RB30 Engine Builds: RB25/30 & RB26/30

The RB30 is the odd one out in Nissan's RB family: a 3.0-litre single-cam iron block that came in the Holden VL Commodore and the Australian R31 Skyline, not the twin-cam turbo motors the badge is famous for. But because it shares its 86mm bore with the RB25 and the RB26, you can bolt one of those twin-cam heads straight onto the big iron block and end up with a torquey, near-unburstable three-litre six. That is the RB25/30 and the RB26/30, and it is one of the great open secrets of Skyline tuning. This is a plain-English reference to what the builds are, how to tell a turbo RB30 block from a naturally aspirated one, and the gotchas that catch people out.

Why go to the trouble

The RB26 is a brilliant engine, but at 2.6 litres it is short on torque off boost, and the RB25 block is known to crack its ring lands once you push much past roughly 400 horsepower. The RB30 answers both. Its longer 85mm stroke adds capacity and low-down torque, it spools a turbo sooner, and the thick cast-iron block shrugs off abuse the factory turbo blocks will not. You keep the twin-cam head's breathing and bolt it to a bottom end that does not flinch. That is the whole appeal, and it is why the RB30 turns up under everything from mild street cars to seven-second drag Skylines.

Identifying an RB30 block

Is it even an RB30? The giveaway is deck height. The RB30 is noticeably taller than an RB25 or RB26 block, with obvious extra metal between the deck and the water pump, to make room for the longer stroke and longer rods. Once you have seen a 25 or 26 block sitting next to a 30, you will never mistake one again. Confirm it by reading the numbers stamped in the block; there is no single famous “RB30 casting number” to look for, and be wary of old L-series codes floating around online, because those belong to a completely different engine.

Turbo block or NA block? The bit everyone asks

Here is the answer that surprises people: the naturally aspirated RB30E and the turbo RB30ET are the same casting. Nissan did not make two different blocks. The difference is factory machining and internals:

• The tell you can see: look at the back and side of the block. A factory turbo block has the turbo oil feed, the turbo oil return and a rear coolant line already machined and tapped. A naturally aspirated block has blank, undrilled bosses in those same spots. Turbo ports present means a turbo block; blank pads means an NA one.
• The tell you can read: the engine-number suffix. An A8 is the turbo RB30ET; an A6 is the NA RB30E. Treat it as a strong hint rather than gospel, but it lines up in practice.
• Inside: the factory turbo ran dished, lower-compression pistons (roughly 7 to 8 to one), the NA higher. If you are building the engine you are fitting your own pistons anyway, so this matters most for spotting an unmolested original.

For a build, a factory turbo block (Series 2) saves you drilling those oil and water provisions yourself, so it is the nicer starting point. But it does not make or break the engine, and plenty of strong builds start with an NA block that has had the provisions added. Worth knowing: the factory-turbo RB30ET only ever came in the VL Commodore Turbo. The R31 Skyline only got the naturally aspirated RB30E, so a “factory turbo R31 block” is not a real thing.

RB25 head or RB26 head?

This is the single biggest source of confusion, and it comes down to oil.

• The RB26 head bolts on clean. It has no variable cam timing, so its oil and water galleries line up with the RB30 block with no extra plumbing. That simplicity is a real point in the RB26/30's favour.
• The RB25 head needs a fix. The R33, R34 and NEO RB25 heads have variable cam timing (VCT), and its oil feed lines up with a water gallery in the RB30 block, which is obviously no good. The usual answer is an external VCT oil feed with a small restrictor, or deleting the VCT. It is a known, solved job, just do not skip it.
• The easy exception: the plain R32 RB25DE head (non-turbo, no VCT) is the simplest of all and bolts straight on.

If you want the last word in top-end flow and do not mind the six-throttle-body complexity, the RB26 head is the one. If you want a cheaper single-turbo engine and are happy to sort the VCT feed, the RB25 head is the value pick.

The gotchas that cost people engines

None of this is hard, but the RB30 build has a handful of traps worth knowing before you start.

The oil pump and the crank collar

This is the famous one, and it is usually explained wrong. The problem is not the diameter of the crank snout, it is the width of the oil-pump drive on it, which is narrow on RB30 (and early RB25/26) cranks. A wide aftermarket oil pump then rides on a tiny contact patch, wears it, and eventually loses oil pressure. The fix is a crank collar, which gives the pump a proper wide drive to grip. The catch that bites people: once you fit a collar you cannot use the standard single-cam RB30 oil pump any more. You must run a twin-cam RB25 or RB26 pump, or an aftermarket billet one, and you need to match the collar and the pump so they physically clear each other.

Getting oil to and from the head

The single-cam RB30 was never plumbed for a twin-cam head. You plug the old rear oil gallery, put a restrictor in the head feed (a range of roughly 1.1 to 1.5mm depending on the build and the pump, not one magic number), and, most importantly, you improve the oil return so it does not pool in the head at high rpm. That last point is the RB30's real weakness, and the good builds add drain capacity, sometimes an external drain back to the sump. Take your time drilling, too, because the castings can be thin around those holes and people have broken into the water jacket and scrapped a block.

Deck height and the timing belt

Because the block is taller, the timing-belt path is longer. You need a longer belt (around 150 teeth), a relocated tensioner, an extended upper cover and a plate to fill the gap the tall deck opens up. There are off-the-shelf kits for exactly this. And do not reach for a “head spacer” on a genuine RB30: that is a trick for stroking a shorter RB25 or RB26 block, a different thing entirely. On a real RB30 you set compression with your piston choice and by decking the block. A half-inch ARP head-stud conversion is the standard upgrade (which means drilling and tapping), and the head gasket is easy because the RB26 and RB25 gaskets share the RB30's 86mm bore.

How much can it take?

The stock RB30 casting is genuinely tough. This is an engine that did 400,000 kilometres in family Commodores, and the “RB30 blocks crack” reputation is overstated for the casting itself. Where they do let go under big power it is almost always the main caps “walking” or hairline cracks near the head-stud holes, which is why serious builds add a girdle, billet main caps and bigger main studs. Tellingly, the most famous “cracked RB30” story on the forums turned out to be a broken engine mount hammering the block, not the power. As a rough guide, a stock or lightly-prepped block is happy to somewhere around 500 to 1000 horsepower, a properly-prepped cast RB30 or stroked RB32 has been proven well past 1200, and beyond that people move to billet blocks. For context, one of the most famous examples, RIPS Racing's “MGAWOT II,” is a 3.2-litre RB30 with an RB26 head making around 1500 horsepower on pump E85 and running eight-second quarters.

A note on the numbers: the RB30 world is full of forum lore, so we have stuck to the builder consensus and flagged the genuinely disputed bits (oil squirters, the exact deck-height figure, and where a block really needs a girdle) rather than pretend they are settled. Spot an error? tell us.