Originally Posted by Tim Kelly of Honda Tuning Magazine

BORED TO BE WILD

FOR YEARS MANY BELIEVED HONDA'S ODDBALL H22 BLOCK HAD LIMITED BUILD POTENTIAL BECAUSE OF ITS UNIQUE CYLINDER DESIGN, BUT NOT IT'S POSSIBLE TO SAFELY OVER-BORE THE PRELUDE CASTING WITHOUT USING AFTERMARKET SLEEVES. SRD SHOWS HOW IT'S DONE USING MAHLE'S GOLD SERIES PISTONS, SLUGS ENGINEERED FOR THE H22'S SPECIAL HOLES.

The H22 is Honda's "just had to be different" block. Unlike its other castings, the 2.2-liter doesn't have steel sleeves pressed into an aluminum block. Instead the H22 is entirely aluminum, with no sleeves at all. That's great for weight savings, and the aluminum cylinder walls transfer combustion heat more effectively than steel liners (which may be why early H22s are closed deck). The downside is the distinctive bore design is bad for dropping in any aftermarket pistons.

If you H22 has a problem like a scored cylinder wall, or you're not getting good compression and leakdown, then traditionally, your next step would be to sleeve it or trash it. That's because you couldn't just do a conventional 0.0020-inch overbore and drop in new forged pistons like you could on a B-series engine.

Specifically, the problem is that aluminum pistons don't function well in an aluminum bore. Without a special coating, like thestock pistons and sleeves have, the components will encounter different expansion rates. At their worst, the thermal problems can result in a seized engine. Go through this and your H22 could be finished.

But Honda has a little trick up their sleeve. There are actually OEM 0.25mm overbore pistons available, so the block many thought couldn't be bored out can indeed be opened up some. Converted to American measurements, 0.25mm over is really small, 0.0098 inches, quite a bit smaller htan the 0.0020 mentioned previously. If your H22 has only minor cylinder wall problems, the pistons could be the answer that will save your wallet.

To that end, the German piston company Mahle, which makes OEM and racing pistons - including a good percentage of Formula 1's pistons - now has a drop-in stock and 0.25mm overbore forged piston for the H22. Available in low, stock and high-compression versions, the slugs may be the ultimate answer to a piston refresh for an H22.

The stock Honda pistons aren't forged, which means they can't tolerate much abuse. It's not uncommon to see stock H22 pistons pitted from detonation, 50hp nitrous shots, and bolt-on turbo kits. An inexpensive cast OEM piston just isn't made for that. But the Mahle Gold Series notony have a unique coating that makes them entirely compatible with the H22's aluminum/carbon matrix bore, the three compression versions available make the Mahle's ideal for any high-performance application.

Once you get the pistons, the next challenge is finding a machine shop that can bore an aluminum block. Porsche 911s have the same style all-aluminum piston bores (and surprise, they also have factory Mahle forged pistons), so theoretically, a shop that has experience with those should be able to get the job done. It's a completely different process, one that requires additional tools and materials, so make sure the shop understands what it's doing and that it stands behind its work.

To see just how easy this piston swap is, we ordered up a set of stock bore, high-compression Mahle Gold Series H22 pistons for an upcoming project (the non-Gold Series pistons are for sleeved blocks and are not recommended for a stock casting). We quickly took off the head and oil pan, ready to push in new pistons without having to remove the block from the engine bay. Unfortunately, we found what a lot of H22 owners find - scored piston walls.

The stock diameter pistons would do, but we'd likely suffer leakdown problems, varying compressions, and oil consumption. A bit of measuring and it was decided a 0.25mm overbore would make everything like new. We hit back Mahle for a set of the oversized pistons. After a trip to SS Machine, in Mesa, and the H22 was ready for our main men in Arizona, Superior Racing Development (SRD), to put the engine back together.

When we were finished, wheel-horsepower was up by 10 and the air-fuel ratio was dangerously lean. Detonation could also be heard. An engine ECU designed for 10:1 compression running 11.5:1 compression is asking for trouble. We expected as much and were ready for it, but that's another issure for another time.