Need some input

[tigmeister]

Just got my new stroker together, broke in the cam for 20 min, changed oil and filter. All sounded good, no leaks, AWESOME!

Went for a test drive, about 3 miles, good oil pressure and temp. Had the windows down 'cause it was a nice day. When the wind noise got to a low enough level, realized engine had a pretty hefty knock coming from it. CRAP!

This is a '90 block with a '93 head, new Scat crank, 4.0 rods, ICON 944 pistons, plasti-gaged every main and rod bearing before final assembly, basically checked every bearing tolerance. All was within spec, including thrust clearance.

To make a long story short, toasted the backside of the thrust bearing and at least 2 cam bearings. Haven't got it completely torn down yet, but I'll post a link to some pictures.

Seems that the only thing that can cause this kind of failure is excessive forward pressure on the crank from the torque converter/transmission, which in turn probably took out the cam bearings.

The torque converter was never removed from the transmission, so I got checking the pilot holes (larger outer, and smaller inner) in the new crankshaft compared to the old crankshaft.

On the old one the outer is .835" deep and on the new crank it's .597" deep, which means the new one is .246" more shallow than the old crank. Almost 1/4".

Does anyone know if all the new Scat 4.2 cranks are this shallow compared to a 1993 4.0 crank? If so could this cause this kind of problem?

Thanks.

https://www.flickr.com/photos/77467221@ ... 224529891/

[Retlaw01XJ]

My '01 XJ went together without issue and has a Scat crank....(tho I did need Mopar bearings to get the clearance tighter.)
I'm more familiar with 60's GM transmissions than I am Jeep.....On GM's, after the eng/trans is bolted together, the converter needs to be slid away from the trans a bit (less than 1/4") to mate with the flexplate. If the converter hits the crank/flexplate before the eng/trans is mated, it means the converter slid out a bit and disengaged from the pump tangs.... and could lead to the failure you described.

If you had flexplate/converter contact when the eng/trans was bolted together..., you had a problem at that point.

You mentioned the crank recess issue... that could be a problem if the converter nose is greater than 0.597". I don't know, never checked mine.

Other possibilities include the harmonic balancer not having the proper spacer or snout length thereby putting a preload on the crank, and pulling it forward.

[tigmeister]

I think we've figured out what went wrong, and I thought I'd post for others reference.

First, the snout of the torque converter didn't quite go in the hole of the crank. There's only a small surface, maybe about 1/8" wide on the larger portion of the snout that only goes in about 3/16" to 1/4" into the crank. The front side of the snout has a square flat surface and it's possible to get it slammed against the face of the crank flange and not inside the crank and still be close enough to get the torque converter bolts in. There's tell tale signs on the face of the crank that this is what happened, just a small ring of scuffing about 1/8" wide on the face. This put excessive force forward on the crank which wore out the thrust bearing and maybe then pulled the cam forward also.

The second problem after it was tore down was 3 collapsed pistons. Melted the sides and scuffed the cylinders. This is obviously too much heat in the engine. When I was disconnecting the wiring harness, half of the temp sending unit was broken off and dangling in the plug. We always had a temp reading while we were running the engine, but if it was partially broken, it probably wasn't accurate and the temps were a lot higher than indicated. That could have also contributed to the cam bearing failure if the oil film broke down due to too much heat.

Talking to the machine shop that's built over 150 of these strokers, they only break them in at 1800 rpm for about 10 minutes while watching gauges closely, then shut it down, check everything, then take it out for a drive.

So the blocks back at the machine shop getting bored to .060" over, got new piston, rings, main and rod bearings coming. The rod bearings looked OK, but they're cheap and I'm not taking the chance.

By the way, all the journals on the cam and crank still look perfect. Cam only has .003 max runout.

Live and learn I guess. Only gonna cost me another $800. The machine shop felt so bad, they cut me a great price on the replacement parts. They already made a decent profit off of me the first time.

So, lessons learned:

1 - Not too high rpm during break in.
2 - Make sure all sensors are in good condition and watch temp and oil pressure closely.
3 - If torque converter doesn't spin freely and have some movement front to back after block is mated to bellhousing, it's not in the right position.

[wjtom]

I posted about this before.Had the same problem.Looks right when putting it together but its not.Luckily for me no damage.Test fit the torque convertor!!!

[tigmeister]

Will do. Hopefully I'll get the block back from the machine shop this week. Punching it out .060" over. Sucks though, Had all of 30 min on this engine and toasted it. At least I won't have to break the cam in again.

[SilverXJ]

Did you keep the lifters in order?

[tigmeister]

Yes, everything bagged and tagged.