Piston choice for Stock 2000 4.0 rebuild?

I would appreciate knowing what pistons people have used to rebuild their stock (non-stroker) engines.

I chose the OEM Tech forum, since the only possible modification I will do is aftermarket pistons. If the moderator thinks this belongs someplace else, go ahead and move it.

I have a bone stock 2000 XJ 4.0 that I have driven with a piston slap for the last 5K miles. At 162K miles the head cracked, so I need to replace that. I took the pan off and found (not surprisingly) the skirt broken off piston #5. All cylinder walls show very little wear. I will mic the bores after I get the head off. I hope to do this work with the block in the XJ. I am reluctant to replace just this one piston with another factory piston, because of their reputation for cracking. I will probably replace them as a set, to keep consistent compression ratios on all cylinders.

When I look at all the available aftermarket pistons, I see the ones with the tallest compression height are still .009" lower in the block than the factory pistons. If I install the .043" MLS gasket, I still will loose 0.2 on the CR, because of the increased dish volume. I really don't want to mill the block. If I was to go to that trouble, I should just start looking for a 258 crank.

What pistons have you used in a stock rebuild?

I assume by your term,"stock rebuild", you mean retaining the same CR, bore sizes, stock heads, etc., but using a better quality OEM piston? If that's the case, then in my view and experience(since the 1950s) I have a very low opinion on OEM pistons. After many years of cracked OEM pistons, i.e. gross cracks, cracks between the ring grooves, skirt cracks, thermal expansion fatigue cracks from steel inserts cast into the piston, etc., I now always used forged pistons. Once you have to pull an engine, strip it down, replace pistons/and or re-hone scratched cylinder walls/and or sleeve cylinders and/or replace the block, re-assemble everything, re-install the engine, break it in again- stronger quality forged pistons are very inexpensive and a cost-effective choice. Forged pistons with dished heads can be purchased to retain your stock CR. Likewise, quality ductile or stainless steel rings virtually eliminates future ring breakage. Also forged pistons run at significantly cooler temperatures.

Good luck on your stock rebuild!

Best regards,

CJR

CJR said:

I assume by your term,"stock rebuild", you mean retaining the same CR, bore sizes, stock heads, etc., but using a better quality OEM piston?
CJR

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That is exactly what I meant, sorry if I was not clear.

CJR said:

Forged pistons with dished heads can be purchased to retain your stock CR. Likewise, quality ductile or stainless steel rings virtually eliminates future ring breakage. Also forged pistons run at significantly cooler temperatures.
CJR

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I plan to run moly rings. The only forged pistons I can find for the 4.0 are the IC944 & IC945 and these won't work with the stock 4.0 crank.
Can you recommend other forged pistons with a 1.60 compression height?

Make sure the moly rings are ductile iron and not ordinary grey cast iron with the moly coating. I've used Total Seal rings a lot and they offer stainless steel rings which are also good. Also, I've used a lot of Venolia forged pistons in the past and assume they're still in business. If you go with Venolias, make sure they put enough oil drain-back holes in the oil ring groove. If there are not enough oil drain-back holes, the oil will sit in the piston groove, cook, and set-up to lock the oil control ring closed. I've found that problem in a lot of newer engines, i.e. Mitsubishi V-6, where at 100,000 miles five of the six pistons had oil control rings frozen closed. Not a good scenario.

Best regards,

CJR

Thanks for the tip on Venolia pistons. It appears they are still in business. However, they don't list any Jeep applications and all AMC & Chrysler applications are for their older V8 engines.

What is your opinion on Hypereutectic pistons? I have read they can be noisy until the engine warms up. I have only used cast and forged pistons before in other engines. I am now considering the 3241HC piston. Summit has them for $166 a set. These would only drop the CR .06 with the MLS head gasket.

Venolia supplied the forged pistons for my last 4.0L rebuild. In the old Venolia catalog I have, they listed two pistons for AMC 232-253 6 cyl.; #6610 (Flat-top) and the #6620 (Dome or dish). They machined a set of #6610 for my 4.0L. My POC @ Venolia was "Dale"; give them a call and tell them what you want to do. They're very helpful.

Though I have an in-depth knowledge of cast "hypereutectic" pistons, I've never used them and don't plan to in the future. In my view, the term "hypereutectic" is just another clever way to market "cast" pistons made of some proprietary aluminum alloy. I started playing with engines in the 1950s and learned the hard and painful way $$$$$$$ about pistons. If they're not forged, I don't use them.

Good luck on your piston choice and rebuild! It'll be a lot of fun.

Best regards,

CJR