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Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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i just finished re building my 2jzgte engine with CP piston. im using the 86.5mm piston and gapped with 1st and 2nd piston ring according to their formula.

after 600kilometer driving (broken in as per HPacademy process during the first 100km) i did my 3rd oil change to fully synthetic. i noticed im experiencing a faint constant smoke coming out from both of my valve cover breather when idling.

top ring gapped at 0.0019" and 2nd at "0.0024"

is this normal or i cracked a ring land?

i did a compresion checked all is healthy at 160Psi.

and leakdown all is around 4-5% .

headgasket is still healthy.


I think that's normal. We've gotten used to no oil vapors since PCV systems became common 50 years ago. Race engines still emit vapors through breater filters even when fresh.


Hi Madnoor,

You’ve done all the right things. If there was anything a miss the leak downs would be where you’d be able to find any issues. When doing the leak down it’s a good idea to inspect the breathers and how much gas is leaking past the rings (as well as valves)

Just for some info; you can use the leak down kit else where if you suspect heavy oil breathing; for example make a intake and exhaust port blanking plate with a port in each runner to attach the leak down kick; this allows you to see how the stem seals and valves are doing with pressure against them.

hope that helps put your mind at ease.

Everything is pretty much what I would expect. At idle the pistons and rings are relatively cool and this means the ring gap hasn't closed up to remove most of the clearance, and the pistons are rocking slightly in the bores which can cause the rings to lose some contact with the bore. Under load, the rings are held harder against the bore by gas pressure behind them, so that's also missing.

What all that means is that there is going to be a small amount of gas weeping past the ring seal and exiting out the breather(s). If it is steady, it suggests it's even between cylinders - what would worry me would be if it was 'puffing', as that would suggest a single cylinder was the problem.

You can purchase gauges/meters to measure this blowby but, as it's more of a concern when under load and you can really only do that on the dyno' or actually driving, the hand-held ones have limited use.

What I did find, that should be a very useful addition to any modified vehicle, is this - https://performancetrends.com/Blowby_CFM_Flow_Sensor_Meter.htm (I expect there are many other brands) - as it can be permanently installed and, by using the logging function in the ECU or stand-alone, one can monitor blowby and hopefully catch any sealing problems before they get serious. For example - it may catch a failing piston before it breaks completely and the con' rod takes out the whole bottom end of the engine.

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