Out of round piston??

butch27 · March 16, 2010, 09:44:38 PM

On my '78 Ford 302" in my "T" I had NO compression on #1 cylinder.It ran o.k. sounded like a big cam in it.LOL I pulled the head and put a feeler gauge between the piston and wall. Had about .005 on one side until it hit he rings and on the OTHER side (180 degrees across) I couldn't get he gauge in at all. The engine is a fresh rebuild bored 30 over. Any ideas? Strange to me.

Fat Cat · March 16, 2010, 09:58:49 PM

Are you sure the bore is round?

butch27 · March 16, 2010, 10:37:40 PM

Man I sure hope so. I'll have to check every thing. Still should have had SOME compression ??

enjenjo · March 16, 2010, 10:43:13 PM

Bent rod?

butch27 · March 16, 2010, 10:56:56 PM

Taking it (the piston) Thursday.

brucer · March 16, 2010, 10:59:56 PM

Quote from: "butch27"On my '78 Ford 302" in my "T" I had NO compression on #1 cylinder.It ran o.k. sounded like a big cam in it.LOL I pulled the head and put a feeler gauge between the piston and wall. Had about .005 on one side until it hit he rings and on the OTHER side (180 degrees across) I couldn't get he gauge in at all. The engine is a fresh rebuild bored 30 over. Any ideas? Strange to me.

you'll have clearance at the top of the piston, the skirt has the closer tolerance. check bore for roundness and size.
what the valve springs look like? sounds like you didnt have a valve closing.

chimp koose · March 16, 2010, 11:35:53 PM

There should be a lot (.040) clearance in the ring land area. Are you sure there isnt a bent or stuck valve?

Jokester · March 17, 2010, 12:07:52 AM

If I remember correctly from tech school (40 years ago), pistons are not round. They are slightly oval so that when they heat up they become round. Anyone else confirm this?

.bjb

chimp koose · March 17, 2010, 01:24:05 AM

Pistons are generally narrower across the pin as there is more metal to expand in that direction when at operating temp(on a full skirt piston).Likewise the piston is much smaller in the top as the piston is solid and will expand much more than at the skirt.

enjenjo · March 17, 2010, 09:16:16 AM

Quote from: "Jokester"If I remember correctly from tech school (40 years ago), pistons are not round. They are slightly oval so that when they heat up they become round. Anyone else confirm this?

.bjb

Kinda. At the top the piston is generally round, but the skirt is not. It's wider at 90 degrees to the pin, that it is measured at the pin, as the pin area of the piston retains more heat, and expands more. So ideally, at operating temp, it will be round.

butch27 · March 17, 2010, 10:46:47 AM

About that bent or stuck valve-I did have valve problems and have got new heads. Would this have HURT the piston or the bore that much , or can I just put the heads on and run it?

enjenjo · March 17, 2010, 11:04:47 AM

I would pull that piston and rod, to do further inspecting. As I said, I suspect you have a bent rod, which can be caused by a stuck valve. Another possibility is a collasped skirt, which can also come from a stuck valve.

butch27 · March 17, 2010, 08:44:53 PM

Pulled it! The 2 compression rings were aligned TOGETHER I turned the so many degrees apart, I think the reason I had NO clearance on one side is because of this. ?? The rods are new . How do I check for bent?

brucer · March 17, 2010, 09:14:19 PM

Quote from: "butch27"Pulled it! The 2 compression rings were aligned TOGETHER I turned the so many degrees apart, I think the reason I had NO clearance on one side is because of this. ?? The rods are new . How do I check for bent?

you could take a straight edge if you can fit it between the piston and the small end, and run it from the big end of the rod to the small end and see if the rods flat on the ground surfaces.. then i guess just eyeball it..

the ring end gaps should be misaligned and not in-line with each other....

i think you need to look at your valves, springs,lifters and pushrods on that cylinder... and look at the lobes on your cam, you mighta wiped them on that cylinder when you did the break-in..cam and lifters have been a common problem since they took the zinc out of the engine oil.

enjenjo · March 17, 2010, 09:17:04 PM

Quote from: "butch27"Pulled it! The 2 compression rings were aligned TOGETHER I turned the so many degrees apart, I think the reason I had NO clearance on one side is because of this. ?? The rods are new . How do I check for bent?

Best way, is to remove the piston, and check it in a rod fixture. It would not be the first time I have found a new rod that was damaged. Any automotive machine shop can do it.

If the rod is bent, that can cause the rings to rotate like yours have.