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Engine Bore size to oversize or not to

Engine Building Fundamentals

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Hi there,

In the midst of upgrading an 4g63 evo VIII engine and to replace them with an aftermarket 2618 pistons (most probably it will be JE or CP). The block is still with it's stock bore and there's no issue with the block. Today, i measured the block and all bores measured around 85.02mm (bore 1/2) - 85.03mm (bore 3/4) (measured using a bore gauge zeroed at 85mm). Took 4 measurements (2 cross sections x 2 positions top and mid part).

Need advise if i should send to the machineshop to overbore it and use a 85.5mm pistons or can i just buy 85mm and bore them with the recommended clearance. JE recommended clearance for 85mm pistons at (0.0030 - 0.0035 inch) where i know the clearance in inch based on the above measurement is at 0.0008 inch - 0.0012 inch. In other words, i still have meat in the cylinder wall to make up for the clearance if i purchase the 85mm pistons.

Some might be wondering why not just overbore it to 85.5mm, well i just want to save up the meat in the block for long term upgrades say if the current setup failed then i can still go for 85.5, 86, 86.5, 87mm etc. This would allow me to keep on rebuilding the same block before forking out money to buy a new block.

The most important things are proper geometry of the cylinder and proper piston to wall clearance. If you can maintain both those parameters and keep the piston size 85 mm there should be no problem with that approach...

Just to clarify, clearances are NOT built into the bore, they're built into the pistons which will be very slighly smaller than the bores. That is, the bore would be 85.000mm but the pistons would be 84.900 to 84.975 depending on design, use, material, etc - however, even with modern manufacturering techniques, there are tolerances and this is why it's still good practice to size the bore to the piston going into it. However, that's with a fresh bore and your wear should make that un-needed.

As Georg said, if the bores are in good enough condition, you should be fine, especially if running relativel low boost levels. My normal preference would be to re-bore, especially with forced induction, but I do understand your reasoning. If I may make a suggestion, some blocks are used in different engines and you may be able to pick up another, stock bore*, block cheap. This would mean if you did get oversized pistons you would have the option of later using them in the rebored spare block, also with correct clearances.

*Possible a 4G64 based long rod/stoker engine - https://www.maperformance.com/blogs/tech/4g63-engine-building-basics-101

The other thing to remember is cylinder wall honing which is required for good piston rings breaking in. Normally it is additional 0.01 mm everywhere when done with torque plate.

Good point, georg.

Seems there may be quite a few options to check out for the engine blocks, might reward a few weekends going through the local engine dismantlers/wreckers/on-line trading sites - https://en.wikipedia.org/wiki/Mitsubishi_Sirius_engine