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Unread 04-24-2009, 02:18 PM   #1
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1991 YJ Wrangler 
Join Date: Apr 2009
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4.0 piston size?

I have a 1991 4.0 with pistons marked 50. Anyone know what size this is? Thanks Dave

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Unread 04-24-2009, 03:23 PM   #2
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1989 YJ Wrangler 
Join Date: Apr 2009
Location: Phoenix, AZ
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Re: 4.0 Piston size

Typically pistons marked with a numerical like 010 or 10 are indicating they are an oversize piston,

010 for .010"
20 for .020"

.050" oversize seems larger than normal, but i am not a machinist so I can't say whether the 4.0 bores can accept that large a piston for sure.

Not a ton of info here, but HTH.

Last edited by BDKR; 04-24-2009 at 03:24 PM.. Reason: crappy spelling
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Unread 04-24-2009, 04:02 PM   #3
from here it gets rough
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1993 YJ Wrangler 
Join Date: Dec 2007
Location: Beyond Honolulu
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98.4 mm or 3.87 inches is what I found online. Are you able to mic it and see how this corresponds to what is listed as stock specification?

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