Jeep Strokers

I have a knock that I can't figure out. First off the build......'95 Cherokee 4.0 with a '90 4.2 crank, 4.0 rods and IC944 pistons .030" over. It’s running in a ’90 wrangler with a TF999 trans. I drilled out the casting to add the crank sensor, switched to a wrangler flywheel, and running the Cherokee ECU. I’m running the melling high volume pump along with the comp cams 68-231-4 cam and lifters. I added a ’91 wrangler fuel pump to the tank and a ’91 wrangler VSS for dual signals. I have cut the ears on the dizzy and I’m running an AEM wideband o2 sensor located about 6” down from the stock o2 sensor. It idles good at 14.7 and tracks pretty much like it should. I originally ran with the fuel pressure regulator hooked up at about 30 psi fuel pressure but pulling the vacuum off I get a solid 40 psi. That’s what I finally wound up doing at the end, however the knock started long ago. Quench is around .059”- .062” using the fel pro 26211…..I had .015” - .018” on the deck and didn’t have any work done. My fel pro actually mic’d at .041” but I went with the .044” that everyone else seems to say for my calcuations. All crank and rod bearing tolerances were .002” or .0025”. I might have had one pushing close to .003” but I can’t recall right now.

The initial break-in went fine and I had no issues. I drove it around town for approx 20 miles before the rear leaf spring broke at the shackle…little more torque than what they were used to with the carb’d 4.2 I think. I had 4” lift springs lying around so I had some downtime for about 3 weeks getting the SYE and a new shaft installed before I got it back on the road. A few miles around town and then decided to drive it to work….51 miles each way. The drive down was fine but starting it up in the morning for the trip home it was knocking. I didn’t sound like a hard knock, more like piston slap, but since I’d never heard it in person I got pissed and hammered it on the way home. I figured it would blow or I’d make it home….made it home fine. The weather turned south bad so I left it in the garage for almost 2 months….no heat in the garage and I was still pissed.

Finally pulled it back down and I had marks inside of #1 on the cylinder wall on the passenger side only….scoring on the piston too of course. I pulled that rod and piston and it seemed a little too stiff at the wrist pin so I thought maybe that was the cause. The machine shop worked on it for me and I honed the cylinder to clean it up. It’s not perfect but it still holds 145 psi on a compression test afterward. I got it all back together but it was still knocking. Still pissed I let it sit for a few more weeks before pulling it back apart. The pistons were ordered through Jegs and shipped straight from UEM as a six pack, all stamped .030”. Imagine my surprise when the #1 piston checked out at 3.872” instead of 3.902”….and it’s stamped .030. My fault for not checking them all but I didn’t…..I only checked the first one. Same with my machine shop. I emailed UEM and was told I’d have to go back through Jegs but it’s already been 7 months since I ordered them so I said screw it and ordered a single piston through a local supply house.

Got the new slug installed and did a short break-in again for about 10 minutes…..all sounded good. I started driving around town some more and probably got 100 miles on it when I started to get another knocking…different this time though, more like top end. Sure enough I dropped a lifter. I pulled it all back down to get the lifter out….great design btw….and found it had shattered inside. To be safe I pulled the pan and didn’t find anything. I ordered a new set but only installed the one since the cam only has around 250 miles on it and shouldn’t be a problem. I double checked the preload and it’s still about .058” using the flat edge and marks on the pushrod system. It’s the same as it was on the original build…..kinda high but still under .060” so I left it alone. The initial break-in used all my comp cams break-in oil so I used their zddp additive for this 20 minute break-in. Everything seemed good and back on the road I went.

Since I had pulled the rod twice now and checking wear on the rod bearing I decided to put in a new bearing in case I had damaged it. It plastigauged at .0025 same as the rest. Maybe 40 miles later I have knocking again….the same as the very first time. I didn’t waste time though so I tore it back down. I haven’t pulled the pan yet but #1 piston is rocking in the bore while the others are not. I have a bore gauge and it’s reading .0005” larger than the others but pretty much consistent all the way around and at different heights. I forgot to actually check the bore gauge with my calipers but the top of the cylinder is 3.902” without any contamination at the top. It’s hard to tell because I didn’t really run it hard afterward but there doesn’t seem to be a lot of wear in the cylinder wall like the first time.

I’m starting to think crank or rod issues. The knock follows the timing light of the #1 firing but doesn’t go away if I pull the plug wire. The tone changes slightly when I do that, but not at all if I pull any other wires. I’m not really sure which direction to go at this point until I pull that piston tomorrow.

Thoughts?

I pulled the new piston and it has scuffing on the cam side just like the first one....and it's snug on the rod and not as loose as it should be. I'm beginning to think I'm having oiling issues but I don't know. The other 5 are nice and snug in the bore but #1 definitely rocks. I checked the piston before installation and it was the right size....now it checks at 3.885 in the same spot. The scuffing makes it hard to measure but I'm betting the rest of it was there and has been worn away.

Did your rod bearings have oiling holes in them, and were they facing the cam? I could be wrong, but I thought I read something about rod bearing oiling holes were also used to splash oil up to the pistons, as well as to the cam. I also remember reading that some rod bearings for Jeeps came without the oiling holes.

The first set did but this last one did not. I read some of that same info and was seriously considering not using them or trying to dremel a hole. There were some opinions that it wasn't needed anymore so I accepted that and installed them. I'm also beginning to think that my first piston wasn't the wrong size after all. Other than splash, I can't see how the wrist pin is getting oiled. Is there a path for oiling the wrist pin through the rod via that hole?

That hole in the rod is for splash on the camshaft and lifters.
Although not totally necessary, it doesn't hurt to put a little notch in bearing to get more oil to the cam.

Is there any witness marks on the side of the piston that would indicate pins were pressed in through a fixture?
The pin fit should always be checked by the shop that hung the Pistons. Improper assembly can distort the housing bore.

The piston pin bosses are oiled through a hole on the backside of the oil ring. As the oil scraper ring is squeegeeing the oil from the cylinder wall it pressure feeds oil to a connecting hole in the wrist pin housing bore.

Russ, there is a very small indentation mark below and left of the wrist pin where it could have been in a fixture. I haven't seen exactly what kind of fixture the shop uses though so I can't say definitely. The original piston has some different marks that could also be same thing but they're not in the same location. I've seen some pictures of other rods where the ends show heat discoloration from the piston install. Mine don't show any such thing. There was a 2 day delay before I could install the piston when I had it done last, and it was still nice and loose.....although the garage temps were still in the low 30's.

I'm starting to lean towards lack of oiling and not install issues. I noticed the oil rings had moved slightly when I popped the piston out so perhaps they had moved during install causing a lack of oiling...? I'll get a new set of bearings and make sure they have the oiling hole this time along with a new set of rings.

Just as a comment for consideration/observation.

You have mentioned the piston having been scored on the cam/passenger side. That also happens to be the thrust side of the piston.

It's looking like the piston bore was distorted by indication of the marks on the side of the piston.

I'm going to order a new piston but there's not many options for machine shops around here without driving an hour away. I'll see if I can find a different shop in the meantime.

Just make sure none of the Pistons are tight on the pin.
Otherwise you'll carefully have to have them press the Pistons off, then pin fit the piston and reinstall by heat method.
.0005 to .001 is the clearance you should ideally have. No more than .001

Will do Russ, thanks.

Just dropped it off with a new machine shop. He didn't really like the way the pin felt with the new piston so he's gonna pin fit it but he also thought I might be having a sticky fuel injector issue. He checked out both of my old pistons and the rod and thought I might have too much fuel. I told him about the cam and larger injectors, as well as my wideband readings, so he said to check the injector.

Raggedyman wrote: I didn’t sound like a hard knock, more like piston slap, but since I’d never heard it in person I got pissed and hammered it on the way home.

I hope you didn't do that on a cold engine with forged pistons.

Imagine my surprise when the #1 piston checked out at 3.872” instead of 3.902”….and it’s stamped .030.

I forgot to actually check the bore gauge with my calipers but the top of the cylinder is 3.902” without any contamination at the top.

So, what was your piston to wall clearance set at?

Sure enough I dropped a lifter. I pulled it all back down to get the lifter out….great design btw….and found it had shattered inside. To be safe I pulled the pan and didn’t find anything. I ordered a new set but only installed the one since the cam only has around 250 miles on it and shouldn’t be a problem.

What brand lifters? I have had comp cam lifters com assembled wrong in the past.


Also, you mention possible problems with the injectors. Where did you get them? Were they rebuilt?

Wall clearance is .0025 and I've verified that with a bore gauge.

The lifters were comp cams and came with the cam kit. I used their lube for assembly and their break-in oil.

The injectors were 24lb rebuilt bosch units I got off ebay.....fuel injection connection. They seemed to have the best feedback and seemed the most knowledgeable in their write-up.

I just got home with the new piston and after talking with the machine shop.....old school guy, very knowledgeable.....he thought about heat issues and possibly having an air bubble trapped around #1. My block does rise a little higher in the front so I think I'll pull the thermostat on the initial build just to make sure I eliminate any air trapped in that area.

I'm still concerned about the rod bearings though. The new ones don't have the cut out for the oil hole and the three shops I called all order the same parts. Should I try adding the cut out or go with the current industry thought about it no longer being necessary....?

It wouldn't hurt to add the notch. Don't use anything abrasive though as it the medium can become embedded in the bearing material. I used this bit for mine: https://www.dremel.com/en-us/Accessorie ... x?pid=9901