7.3 piston gap - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 20 (permalink) Old 03-22-2018, 10:16 PM Thread Starter
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7.3 piston gap

So I have a 95 7.3 with 230,000 miles. I have the motor tore apart to rebuild it and there is a sizable gap at the top of the piston and cylinder wall, I'm relatively new to diesel and I'm not sure if it's supposed to be this way or not. Kt performance offers rebuild just with stock pistons and kits with pistons up to .003 over stock and I don't know if I should go with a stock size or something bigger. Any advice is appreciated.
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post #2 of 20 (permalink) Old 03-22-2018, 11:16 PM
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Re: 7.3 piston gap

So you're gonna have to take the piston out and put a ring in there all by itself and measure the end gap at the top of the bore. Compare to spec. No telling what the end gap is supposed to be but .003 is pretty typical for clearance.

Then do the same thing at the bottom of the bore. compare the two and see how badly it's tapered. That's a quick and dirty way to tell ballpark of it needs bored or not. Can't just install oversized because it won't be worn at the bottom of the cylinder like it is at the top.

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post #3 of 20 (permalink) Old 03-22-2018, 11:20 PM
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Re: 7.3 piston gap

Looks like spec for the 7.3 is actually .004 - .006 clearance

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post #4 of 20 (permalink) Old 03-22-2018, 11:29 PM
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Re: 7.3 piston gap

Or easier yet, just put a ring about an inch down in the bore, and measure the gap with a feeler gauge. If the gap is greater than .024 the cylinders must be bored oversized.

Edit: those are rough measurements. End gap is not the best way to measure diameter, but it'll tell ya what you need to know.

Could also be the piston itself, there's a spec for that too.

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post #5 of 20 (permalink) Old 03-23-2018, 12:19 AM
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Re: 7.3 piston gap

Looks fine to me. All powerstrokes i've seen inside of look like that.

Shoulda bought a Cummins.

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post #6 of 20 (permalink) Old 03-23-2018, 12:33 AM
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Re: 7.3 piston gap

The "crown" of the piston, where your ring grooves are, is a bit smaller diameter than the lower section. Usually called the skirt. You measure the skirt in line with the connecting rod, most of the way down, just about every piston you will see is "barrel" shaped. You would then measure the bore somewhere in the middle, that's where most of the wear will be. I'm going to assume these engines have forged pistons, so piston to cylinder clearance is a bit loose. Forged pistons expand with heat faster than the bore.

Gas engines use a rule of thumb, .003 per inch of bore for the compression ring gaps. I have no idea what it would be for a PSD.
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post #7 of 20 (permalink) Old 03-23-2018, 01:35 AM
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Re: 7.3 piston gap

I've not heard of a rule of thumb on ring gap but the gap in the manual said I think .014 - .024. quite a bit but it is a turbo diesel, so I guess that makes sense.

I was surprised to see the spec on piston to cylinder wall clearance. I thought it would have been higher, but nope.

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post #8 of 20 (permalink) Old 03-23-2018, 02:50 AM
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Re: 7.3 piston gap

The best way to figure things out it is to have a bore gauge or inside mike measurement , machine shop, to know what it is. No guessing that way. Way to much time and money to be guessing.
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post #9 of 20 (permalink) Old 03-23-2018, 03:14 AM
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Re: 7.3 piston gap

My theory with this. If you have to ask the question, best to leave it to the professionals then. Hence why I just pulled and installed the motor, I let the shop do all the hard work.


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post #10 of 20 (permalink) Old 03-23-2018, 04:04 AM
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Re: 7.3 piston gap

Quote:
Originally Posted by jamiesaun View Post
I've not heard of a rule of thumb on ring gap but the gap in the manual said I think .014 - .024. quite a bit but it is a turbo diesel, so I guess that makes sense.

I was surprised to see the spec on piston to cylinder wall clearance. I thought it would have been higher, but nope.
You kind of take it with a grain of salt. That rule probably works more often than not, unless it's going to see No2, turbo pressure, etc. There are some newer ring material and design that are set up a bit different.

Piston to cylinder clearance is different for cast, hypuerectic (spelling?) or forged. Different heat expansion.

I build a LOT of engines, but none of them diesel so YMMV.
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