Yep it could be....i think a leak down test would be better option for you

I have 12 hours on the new motors. I followed the break-in procedure to the letter.
These are new GM marine crate motors. The seller says the normal compression range is 150-180 on these motors. My old engines averaged 210.
The throttle was wide open for the cylinder compression test.

I will do a leakdown test.

What do you folks make of the blow-by coating the TB with oil?

Have to narrow down where the blow-by is coming from.....could be past the piston/rings, could be a bad head gasket.....could even be a bad exhaust valve guide or valve stem seal. Just have to investigate more. There is no direct answer. Engines are not complicated but any number of possibilities can cause the same issue

Thank you, Wally, for the input, much appreciated.

210 psi ?
Nahhhhhhh.
You’d have to have 10:1 compression or so for that.

I spent the day yesterday performing some tests on the engine and specifically, cylinder #8.

I took a baseline compression test with the engine warmed up and got the same 151 psi result. I then added some oil to the cylinder, retested, and got a reading of 171 psi.
I performed a visual inspection of the cylinder using a boroscope and nothing appeared amiss.
I ran a combustion chamber leak test on the coolant reservoir but the blue test fluid did not turn yellow, so no issue there.

I then performed a leak-down test on cylinder #8. I used a boroscope to verify TDC.
The results are as follows: At 75 psi (left gauge) the right gauge reads 26 psi.
It appears the blowby is going into the crankcase and exiting the valve cover breather hose.
I do hear a soft hissing airflow sound from the TB but I can't feel any air passing through there.

I hooked up and ran the Diacom software and I did not see any knock retard as I ran the engine from idle to 3k rpm.

Thank you all for your help and I'm looking forward to reading your thoughts on the matter.

I did find the cause of the blowby coming from the breather hose. It turns out the (new) hose connecting the PCV valve to the TB was defective and had an internal obstruction. I swapped out the hose and the blowby is gone.

I did the compression leak-down tests on cylinders #4, 6, & 8.
#8 - 89 psi in, holding 44 psi
#6 - 89 psi in, holding 73 psi
#4 - 89 psi in, holding 58 psi

Here is a photo of the inside of cylinder #8: https://photos.app.goo.gl/xFaNjeQGo28dVfWz8 - is that a spot of rust?

Pistons are alum and don't rust. Not sure what that is. Maybe some ring land damage during assembly? Compression ring end-gap not set correctly?? Hard to tell from a pic.

Had a SBC years ago with low compression on one cyl after break-in. Wound up being a cracked ring land. Never did figure out what happened.

Think I'd run the compression tests a few more times, avg the results and contact the supplier with the data. Maybe a warranty issue?

Hi All

Here is a link to the original eBay listing, this was back in February when I purchased the engines (I guess they don't offer the GM crate engines anymore). Sorry, I didn't notice the engine listings had changed:
https://www.dropbox.com/scl/fi/hzlox...6r4upt5h5&dl=0

BTW, from the eBay seller: The only degree info I have is: Centerline, deg BTDC - 116

I'm going to properly photo-document the cylinders this week. I purchased an articulating boroscope for this purpose, and I will post the findings as soon as I have them.