Forged piston expansion

[fast400]

I have a set of forged Ross pistons in a new rebuild. I'm wondering how much the block vs piston expands. The reason I ask is I decided to do a compression check at 1000 miles. I opened the throttle, but the engine was only lightly warm to touch. The readings were consistent at 105 psi on all six...it's the first time I've had readings this consistent. My only worry is the slightly low reading. Spark plugs looked textbook normal.
The car is really strong, and chirps the tires at 60 mph at wot.
Should I consider things all good, or worry?

 

[earlbrown]

it's the rings that seal not the expansion of the pistons. That PSI is on the low side. Any chance you have a huge ass cam and/or hi-ratio rocker on your engine?


Also: do you have 100% faith in your compression gauge? Stick it on the family truckster if you don't and see what it reads for comparison.

 

[Phoneguy]

Did you throw some boost at it after firing it up??? That would have helped with getting the rings seated. 105 is definitely low...mine are 175

Bryan

 

[fast400]

Verified the gauge, it's accurate. Other than harland sharp rockers and a 206/206 roller cam, it's stock. It only bothers me because it runs so darn strong..no fouled plugs or crappy idle...nothing!

 

[1KWIKSIX]

I'd warm up the engine to full operating temperature & then re-check your compression in all cylinders. Of course , be sure to hold throttle wide open during cranking. If you don't find much appreciable difference in the readings from your first test, best to perform a leak down test which will tell you alot more.

 

[TheCamel]

Are you checking with the rest of the spark plugs in, or all out? Engine should have fuel and spark disconnected, and throttle open. I am assuming you have already done all that. But at the same time the engine should be cranked a minimum of 6 times for each cylinder. If you are worried about rings sealing, after doing the dry test, you need to put about 1 ounce of motor oil down each cylinder and run the test again, if the oil seals the rings and your compression goes up by 20% or more, your rings have not sealed.

 

[forzfed]

I have a set of forged Ross pistons in a new rebuild. I'm wondering how much the block vs piston expands. The reason I ask is I decided to do a compression check at 1000 miles. I opened the throttle, but the engine was only lightly warm to touch. The readings were consistent at 105 psi on all six...it's the first time I've had readings this consistent. My only worry is the slightly low reading. Spark plugs looked textbook normal.
The car is really strong, and chirps the tires at 60 mph at wot.
Should I consider things all good, or worry?

What is your compression ratio, heads, and what size of cam are you running?