Hi there,
Recently bought a 30 over 302 65 ranchero. As I drove it, I heard a pop every once and a while (being my first old car I assumed it had to do with the poorly done and unsealed exhust). Soon smoke started coming out of the valve covers in at an increasing rate (this all happened over several trip maybe around 100 miles). So I stopped driving it and did a compression test. Found that when the engine was cool I had 170-180 psi in all cylinders other than number 2 which only had 54. I was hoping it was bad ring but taking it apart I found it had a missing skirt, a deep crack near ring landings, the piston was hitting the head, a missing landing and ring, and finally there are minor scratch marks on the skirts and cylinder walls. I asked a friend and he said it may be that the cam was ground off center, however I am think there was detonation which cracked the pistons and broke the skirt which then destabilized the piston which is why it was touching the head. Any ways here are the pics and if yall have any other ideas of what may have happened I am all ears. Additionally if my theory is true what exactly caused the detonation and how can I fix it? Was it the high compression? The hot cam? The cast heads? The 87-89 fuel I was using? For me has always ran below 185f and could go down to 150f when cruising on the high way. I also installed an afr gauge and found it running lean on deceleration (16 afr range) but when cruising or accelerating it seemed to run normal (13-15.5 afr). I believe the block is a 1968 while I think the head is a 1966 289.
Thanks in advance,
Vince

Possible detonation marks? No carbon build up where I assume the cylinder was hitting

Cylinder wall vertical scratch from skirt

When I initially took the piston out the only missing parts were part of the oil ring and the skirt. I took out both rings and the landing after the fact. They are all seated relatively well.

Cylinder 2 is clean from hitting marks

scratch marks piston and wear near the striking face.

If anyone has any knowledge on these piston markings it would be greatly appreciated if you could share, so I can buy another one of the same set.

 

Broken ring lands like that are usually caused by detonation but is is also possible that the piston was damaged when the engine was built(dropped and skirt cracked). To me those marks on the piston top were there from when the piston was new and are no problem. A 302 with a piston and a cylinder head like that one will be around 10:1 compression give or take. With a cam of about 205-210 @ .050 duration it should run fine on premium pump fuel with a proper tune up.

The scores in the cylinder won't cause a problem but it would be interesting to see what it looks like after a few strokes from a rigid hone. It might be beat up pretty badly from the pounding it got from the broken piston. Might have to take that one out to .040.

 

Thanks for the info. Being a college students in CA, gas is $5-$5.30 for premium so im thinking about going with a mild cam either an extreme energy comp cam or summit classic like you (right now I have an unknown aggressive cam) and getting a new set of DSS e series forged pistons. I am also considering getting domed pistons and maybe some used 351 Windsor heads to reduce compression to allow for lower octane fuel.

 

Sadly, gas is more expensive where I live. No, I am not bragging. It's what you should expect when you live on the west coast.

 

Domed pistons are bad with production heads on these engines. It just doesn't run very well.

 

Late to the party here, but the big scuff mark at the top land above the broken skirt makes me think the piston was rocking enough for the top land to drag on the bore hard and may have been causing those lands in that area to flex and eventually break. The dish in that piston should have kept the CR from getting too terribly high.