I need pistons for an rb25 buildup using rb26 crank and rods.

I've been looking at some used rb26 wiseco forgies but have found out that wiseco spec their compression ratio at 7.9:1 or 8.25:1 if you deck the max off the block.

I was hoping to keep comp. at 8.5 for off boost response.

Dome volume is listed as 14cc compared to ACL of 16cc for 8.5 CR

Wiseco list the rb26 chamber volume as 70cc and I thought it was less than this.

Can anyone who has used wiseco pistons in their rb26 please comment on CR?

Checking up seems to indicate rb25de and rb26 heads both have chamber volume around 63cc.

Where does that leave Wisecos CR specs when they list volume as 70cc?

Checking up seems to indicate rb25de and rb26 heads both have chamber volume around 63cc.

Where does that leave Wisecos CR specs when they list volume as 70cc?

Strangely enough, very close. I've used heaps of Wiseco's in RB26's and they run the same size dome as a standard piston. I've run them up to 9:1 with a 1.2mm head gasket, the piston flats .005" out of the block and milling the head to suit. I wouldn't put too much faith in the catalogue.

For RB25's though, they have supplied us 2 different pistons, one is very similar to the factory piston dome wise and the other only runs a 1.5mm or so dome which doesn knock out some compression that you can make up for milling the head. Either way they are a top quality piston, most now come skirt coated as well which is an extra $200 or so worth for free.

I used a set of Mahle Motorsport pistons in the last 26 I did, nothing compares to them for quality and design, they are completely different to any piston that I've seen for a 26 before. They are a proper slipper skirt, very short pins, very short skirts, "graflon" coated skirts, 1mm steel top ring, Mahles new design locks and are nice and light. They do require some modification of the cooling squirter but are well worth the effort. They are twice the price of a lot of other forged pistons but you do get what you pay for.

I went to two local engine builders and both tried to sell me 'mahle' pistons.

The second guy pulled one out of a box and I said that looks very similar to an ACL piston.

He said it is but it's made by mahle.

I've heard the companies are related but do mahle make the forged pistons for ACL?

It's obviously not the same as the Mahle pistons you used.

I went to two local engine builders and both tried to sell me 'mahle' pistons.

The second guy pulled one out of a box and I said that looks very similar to an ACL piston.

He said it is but it's made by mahle.

I've heard the companies are related but do mahle make the forged pistons for ACL?

It's obviously not the same as the Mahle pistons you used.

ACL piston products is owned by Mahle. The ACL race series pistons in the yellow boxes are made by Ross for ACL and are a nice piston. The Mahle pistons for the RB26 are new, I used one of the first sets and they are made by Mahle in the US, Mahle is a German company but Mahle Motorspot pistons are made in the US.

I've got some photos of them here that I can upload if you like.

ProEngines: please do post some pics also what do you mean by modifications to the oil squiters?

Some pics are hopefully attached. Because the pistons are a slipper skirt design, the strut that runs from the pin boss out to the skirt interferes with the squirter in its normal location. It's just a matter of rotating the squirter slightly to clear the piston. They are making a running change on the pistons and machining a small relief in the skirt to clear the squirter once it's moved. These are one of the first sets they have made so it was a little R&D as we went.

ACL/Mahle give fantastic support, I spoke to Paul in Sydney about the squirters one afternoon at almost 5PM and by 8AM the next morning I got an email through from Mahle in the US agreeing with what to do with the squirters and they had made a running change to the design to suit.

Very nice looking. PM sent

Wiseco say deck 0.5mm with 1.2mm head gasket will give 8.25:1

My gasket will be 1.3mm

How much can safely be machined off the deck?

I'm not keen on taking much off the head.

Would machining deck/head retard cam timing significantly?