502 bbc's w/ B&M 174's

Engine developed an issue halfway through last season, engines produces a copious amount of blowby under boost only. Idles and cruises fine with no issue and no oil burn right up to 4,000 rpm (where boost starts to come on), then i get blowby through both breathers and the dipstick if i run it hard. Suspected bad rings or a head gasket pushing boost into the crankcase. Pulled engine and performed compression check.

(1) 150# (2) 145#
(3) 145# (4) 140#
(5) 150# (6) 150#
(7) 100# (8) 145#

squirted some oil into #7 and re ran compression check. This time compression was 150# Assuming scored wall or broken rings. Pulled head and see no obvious scoring or broken ring marks. No obvious failures with head gaskets (felpro 17049 Marine MLS gaskets). The low compression makes this cylinder suspect but all looks good, however the combustion chamber looked a little dirtier on #7 than the others.

Thoughts?

Sounds like it detonated and lost ring seal.

Head:


Cylinder:

174 on a 502. What kind of boost you running? Your not running any pvc valves are you. When I was running 420's mb's on 454's I had a problem with the dipstick issue but not through the breathers. Springs did the trick for the dipsticks but sounds like in your situation you have to get to the root of the problem. My guess like black baja would be rings.

Looking at pics above through my cell ph but doesn't look like it would take much boost to blow by that piston. Has your boost gauge remained readings throughout before/after problem with crank pressure? I can't tell for sure by pic/shadow but looks like broken ring moving around in an isolated area causing abnormal cylinder wear off to the right at 2:00-3:00.

5 psi. No PCV on both engines. boost looks good through RPM range. Cylinder/wall clearance is the same for all the remaining pistons.

That mark at 2-3:00 is from my finger and wipes off.

Put a plate on top of that cylinder and do a leak down test , if you can't get a plate bolt the head back on without and rockers or intake manifold and do a leak down test .

That piston is nice and detonated. Order a piston and rings.

#5 doesn't look so good either. Make sure you measure the ring lands.

#5 intake valve looks tuliped.edit in,those are not mls head gaskets.