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Message: <blockquote data-quote="waltertan" data-source="post: 1064605806" data-attributes="member: 112336">For 4G13 engine block the maximum oversize pistons limits is 74 mm, which is originate from 4G12 engine. Original 4G13 pistons size is 72 mm and is high compression. You need to bore your block cylinder another 2 mm which is acceptable. Some of the user install 4G15 pistons and the size is 75.5 mm, but you need to take the risk due to cylinder side wall become very thin, and is really depends on luck whether the side wall crack in high rev. Even the cylinder side wall is not crack you still need to face high temperature issues due to 4G13 radiator is small in manual car. if you use 4G12 pistons in 4G13 block, due to it pistons is low compression it is good for turbo setup and you will gain extra boost, but for NA setup you will face problem in loss pick up and loss hp. I can feel that after i install the 4G12 piston in my 4G13 engine, the power is worst than original 4G13 pistons. Plus high fuel comsumption due to your engine loss power. Therefore, for my opinion go for 4G13 73 mm high compression pistons.If you insist want to install 4G12 pistons in your 4G13 engine, you need to skim your engine head for another 1 mm. This will lower down your engine head for more compression, and regain the power loss. But you will face tempreture issues due to extra heat, you need to change your radiator to wira 1.6 double layer size radiator to stand the extra heat.After you have skim the engine head for 1 mm without any problem, you can try to skim another 0.5 mm. But I'm not sure whether the pistons will hit the valve or not. For my experience i buy another 4G13 head to skim for 1 mm, and after they have install everything and before they starts my car i was sweating and praying hopefully does not having any problem. Therefore, to skim another 0.5 mm is really a challenging, and if the pistons hits the valve please don't blame me if your valve become banana size or your piston surface has crack. Even you have pass this, you still need to see whether or not the water temperature raising with your big size raidator can stand the heat or not. Therefore, if you face any 1 of the both issues that i mentions, you can say bye bye to your engine head, once skim there is no going back. So try at you own risk.For my humble opinion, get the 4G13 73mm oversize high compression pistons. You will get less hassle, less worry and less money spend. Even if you do all the works for installing the 4G12 pistons in your engine, from my estimation if your put 4G13 73 mm oversize pistons the compression ratio and the hp will be the same or maybe higher.Therefore, compression ratio is very important for NA car, below is the compression ratio for 4G13, 4G12 and 4G15 for your preference:4G13Piston size 72mmPiston surface raise + 1.9mm - 2 mmCompression ratio 9.5:14G12Piston size 74mmPiston surface down - 1.9mm - 2mmCompression ratio 8:1 or 8.5:14G15Piston size 75.5mmPiston surface flat 0mmCompression ratio 9:1---------- Post added at 10:29 AM ---------- 6 hour anti-bump limit - Previous post was at 09:45 AM ----------This are my pass year experence for tuning my Proton Wira 4G13 SVDO AerobackBy the time my car is in age 7 or 8 when to dyno test:4-2-1 extractorStraight flow mufflerWira 1.6 Fly Wheel (without skim)HP is 74Torque is 12Install piggyback ECU Unichip after tuningHP 85Torque is 14Few years later overhaul engine skim flywheel extra 1 kg, go for dyno againHP 94Torque is 12another few years later port & polish engine head manifold intake and out, go for dyno againHP 98Torque is 12.5another few years later engine got white smoke rebore cylinder block install 4G12 pistons, go for dynoHP 78 (Is like SHIT)Torque is 11Now skim engine head 1mm, feel the power is coming back. will go to dyno later</blockquote>