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This is a 383.
Can insert an .030 feeler gauge in there! Is this normal? I understand temperature expansion and contraction, and it's an obviously cold motor. But THAT MUCH of a gap? Never burned oil. No obvious ridge near the cylinder's top edge. 13K miles on the engine.
Comments please.
That's NOT how you check piston to wall clearance. Pistons are tapered and not round on the bottom. they are checked at the bottom of the skirt and only accross the thrust surfaces.
Your piston should have a gap like this at the top. Allows the rings to function properly.
That's NOT how you check piston to wall clearance. Pistons are tapered and not round on the bottom. they are checked at the bottom of the skirt and only accross the thrust surfaces.
Your piston should have a gap like this at the top. Allows the rings to function properly.
True, but just eyeballing it looks like it's got 100,000 miles not 13,000.
I purchased a Z-28 once in which the owner said it had a 'broken' piston. The engine presumable had very limited miles as it had been dealer installed as a replacement for a blown OE engine. At least that was the story.
Upon disassembly I discovered one bank to be sized correctly but the other was oversize with the smallest cylinder oversize by .009 and the largest something in the high teens. (I don't recall the exact numbers - it was 50 years ago!). The failed piston had a broken skirt.
I know piston crowns are smaller than the maximum diameter closer to the piston bottom. It's also true that forged pistons typically require more clearance that cast pistons. Furthermore I don't know how much oversize a cylinder can get before piston slap will break a skirt. With those unknowns, I'd suggest measuring the cylinder diameter of each cylinder. If they appear to be sized correctly (i.e. OEM bore size, ,030 over, etc.) you probably have no risk. If the bores vary more research is probably warranted.-
Here's a side note: I was young and didn't have a lot of money so I had only the one bank bored oversize. I ran standard size on one size and .030 over on the other. This was a 302 small block - standard Z-28 engine. Once it was back together, you'd never know the difference. It was a street car so very limited stress exposure, but I drove it a long time with zero problems.
I purchased a Z-28 once in which the owner said it had a 'broken' piston. The engine presumable had very limited miles as it had been dealer installed as a replacement for a blown OE engine. At least that was the story.
Upon disassembly I discovered one bank to be sized correctly but the other was oversize with the smallest cylinder oversize by .009 and the largest something in the high teens. (I don't recall the exact numbers - it was 50 years ago!). The failed piston had a broken skirt.
I know piston crowns are smaller than the maximum diameter closer to the piston bottom. It's also true that forged pistons typically require more clearance that cast pistons. Furthermore I don't know how much oversize a cylinder can get before piston slap will break a skirt. With those unknowns, I'd suggest measuring the cylinder diameter of each cylinder. If they appear to be sized correctly (i.e. OEM bore size, ,030 over, etc.) you probably have no risk. If the bores vary more research is probably warranted.-
Here's a side note: I was young and didn't have a lot of money so I had only the one bank bored oversize. I ran standard size on one size and .030 over on the other. This was a 302 small block - standard Z-28 engine. Once it was back together, you'd never know the difference. It was a street car so very limited stress exposure, but I drove it a long time with zero problems.
That's a very interesting story.
Thanks for the insight.
I was going to type a long comment......but here is all I have to say about this......
The crown of the piston is substantially smaller than the skirt...this is what you are seeing..... If a skirt has more than like .005 on a Hyper or .009 on a forged.....it will slap when it starts up cold. Did it slap?
What does the ridge look like? Are those hone marks?
Pull one and measure the skirt......or put it back together and run the f%&k out of it.
Jebby
Last edited by Jebbysan; Dec 21, 2021 at 07:55 AM.
I would agree with Jebby, if you wish to pull one and measure correctly, fine. otherwise put it back together and run it. Advise saying it needs a bore job from the photos you put up is pure bullsh!t.
With that much piston wear and a blown head gasket at 13,000 miles are you sure you didn't have any detonation going on that you didn't hear?
Honestly don't know. Was always noisy and never sure of what to listen to anyway. Ran nothing but 91 Octane (California blend ) and an occasional tank of 100LL avgas.
I was going to type a long comment......but here is all I have to say about this......
The crown of the piston is substantially smaller than the skirt...this is what you are seeing..... If a skirt has more than like .005 on a Hyper or .009 on a forged.....it will slap when it starts up cold. Did it slap? Don't believe it did. Standing over the idling motor and goosing the throttle didn't reveal any abnormal sounds that I could detect anyway.
What does the ridge look like? Ridges barely detectable. Are those hone marks? Block was bored before pistons were ordered and cross hatched afterwards.
Pull one and measure the skirt......or put it back together and run the f%&k out of it.I like that recommendation.
Measure the movement at TDC. The piston should rock a bit from side to side but that one looks to have too much clearance.
This also one way to do it......but you will need a mag base and indicator or a bridge with indicator......and you will have to do a bit of math to accommodate how far the wrist pin is from the top.....and you have to know what that number is (Piston C/D).
I would agree with Jebby, if you wish to pull one and measure correctly, fine. otherwise put it back together and run it. Advise saying it needs a bore job from the photos you put up is pure bullsh!t.
Thanks mate.
The car has been down for over half a year. Mostly my fault because I choose to do the work myself instead of a shop. Will it be perfect?: No. Is it considered proper to not make it perfect on reassembly?: No.
Do I want to drive this car?: Of course. Does a perfectly rebuilt car start to wear as soon as the key is turned on?: Yes.
This project keeps me home during this fu@&in epidemic, which right now looks unending and I'm considered high-risk.
I appreciate all the feedback here. You guys know your s#*t. It's a case of weighing everything said here, going from there and doing my (our) best with it.
Life is kinda short.
Piston to Cylinder Wall Gap.
) and an occasional tank of 100LL avgas.
