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Old 09-01-2019, 01:37 PM   #1
Gwill65074
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Default What size are my pistons

I'm trying to determine what size pistons I have in order to order new rings. The pistons do not have any size markings on them on top or inside the skirt. My pistons are measuring 3.866 to 3.868
Thanks for any help.
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Old 09-01-2019, 01:42 PM   #2
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Default Re: What size are my pistons

Hmmm, standard bore is 3.875".
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Old 09-01-2019, 01:46 PM   #3
Gwill65074
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Default Re: What size are my pistons

So does this mean my pistons are standard size?
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Old 09-01-2019, 02:04 PM   #4
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Default Re: What size are my pistons

Can you measure the bottom of the cylinder, where there is less wear? I would venture to say you have standard pistons with .007" - .009" wear?
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Old 09-01-2019, 02:07 PM   #5
Kurt in NJ
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Default Re: What size are my pistons

You need to measure the bore--at the bottom, I have seen standard size pistons with the bore worn .040 at the top, tapering to near standard size at the bottom
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Old 09-01-2019, 02:20 PM   #6
Gwill65074
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Default Re: What size are my pistons

So with .007-.009 wear are my pistons ok to reuse in my engine?
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Old 09-01-2019, 02:29 PM   #7
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Default Re: What size are my pistons

Hallo, at Ford in Germany, the piston size was stamped on the piston crown. Was not that in the USA?
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Old 09-01-2019, 02:47 PM   #8
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Default Re: What size are my pistons

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Hallo, at Ford in Germany, the piston size was stamped on the piston crown. Was not that in the USA?

Not on original manufacturer pistons. Aftermarket are almost always so marked.



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Old 09-01-2019, 02:54 PM   #9
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Default Re: What size are my pistons

the top of the piston where the rings go is usually in that range smaller than the skirt
not all pistons have the size stamped on top

If you have it apart take a ring, put it in the bore at the top, measure the end gap, then push the ring to the bottom, measure the end gap, the cylinders taper is about 1/3 the ring end gap change---this is a crude method that comes out close
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Old 09-01-2019, 06:13 PM   #10
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Default Re: What size are my pistons

With the standard bore being 3.875", by the time we allow the piston clearance of about 0.004", I would say the pistons are worn about 3 or 4 thou. So long as the bore isn't too tapered, I'd go for a new set of rings. I have just put an engine with 0.008" wear in the bores into a car . The cylinders have been honed back to parallel, the pistons stretched accordingly and new rings fitted. Now that it is in, I find the #3 gudgeon is knocking so I'll take that rod out and replace the bush. It runs very well. Who knows how many miles it has done and how many will do yet.
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Old 09-02-2019, 09:56 AM   #11
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Default Re: What size are my pistons

Measure the taper as Kurt said, then post your findings before deciding how to proceed.
It may need to be bored to the next oversize.
If the taper is OK, then use a long thin feeler gauge to measure piston to wall clearance.
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Old 09-02-2019, 11:39 AM   #12
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Default Re: What size are my pistons

Where are you measuring? As noted by Kirk, the top is smaller than the widest part of the bottom of the skirt. A 3.875(standasrd) bore should have a 3.872 piston diameter measured at the widest part. Measure the bottom of the skirt 90 degrees from the direction of the pin. That should be a few thous less than the bore. Also measure the bottom of the skirt in at the same direction as the pin. That should be .005 or so less.(This is called cam ground, ie, not perfectly round.) If the piston is the same diameter all the way around there is going to be trouble unless you have really large wall clearance. A buddy's engine froze up, tore it down the pistons were all scored, and were the same diameter all the way around.
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Old 09-02-2019, 12:39 PM   #13
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Default Re: What size are my pistons

I figure the general rule at .003" over normal bore size for wear (and that's accounting for normal piston clearance). Even .005" over normal is risky on whether the rings will seat well. Even honing will take out a little more. Sometimes it works and sometimes it doesn't. It cost more to bore the cylinders and purchase pistons but at least the chances are a way lot better that it will last a long time.
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Old 09-02-2019, 05:32 PM   #14
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Default Re: What size are my pistons

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Originally Posted by PC/SR View Post
Where are you measuring? As noted by Kirk, the top is smaller than the widest part of the bottom of the skirt. A 3.875(standasrd) bore should have a 3.872 piston diameter measured at the widest part. Measure the bottom of the skirt 90 degrees from the direction of the pin. That should be a few thous less than the bore. Also measure the bottom of the skirt in at the same direction as the pin. That should be .005 or so less.(This is called cam ground, ie, not perfectly round.) If the piston is the same diameter all the way around there is going to be trouble unless you have really large wall clearance. A buddy's engine froze up, tore it down the pistons were all scored, and were the same diameter all the way around.
Since the pistons in question have been running long enough to be considered for a re-ring they are not likely to seize. The matter of round pistons isn't necessarily a problem as years ago Model A's racked up millions of miles on non-camground pistons. It depends on the alloy and design of the piston and some manufacturers continued to make and sell fully round pistons into the 1940's and maybe into the 1950's. I have a couple sets of them that I picked up at a garage sale.
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