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Old 05-16-2005, 23:53   #1 (permalink)
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Pistons .040" over?

Howdy folks, another one of my never-ending questions on my engine build. I cleaned up one of my pistons in the hopes of finding some markings to suggest their size. There is nothing on the skirt or under the piston but I found this marking on the top under all the carbon;



Does this suggest these pistons are .040" over? I need to know becaue I'm buying piston rings very soon, hopefully this week. Thanks guys!
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Old 05-17-2005, 00:07   #2 (permalink)
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Re: Pistons .040" over?

yes I believe that means they are .040 oversize
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Old 05-17-2005, 01:13   #3 (permalink)
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Yes,
It means .040" oversize. The no. 5 would have stamped by the engine builder and just means that piston goes into No. 5 cylinder.

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Old 05-17-2005, 01:13   #4 (permalink)
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Re: Pistons .040" over?

Sure does.
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Old 05-17-2005, 01:37   #5 (permalink)
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Re: Pistons .040" over?

I would definatly check the bore size, and make sure it is still in spec to take a new 40thou piston. If it isn't, now is the time to get a cheap std bore block, and start sith 10,20 or 30 over depending on piston price/availl. I went 30 thou on bore, as i got new TRW 30thou forged pistons cheap.
But, if your current bore checks out ok, it will be fine.
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Old 05-17-2005, 02:18   #6 (permalink)
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Re: Pistons .040" over?

Thanks for the help guys! Actually I'm not sure what the "5" means but it's not the cylinder number. That piston is from No. 1 and they all seem to have the "5" stamped on them (well I've only cleaned three so far, it's tedious work!). I'm bringing the block and crank in for machining on Thursday so I'll get them to measure the bores but I'm hoping they are good because I want to use these pistons, as this is a budget build.
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Old 05-17-2005, 15:26   #7 (permalink)
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Re: Pistons .040" over?

The number 5 doesn't necessarily refer to the cylinder number, no. I found that when I pulled my 351 down, I had numbers ranging from 1 through to 6, totally at random and not consistent with any of the cylinder bores. It had never been opened up before, so I'm none the wiser. Maybe they are casting numbers, although they stamped, so... I dunno...

Just make sure you keep the pistons in order as bores and pistons will wear at different rates, says the engineGod that is Tom Monroe. I'm sure you know this, but just in case you didn't... I re-used my stock pistons in my build as well. Expensive aren't they..?
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Old 05-17-2005, 15:56   #8 (permalink)
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Re: Pistons .040" over?

I have some cheap parts that cvould help you out, But i am guessing post to nz kills that idea!
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Old 05-21-2005, 06:00   #9 (permalink)
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Re: Pistons .040" over?

0.040" over is a bit much for a hi-comp motor but if mild will b fine. Show me the underside and skirt and I might b able to tell u what brand it is. Its a cast piston for starters.
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Old 05-21-2005, 07:18   #10 (permalink)
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Re: Pistons .040" over?

Mines .040" over. No problems here
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