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Unread 02-11-2010, 01:11 PM   #1
stormkaster
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help with pistons

Im doing a rebuild on a 4.2L 258 for my yj and i am going to get it bored out to a .030 over bore, now the problem i hae now is selecting what pistons to use i have the option of the Kohler, Brigg & Stratton and Tecumseh also what HP raiting would i need (8, 10, 12, 14, 16 i think i have the option of an 18 as well) cant find any info on the net and my local Jeep dealership is as dence as the front bumber on my XJ!

and help would be appreiciated
Thanks

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Unread 02-11-2010, 01:49 PM   #2
mdm
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Are you rebuilding a Jeep engine or a lawn mower? I've never seen AMC 258 pistons made by any of those companies or a Jeep engine with 8-18 HP.
??
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Unread 02-11-2010, 01:53 PM   #3
stormkaster
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its just the piston hp rating not the engine hp rating, if it didnt make a diffrence im sure i would be given the option
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Unread 02-11-2010, 01:57 PM   #4
mdm
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Do you have a link for any of these pistons?

I just rebuilt my 4.0L and did a bit of research on available parts and didn't see any pistons made by those companies or that were listed by HP rating.
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Unread 02-11-2010, 02:01 PM   #5
stormkaster
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dont have a link, going on what is infront of me on the computer of course NAPA and the dealer could be full of it! Unfortunatly i cant seem to down track anything to say otherwise.
like i said help would be appreciated
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Unread 02-11-2010, 02:06 PM   #6
stormkaster
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would the 4.0L pistons work in the 4.2L?
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Unread 02-11-2010, 03:07 PM   #7
CJ258
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If your pistons are in good shape you could just run over sized rings. That was a common practice at one time and could save you a bit of cash for other upgrades.
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Unread 02-11-2010, 03:33 PM   #8
stormkaster
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i spent a lot of time cleaning the crap off the pistons and there all in grat condition, just using over size rings wont cause any knocking in the cylinder? sorry if its a stupid question, first rebuild and learning as i go
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Unread 02-11-2010, 05:50 PM   #9
Reelm n Rubi
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4.0L pistons won't work because they have different pin heights.

lots of choices here's just 1 source. Jeep Parts PISTONS WITH PINS

I'd try Summit also, and if your going to bore it go big! .060 over!
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Unread 02-11-2010, 07:37 PM   #10
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Quote:
Originally Posted by stormkaster View Post
Im doing a rebuild on a 4.2L 258 for my yj and i am going to get it bored out to a .030 over bore, now the problem i hae now is selecting what pistons to use i have the option of the Kohler, Brigg & Stratton and Tecumseh also what HP raiting would i need (8, 10, 12, 14, 16 i think i have the option of an 18 as well) cant find any info on the net and my local Jeep dealership is as dence as the front bumber on my XJ!

and help would be appreiciated
Thanks


LOL WAT?
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Unread 02-11-2010, 08:24 PM   #11
CJ258
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It was a common practice and I'm sure machine shop still do it. If your bores aren't out of round you may just have them honed. Talk to the shop prepping your block and see if they are within tolerances. Can't hurt to ask.
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Unread 02-11-2010, 10:25 PM   #12
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Quote:
Originally Posted by CJ258 View Post
If your pistons are in good shape you could just run over sized rings. That was a common practice at one time and could save you a bit of cash for other upgrades.
You should be saying "if your cylinders are in good shape". Generally speaking this is correct, however, the reality is if your going to be keeping the Jeep for a while, your are money ahead to to go ahead and bore the cylinders. This insures they are true and within specs.
As for your piston choices, I think you clicked on the wrong button. Most pistons are offered by differences in composition, not HP rating. For slow spinning engines such as ours, you don't need high end forged slugs. This goes for a cam as well. I suggest you stay within the stock or RV grinds to see the best drivability. My recommendation is to purchase a master engine kit from a reputable vendor. This will include a ground crank, proper pistons, cam, all bearings and seals, pretty much everything you will need in one package.
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Unread 02-12-2010, 12:46 PM   #13
stormkaster
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im statrting to think they typed the wrong criteria into the computer at napa, i should check with the shop to see if my engine can be bored out to a .060
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Unread 02-12-2010, 01:18 PM   #14
EJames1226
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Make sure you stagger your rings when you put em back in.
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Unread 02-12-2010, 02:21 PM   #15
stormkaster
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thanks for the help guys, went back to napa and just went old school and looked threw the Clevite parts guide and found all the right part numbers... if i change the pistons and rings will i need to change anything else?

all the berrings all still fit well using the STD. size and the crank is still in balance..
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