All Small Six lost compression on 2 cylinders.

[Dragonlich1961]

Unfortunalty- looks like my Sunshine is down for the count. Awhile back I drove her to work, and went home on a trailer. funny oil pressure, and a gallop when on the starter. Compression test revealed 70(1),40(2), 165-170(3-6). Leakdown test seams inconcluive. No leakage through the intake or exhaust. No cross leakage- so head gasket seams ok. So that leaves- rings or hurt pistons. GRR. Good news, i get an excuess to use the Aussie head on my shelf. about 4k mileage the build.

Any ideas on what failed, I don't have the space to pull it apart father at the moment(currently have my daily gen 3 hemi apart for cam failure). No aparent marks on the cylinder wall that i can see(small bore scope), decent amount of oil on the plugs but i've been fighting the vintage inlines cover. (I thought my welded in baffles were enough but no.)

 

[DON]

I guess its the ol’ good news/bad news

 

[drag-200stang]

Could be a valve train issue, pull valve cover and take a look. It can cause a symptom like that.

 

[B RON CO]

Hi, sounds like a blown head gasket. Good luck

 

[Dragonlich1961]

Update-- long over due, I've completed a leak down test, with passed- intake, and exhsaust. All blow by in crank case. Pulled the head, and pistons look ok, cylinders look ok. So some how i've pinched or fired the rings in 2 cylinders. Grr only thing i can think of is my early stuggles with pining severly. Everything looks good, so i'm hoping to get away with rings and a hone. Looks like i get to finally toss my 250 2v head on.

 

[Dragonlich1961]

Update-- round 2. Finally found the time to deal with repairs- spent most of my winter redoing the garage so my Sunshine has been parked in the corner while i work around it.
Finally got a chance to pull the oil pan and remove pistons 1 and 2. Tally- not 1, not 2, not3, but 4 Broken rings. Some how I've managed to break both the primary and secondary compression rings on both pistons. The only cause for this that i can think of is the seroius run of detenation that i have been fighting. Review some of my old posts( about 4-5 years ago) and i was struggling with bad lean conditions into the 16/17:1 range. (i have a afr gauge). If 17 is the average then there has to be a number higher, and I think we found where.
I'll be ordering a full ring set and re ringing all 6.

also discoverd that pistons 1/3 have a mark in the top of the pistons, likely from the valves. This most likely happen years ago when the timing chain came loose(my 19 year old self's fault- I put the cam wedding ring on wrong). Never ran weird after the repair- but not something i would have thought could have happen.

 

[Frank]

glad you're making progress! Always feels good to get back on a waiting project.

#2 cylinder is always leanest on all I6 engines with a common-feed intake, caused by the physics of firing order.

you may have had too small a ring gap, another possible cause. Since those cylinders were too lean= hotter, rings may have butted.