258 to bore or not? HELP - Page 2 - JeepForum.com
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post #16 of 19 Old 01-30-2013, 02:12 AM
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No, it is such a minor increase in bore that you will not measure horsepower gain from the bore.

However a cylinder that is now working at 130 psi will always give more power than one at 80psi.

Looking atthos epics I would replace pistons, rings and bearing shells and stick it back in. 120 psi was good pressure when it was pulled out.

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post #17 of 19 Old 01-30-2013, 05:39 AM
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if your going this far id recommend a semi or full rebuild, just my 2 cents as I did my 258 and it runs awesome. It cost me a little over a grand to do and well worth it. id recommend having the machine shop tank the block and look at the crank for a polish, my crank in the 258 had some fairly good scratches in it and they came right out after a polish and looked great wasn't very exspensive to do either. I agree about cam bearings as well change them as that's what my bad oil pressure problem was. if its apart this far change the rod and main bearings also. this is probably unrelated the the bore question. if the bore mics up to the specs you should be fine as long as theres no gouges or ridge and even the ridge can be cleaned up with the ridge remover tool. Also remember to use plasti gauge when putting the bearings in you don't want to have the bearings to tight, id let the machine shop probably order those so that they get the right ones as well.

1981 jeep cj-7 258 with t-176 4 speed and 31x10.5x15 tires,
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post #18 of 19 Old 01-30-2013, 06:38 AM
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Be advised, it's very easy to take too much material with a ridge-reamer....

My cylinders didn't really need to be bored, but I got a little too aggressive with the reamer, and my machinist recommended boring.

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post #19 of 19 Old 01-30-2013, 10:00 AM
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I'm going to add my two cents and try to keep it short.

That ridge you have to cut out is the size the bore used to be and is correct for NEW original pistons. When you try to get off the hook by just busting the glaze and re-using old pistons, you open the door to some potential problems.

Depending on skirt to wall clearance, the piston will rock back and forth in the bore at TDC and BDC. This can make for noisy start-ups that hopefully goes away when the engine warms up. The rings can also unseat with every rock of the piston. That increases blow-by and can also make for a new engine that uses oil. The ring gap will likely be several thou too wide, also adding to a blow-by problem. It won't build the cylinder pressure it should, either.

If your blow-by is enough to overwhelm your PCV, the crankcase will build pressure and you'll find that your new engine weeps oil in addition to using it.

I'm not trying to give you a "doom and gloom" post but I want you to be aware of what can happen. Personally, I'd bore any old, carbureted engine. You could probably get off the hook (safely) busting the glaze on a modern, fuel-injected engine with under 75k miles or so but there's just too much bore wear on an old anchor. I'd have it mic'd and bored to where ever it needs to be to fully clean up the bores and get new pistons and rings.

Do it right and you won't have to touch it again.


Edit: I see where you stated there was no ridge. I must have mixed this thread up with another one. My bad but these points should still be at least entertained

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