Newbs knock dilemma

[TheAntiChrome]

Hey guys I know this being my first post I should introduce myself so here I am.
Lotsa friends with cherokees, just had to find the right one to learn on.
'90, 6" long-arm, 33s, d30/44... I love it...

Mechanic says broken piston skirt or mild wristpin knock... only sounds bad on DEceleration or if I get really into it, like 3,500+ RPM.

so I decided the that I could grab a junkyard motor and hope for the best, rebuild mine, or scavenge a stroker together. From what I have been reading its a lot like rebuilding it with the wrong crank and rods... oops.

I have access to...
-an unknown year ("90 or 91" says the mechanic) 4.2 from a YJ,
-an 87 4.0,
- and I can trade some stuff for a 93HO with high-mileage

questions...
which combination of parts would you go with?
-what to give to the machinist and with what instructions?
will my stock (renix?) engine management be adequate?
is the aw4 strong enough? I got the jeep with a bad tranny, swapped it myself, but am hesitant to trust a mystery junkyard tranny

I am not a heavy-footed mudder, the occasional rocky hillclimb or twisty ravine.

The reason this is such a hurry for me is because its my daily driver, I start college in a few months, and I am unemployed.

Go economy!

I cannot thank you guys enough for any advice or guidance. I will continue to search, like all newbs should, and update this thread as I get things sorted out.

...for the record, I have been into early watercooled VW's and turbo Mitsubishis. I am a contributing member of teVWs.com, vwvortex.com, and 4g61t.com. I consider myself a fan of everything motorized.

~joe

ps.

[lafrad]

if you are going to have the mechanic do it, cheapest "worth it" way to do it:

Drop off the 4.2, he needs the Crank and rods. clean up the crank, get new bearings.

Drop off the 4.0. Everything on this gets reused, except for the head and pistons. (inspect the cam, if it can be reused, it can safe a couple bucks)

have him clean up the 4.0, do a 0.030" overbore and get a new set of the "cheap" hyper-eutectic pistons for the new 0.030" overbore.

Put the new pistons on the 4.2 rods.

have him do a bit of work to the combustion chambers, get a few CC's more volume out to lower the CR so fuel quality is less of a concern.

if you can't reuse the cam, get a "new" stock cam and lifters from a similar year to your jeep.

Assemble and have fun

[TheAntiChrome]

well the mechanic is going to be overseeing it, but I will be doing it.

thank you for your quick response.

where is the recommended place to get "cheap" pistons? ebay?
-I don't mind spending money where it is worthwhile

since it is a newer 4.2, will this have the direct-swap crank nose? that is a part of the process that is still unclear to me...

better cam to use, renix or HO?

what is the tool used to cc the combustion chamber? is there a kit somewhere? I have seen it done I just don't know where to get the tools...

thank you again

[John]

Tools, go to a shop that sells welding supplies and pick up a couple of the large lens covers for a welding helmet, they are a clear plastic and cheap. Drill a 1/8" hole, Veterinary supply places can supply a 60cc syringe and needle, the largest diameter they have, a jar of Vaseline to provide a very thin seal beneath the modified lens cover and the head or piston chamber to be measured. 10 to 12 dollars for this basic set up. Dull the syringe needle or preferably cut the very end of the needle off with a dremel tool when you get it home.
John

Edit, drill the hole in the sheet of clear plexi glass at the center of the sheet, you can use tap water with a few drops of food coloring to aid in visibility, add a couple drops of liquid detergent to improve wetability.

[SilverXJ]

I think Jegs and Summit have kits. I work at a sign shop and we use a lot of plexi. I cut a piece of 1/2" thick plexi on the laser and drilled a hole in it to CC. I don't know how thick the welding plexi is but it needs to be pretty stout so it won't flex. You could call a sign shop and see if they would cut you a piece of scrap. Or if you can't find any I could cut one and ship it to you. I drilled the hole in my plexi on the edge so I could tilt the head to get the air bubbles out if necessary. I got my burette on ebay. If you do get a burette get a glass one. I had a plastic one and I was intending to use alcohol to CC the head. Bad idea. I didn't think that the alcohol would melt the plastic burette and it did. John's setup is a cheaper than mine and will get you in the general area. Mine is more precise, but iirc my burette was $30.

[John]

That is a good middle of the road setup, and the upper end of the scale has stands and optical flats (glass) cover plates. All will give repeatable results with careful use. The economy system works best if you add a second much smaller syringe. Fill the cavity with the larger syringe to the closest major graduation on the syringe body, i.e. 55 cc and then finish with a smaller syringe, graduated in.1cc. Everyone knows a nurse or Dr. and those can be had for free. As the lens I described are made for optical use, they are awfully distortion free (flat). The set you buy would depend on the frequency of use.
John

[TheAntiChrome]

alright thanks guys, I will get one of those when the head is ready to be worked on.

which engine management should I get? I have access to a 99 right now (cash for clunkers, nothing wrong with it). I need to know if I should get the earlier HO stuff, or if the Renix I have is adequate for a basic 4.5-4.6 stroker

I have been raping parts from the 99 for two days but I have not grabbed the harness, just the front nose, radiator, complete intake with power-steering pump, and some other little bits like an amp wiring kit, K&N filter, and Accell coil/wires I really want the newer doors, did they ever come in manual configuration instead of power windows?

thanks

~joe

[John]

alright thanks guys, I will get one of those when the head is ready to be worked on.

~joe

Summit sells a burette, burette stand, and a polycarbonate,(plastic) cover sheet for $100
John

[SilverXJ]

I have no personal experience with Renix, but I have heard that is fine to run on the stroker. If anything I would stay with Renix because it isn't as choked down with emissions programming. Also, if you put the 99 PCM on a renix Jeep good luck getting your gauge cluster to work as the later Jeeps use twisted pair to run the gauges.