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piston ring gaping @ temperature

Practical Engine Building

Discussion and questions related to the course Practical Engine Building


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Hi

I am new to engine rebuilding and i am currently on rebuilding my 1st engine, sr20det notchback. I understand that professional engine builders like HP academy may have a temperature controlled room during your rebuilding process. I am wondering if ignoring the controlled temperature, would it cause any issue with the rebuilding?

Specifically, the piston gaps. As much as I wanted to follow the piston manufacturer specification, there is no indication on the temp the measuring should be performed. I assume the aluminium block would shrink slightly at a cold weather together with the piston rings hence following the specification may be less of a concern as the manufacturer may have already factored in the consideration as the block and the rings were of different material. However, in a hotter environment, i am assuming the shrink of the block and piston rings may be lesser and if we follow the gap as specified by the manufacture, would it resulted in a larger piston ring gap at engine operating temp (assuming the piston ring expand lesser at operating temp since it is partially expanded from the hotter temp) which potentially causing huge blow by? Or this is actually a negligible factor? If it does, what would be the recommended temp for the measurement and gaping temp be done at? On the other hand, I was under the impression that if the block and the rings were in the same room and same temp, it should be sufficient. But I could not get an assurance.

Reason I asked is because I was warned by my local machine shop that i should reduce the gaping on my piston rings to avoid huge blow by gas but I was not able to get sufficient information to conclude this. I am also trying to avoid missing engine oil from too much blow by.

I am using a set of WISECO 87mm pistons set. The recommended gap for the 1st ring is 0.005 inch and 0.0055 inch for 2nd ring. My bore is at 3.4252 inch. This gave me a 0.017 inch gap for 1st ring and 0.019 inch gap for 2nd ring. The recommended gap for 1st ring by the local machine shop was 0.015 inch which is toward the high performance street/strip instead of street-moderate turbo/nitrous. I am looking to up boost to approx 26psi with the new forged internal running gt2871r. It is not a track built purpose but do track occasionally. Engine will be tuned with 97 Ron and potentially a Water Methanol Injection Kit. I have yet to identify the tuner but I am interested to explore and learn about tuning.

Appreciate if you can shed some light.

Lionel

The difference is going to be negligible.

When I am machining aluminium I get around 0.01mm difference in a large block of ali between room temperature (say 15-20 degrees C) and too bloody hot to hold (say 60-80 degrees C).

While it's typical for the clearances to be quoted at an ambient temperature of 20 deg C, as Denis has mentioned, in the range of temperatures we're actually going to experience, the differences are going to be minimal.