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Unread 01-17-2013, 10:54 AM   #1
warrior1670
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258 to bore or not? HELP

Ok, got the measurements on my bores and pistons. Needing some help to decide to bore it or just re-ring it. Called a few machine shops, they want to see the block and wouldn't give advise. Hoping JF will help!

bore = 3.752
piston = 3.747
clearance = .0025

NO ridge on cylinder, except for a small bit of carbon at the top. I pushed the piston out very easily. Pistons look ok, the tops look cruddy. No idea how many miles are on the motor, it was a transplant. Motor hat 120 PSI compression, lots of blow by.

img_0852-1-.jpg   img_0851-1-.jpg   img_0850-1-.jpg   img_0819-1-.jpg   img_0818-1-.jpg  

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Unread 01-17-2013, 11:36 AM   #2
CSP
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Any taper in the bores? How many measurements did you take?
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Unread 01-17-2013, 12:19 PM   #3
warrior1670
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Bores are straight. Took lots of measurements. Bores are within all stock specs. 8 measurements in three locations in the bores.
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Unread 01-17-2013, 12:22 PM   #4
Jack61
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Go to page 18 of this, http://theamcforum.com/forum/uploads...ine_Manual.pdf Looks to me like you can hone to break the glaze and be fine.
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Unread 01-17-2013, 02:45 PM   #5
thompsoj22
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hone marks dont look factory?

to me it looks as though it has been honed/ringed prior to your teardown. the hone marks in your cylinders arent at factory angles/degrees. pistons are cheap and if the bores are within tolerance and dont require oversized rings i would hone "correctly", and install a new set of std pistons/rings. that way no cleanup of old pistons!. the bearing does look rough, id have the crank checked by a machinist and go with his recommendations on turning/cleanup of the journals and absolutely replace cam bearings no matter what. all this is inexpensive and youll be glad you did it.
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Unread 01-17-2013, 04:18 PM   #6
warrior1670
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I was thinking the same thing. Would those messed up hone marks eat a set of rings?
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Unread 01-17-2013, 10:30 PM   #7
bruteboy
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in the first photo,it looks like the piston's top ring land has excessive wear and will need to be replaced.
new pistons and an overbore would be best.
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Unread 01-18-2013, 04:43 AM   #8
warrior1670
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Worn ring land....ugh!
image-1188105207.jpg   image-2575705538.jpg  
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Unread 01-18-2013, 08:46 AM   #9
Jack61
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Didn't notice that the first time. It kinda looks like someone else tried to do a ring job and halfassed it. You may be a lot better off in the long run to just bore oversize with new pistons, that way you know it is right and should last a long time.
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Unread 01-18-2013, 10:50 AM   #10
vinnyabs
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Wow. Had mine bored out 30 over. I use that loosely. I thought. Knew what that meant until you guys started talking. Guess ill just stick to what I knw and sound like a pro otherwise
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Unread 01-18-2013, 11:09 AM   #11
uptillnow
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Quote:
Originally Posted by thompsoj22 View Post
to me it looks as though it has been honed/ringed prior to your teardown. the hone marks in your cylinders arent at factory angles/degrees. pistons are cheap and if the bores are within tolerance and dont require oversized rings i would hone "correctly", and install a new set of std pistons/rings. that way no cleanup of old pistons!. the bearing does look rough, id have the crank checked by a machinist and go with his recommendations on turning/cleanup of the journals and absolutely replace cam bearings no matter what. all this is inexpensive and youll be glad you did it.
I agree... These are "Hastings" rings and the second ring is only "half" seated by the looks of the ring. These are cast iron rings and should seat in the worst conditions, given some time and full throttle pulls.
Just below the oil ring on the piston shows some "wear", how much will determine if you need to replace the pistons. "If" you replace the pistons, I would go to the next oversize and bore the cylinders and start with a proper cross hatch pattern, honing out the boreing tool marks.

As i have heard here before, this advice is worth what you paid for it.
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Unread 01-18-2013, 11:18 AM   #12
uptillnow
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Quote:
Originally Posted by bruteboy View Post
in the first photo,it looks like the piston's top ring land has excessive wear and will need to be replaced.
new pistons and an overbore would be best.
The top ring lands appear to have had a tool scrape out the crap, goudging the ring land. This is also a place to measure and see how much clearance between the ring and the ring land in the piston. The piston NEEDS to hold the ring as "square" on the cylinder as possible.
Again, I would replace the pistons, next size up and bore the cylinders due to the existing honing/scratches in the bore.
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Unread 01-18-2013, 05:48 PM   #13
Bigjonh
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Just my two cents.

Any good machine shop will tell you what to do, don't guess at it. Take it to them and get there advice. Maybe just a honning and a ring job might do it.
Hard to tell from pictures, and there advice is free.
Since you have it all apart and if its needed or pretty close to the limits I would spend a few extra bucks, have it bored over, new pistons and then you really have a almost new engine.
If you plan on keeping the Jeep it will be a good investment.
Just my two cents.
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Unread 01-21-2013, 04:15 PM   #14
uptillnow
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I was looking at these pistons again... It looks like there is a crack at your thumb on the skirt of the piston.
About 1/4" long. Yep Yep i would definatley have someone "look at with authority" INSPECT.
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Unread 01-29-2013, 07:53 PM   #15
Bigt0533
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Great thread. Alot of good info. I will soon be replacing the pistons and having it rebored. I also have a 258 on a 1979 cj7 that i have just started to rebuild. My question is, if having it rebored to the next size up does it increase the overall hp? I am a beginner mechanic.
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