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How to Select the Correct Piston Bore and Piston Specs

correct piston bore

When building your Subaru EJ series engine, Mitsubishi 4G63T or 4B11T engine, the first question you're often asked is "what will the engine be bored to?". This can be a tough decision for many first time builders. While pistons and cylinders can be complicated, you dont need to overthink them and their relationship with each other. The most common answer is, "I'll be using standard pistons, so standard bore." While this is a great rule of thumb, not all bores are standard, and not all standard bores are usable. In the guide below, we'll go over a few things to keep in mind when selecting pistons for your performance engine, including a few scenarios that call for oversize pistons.

  • Cylinder Bore / Shape and it's Importance on Piston Selection
  • Piston-to-Wall Clearance
  • Engine Displacement

Cylinder Bore / Shape

Lets start with the cylinder wall finish in the engine block. Cylinders on rebuilt engines are almost always bored slightly oversize to remove any irregularities from the cylider wall. This is almost always done on used engine blocks, as cylinders can become slightly egg shaped after years of use. Without removing these irregularities, standard bore pistons wont hold compression and you'll consume excess oil. If your cylinders are out of round, a machine shop will need to bore the cylinders out .020 or .040 which is typically enough to bring the bore back to a perfect circle. The same holds true for engines that have seen piston failure in the past, as you'll need to remove cylinder wall scratches by overboring.

It's always important to know where your engine block came from, and what led to it's removal from the car in the first place. In most cases you should always try and rebuild you own block, but this isn't always an option, so be sure you get the full scoop from the original owner of the engine you're building and if any horror stories about the engine are told, it's a safe bet to bore .020 over and use oversized pistons

Piston-to-Wall Clearance

A machine shop will also bore oversize to help set a specific piston-to-wall clearance. Piston-to-wall clearance is the distance between the cylinder wall and the piston, and a clearance that is too tight can lead to siezure of the piston, while a clearance too far can cause loss of power and oil blow-by. New pistons will come with a spec sheet with information about the piston diameter, but it's always a great idea to phyically measure the piston yourself.

Machine Shops will need to know this information when they perform the machining of the block. Pistons are usually matched to the cylinder they'll be put in. Specific piston-to-wall clearance can even be different on a car that is track specific or simply a daily driver, so be sure to communicate every bit of information with the machine shop to ensure you're getting the closest piston-to-wall clearance for your applications usage. In rare cases, you'll have to get a custom sized piston to achieve a specific piston-to-wall clearance, but in most cases this clearance can be matched with slightly overbore pistons.

Engine Displacement

The last reason a machine shop will bore oversize is to increase the displacement of the engine. There are many reasons you'd want an engine to be bored over such as increased performance, the installation of stroker kits and other internals requiring larger cylinders, or to strengthen the structural integrity of the engine block for nitrous and turbocharger modifications. You may also need to increase displacement to help set a specific piston-to-wall clearance or clean up cylinder bore shape.

It's always important to discuss your plans with the engine builder so they know exactly what to prepare the engine for. How much should you increase displacement? Any engine builder worth his salt should be able to calculate the exact overbore based on your displacement goals and decide whether aftermarket sleeves or standard sleeves will work. At MAPerformance, we advise not going for "max bore" when using standard or aftermarket sleeves. Doing so will render the block unusable if engine failure occurs, leaving you with nothing more than a fancy boat anchor.

In Conclusion...

Always, always, ALWAYS consider oversize pistons, even on a low mileage engine block. Do not immediately assume your engine needs standard bore pistons. With the large assortment of pistons on the market, the selection of off-shelf pistons is huge and no longer costs tons of money for non-standard pistons. Spending a few hundred dollars on machine work to ensure your pistons are matched to the cylinders can be worth it in the long run when considering the cost of replacing components after they've broken due to negligence. Never max out your bore the first time you build, as it will prevent the ability to increase displacement in the future.

At MAPerformance, we typically never bore over .020 unless specifically requested by the owner. Due to the thin OEM sleeves found on 4b11t and EJ series engines, we avoid overboring over .020 as it can make the block unusable for future builds. If you have any questions about your specific build, feel free to contact us as we're happy to assist!

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