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OEM Piston to wall clearance

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Hi Guys,

I recently purchased a spare 1zz Toyota engine for my car. Since these engine can be bought for almost nothing I thought it will be a easy and low risk start into getting more knowledge into building engines. My plans are to take it apart, replace all worn out components such as bearings, and prepare for a low boost turbo setup in the future.

At this moment the engine is almost fully disassembled and I am still waiting on some tools (for example a bore gauge) to get further into the project. I managed to download a full repair manual for the engine and I was looking into the specs for the cylinder to wall gap. I noticed that standard diameter specs (I assume this means specs from factory) are given 79.000 to 79.000mm with a maximum diameter of 79.133mm and for piston diameters is given 78.955mm to 78.965mm. I noticed that the specs for the piston oil clearance state standard oil clearance 0.073mm to 0.096mm with a maximum of 0.1mm

I don't get how they calculated these numbers, if take the smallest bore 79.000mm and largest piston diameter 78.965mm I calculate a clearance of only 0.035mm, much smaller than stated oil clearance. Can someone explain what I am doing wrong?

Attached Files

You're not doing anything wrong, it's just a mis-understanding.

It's almost impossible to hold the extremely tight tolerances of the bore and piston, during maufacture, at least without extremely expensive production techniques.

What manufacturers do, instead, is manufacture to tolerances they can achieve while minimising costs and, to this end, they will normally measure the components and give them a grade for the actual size. By doing this they can match different grades of pistons and bores to achieve the correct clearance - a larger bore is matched to a larger piston, smaller piston to a smaller bore.

The same process is used for other components, like con' rod and main bearings to journals, valves to guides, matching con' rods' mass, etc.

Sometimes you can see a suffix letter or number for these components, but as a general guide, it can be useful to note which pistons, rods, valves, etc, came from where and refit to the same place - it can avoid some potential risks of mis-matching clearances.

Oh, you've a error for the second(?) bore dimension.

thanks,

Indeed I made a mistake in my post, standard cylinder bore sizes are 79.000mm to 79.013mm as seen in the attachment.

I did not realize that OEMs also use different grades for pistons and bores, this explains a lot.

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