005" further down the bore

[Muskett]

Just trying to sort out this deck height problem, I have two spare standard rb25 blocks here at home both measuring 188.6mm deck height and both have .020" (0.5mm) piston out of the top of the block. So I'm assuming the rb25det has a negative deck height standard. So my current block I'm using is .029" (0.7mm) out the top with the cp Pistons I have meaning only 0.2mm) has been decked from the block. Can anyone confirm this?

[Muskett]

Just trying to sort out this deck height problem, I have two spare standard rb25 blocks here at home both measuring 188.6mm deck height and both have .020" (0.5mm) piston out of the top of the block. So I'm assuming the rb25det has a negative deck height standard. So my current block I'm using is .029" (0.7mm) out the top with the cp Pistons I have meaning only 0.2mm) has been decked from the block. Can anyone confirm this?

Anyone confirm how much negative deck height on a stock block

[GTRPSI]

I wouldnt be too concerned about how far the piston sticks out of the block if its a slight amount.

I be very worried about the piston to head clearance, you need to allow for stretch and rock at change of direction when at song so they dont physically meet.

If your dech was taller (not decked) you would run a thinner head gasket to keep the same head to piston clearance (that you were looking for).

Do not try to radically increase compression by running the head gasket so thin that the pistons and head will meet.

Whatever the dome is on the piston, will not effect compression much far from what they were designed to produce by the manufaturer by your blocks deck height, either way you will need to keep the piston and head appart a certain amount.

If your chasing a compression increase and already have a piston to head clearance of around 1mm (40 thou) with your calculations, start looking for a larger domed piston, going any thinner on the head gasket could end in disaster.

Your engine builder should be aware of all this.....speak to them as they will know the piston to head clearance.

If your piston sticks out 1mm, you can get the decks machined down while leaving the dome alone to increase compression, another way to bump up what your after because you can run a thinner head gasket while maintaining the correct quench between the pistons and head.

Edited March 11, 2015 by GTRPSI

[Muskett]

Thanks for the reply, not concerned with having high compression, I'd be happy with anything between 9:1 and 9.5:1, more concerned with things being too close more than Anything. The Pistons are .029" out the top was just wanting to know where they sit on a standard block. Cheers

[GTRPSI]

Do your calculations then based on a 1.75mm head gasket (or as your builder advises) if chasing a 40 thou quench, how far the pistons protrude above the deck top should not be an issue unless the top ring land position is questionable.

Mine are 0.6mm out of the deck on my RB26/30.....means little as long as there is enough room for assembly stretch and piston rock at high RPM.

But not too much space, you dont want secondard detonation pockets.

You can provide less space if Red line RPM's are kept under check.