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Old June 7th, 2016, 20:07
panzerman200 panzerman200 is offline
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1998 Piston Slap questions/New Piston Install

Evening all. I have a 1998 4WD, XJ, AW4, 4.0, NP231 that's developed that famed piston slap. It has about 100k on it; perfect shape other than a few small quirks. It also has a little bit of valve train noise (sowing machine.)

I've determined that the #5 or #6 Cylinder seems to be the culprit for most of the noise. I have checked and tightened flex plate bolts just to be sure.

I'm going to tear into it this weekend and replace the lifters, oil pump (just because I'm in there), and piston(s). I've ordered all the parts other than the pistons.

My questions are as follows:

1. Should I replace all pistons since I'm in there? My fear is that since most of them are "broke in" and there's only one or two causing problems, that I may just create a problem where there is not one.

2. Can someone recommend a brand of piston? I will have limited time to work on this so quick shipping and quality are key here. I can pick up one for 40 bucks down the road at the parts shop, or 6 online for three times that.

3. Do pistons usually come with new wrist pins to press in? I'll need to hit the local shop for having them properly fitted, of course.

4. As for size of pistons... How common is it to have oversize from stock? The markings for the size are on top, correct? If there's a good chance of having stock pistons then I'd like to order them now and take a chance.

5. If the crosshatch pattern is still there and there is no ring worn into the top of the cylinder wall, can I just drop in the new piston/rings without reaming/honing?

6. Is it worth throwing new rods in? I've heard mixed reviews about that.

7. Is there anything else I can do while I'm in there that people would suggest? I've got a new exhaust manifold already that's going on. I will give everything a good cleaning and use Fel-Pro for everything. Obviously assembly lube on all metal to metal surfaces and Vaseline to prime the oil pump.

8. There's many posts about just dropping new pistons in with minimal honing and it running fine, and other posts about the absolute "Must Need" to measure the cylinder for correct fit and ordering .10 over pistons. Like I said, time is a factor here once I get it apart, so removing the block and having machine work isn't an option; Are there any thoughts about the reliability of just putting the same size piston in as before? My biggest fear is putting 6 new pistons in is just 6x the chance of messing it up if 5 of the six are fine.

Thanks so much for any tips and advice for all this.

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Old June 7th, 2016, 22:29
lazyxj's Avatar
lazyxj lazyxj is offline
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Location: Los Angeles
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Re: 1998 Piston Slap questions/New Piston Install

Most 4.0s, with over a 100,000 miles, make noise.
Live with it while you find/build a replacement engine, being in a rush is a sure fire formula to screw things up.
This would give you time to do it right.
Both XJs are still Real XJs
01 Sport, 4.6L Russ Pottenger. stroker, AW4, 4" lift, 33 X 10.50 KM2, 4.56-8.25 TruTrac, D-30-E-Locker with alloy shafts, Hidden M8000 winch, Vanco front and ZJ rear disc brakes, Frame stiffeners, Lots of armor, Towing baseplate.
1997 Country, Mostly stock, 2" lift, Engine mods, Towing baseplate
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Old June 8th, 2016, 07:05
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old_man old_man is offline
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Re: 1998 Piston Slap questions/New Piston Install

As long as it is really piston rattle, just drive it. Most 4.0Ls rattle, even from day one. I have seen engines that had 400k on them and rattled like hell, but still kept running just fine. Unless the compression is down, I would not touch the pistons.

I would not change the lifters unless you have a specific problem. The same goes for the oil pump.

In other words, if it ain't broke, don't fix it.
Tom Houston - Former Colorado Chapter President , Terminal Adrenaline Junkie, and Rocket Scientist
Loveland, Colorado My teardrop build
You're just jealous because the voices don't talk to you.
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Old June 10th, 2016, 18:11
93blackxj 93blackxj is offline
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Re: 1998 Piston Slap questions/New Piston Install

I bought my '99 XJ off my boss due to clacking noise from motor. He was on a road trip when it started and ended up driving over a thousand miles. i thought it might be a bad head gasket forcing compression back and forth between joining cylinders. Had that happen on a nissan pick up. Ended up being the #6 cylinder piston skirt broke off and was slapping in the cylinder at tdc and bdc. Bought a jy motor for $450 and now have 20000 miles on it.
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Old June 11th, 2016, 22:26
panzerman200 panzerman200 is offline
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Re: 1998 Piston Slap questions/New Piston Install

Thanks for the suggestions guys. It's been getting worse in the last 1000 miles or so (as in went from just sort of there to very noticeable.)

I am planning some long road trips later this summer and can't afford for it to grenade down the highway sometime. I've reworked my schedule to have more time to work on it. I'm going to get into it within a few weeks and see how it goes. It's also got an exhaust leak so I'll replace the headers while I'm at it.The lifters chatter a little; so the idea was just to replace those while I was in there.
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