belvedere

I was talking with the owner of the local salvage yard about the broken piston skirt (kilt) on my 98 Cherokee. He said every one he's ever seen has been either #5 or #6. That was before I told him that mine was #6. Any thoughts?

DFlintstone

Dang, you beat me to the "kilt" reference. (I'm a Scott). We have some engines here for you in N-CA. Please send specs and coordinance for the launch...to SD. Trevoushet?

belvedere

Delivery to SD...that sounds great! Now that the nice weather is finally here, I have to decide what to do with the darn thing.

Radi

From what I've read, #1 and #2 are the other common failures, and 96+ appears more likely to have a problem. I've never had one fail in person or had to fix one that did so no direct experience or theories. The skirts seem too short for an inline motor though. (and this is the one and only time I will complain about short skirts, lol)
Was yours noisy before it failed? Any idea what precipitated the failure?

DFlintstone

O...K.. then. Where are we then. (don't let the "post count" fool you, I'd ask Radi). Do you think, or know that your skirts have been compromised. Junk in the pan? You have pulled the head? Pics?

#6 gets less cooling than #5. How meny miles on it? Doesent sound good.


Gut says, if it is really a broken skirt in #6 ...

belvedere

Reader's Digest version: I bought the Heep last summer to use as a winter beater, knowing it needed various repairs, but nothing major. In about October, I commented to the wife that I thought I had it ready for winter (big mistake!). Though it had 181k miles, the engine sounded great...not even the usual lifter noise was present. One day, when pulling onto the highway, there was suddenly a loud rattling/knocking sound.

belvedere

Whoops, posted too soon. Anyway, now I gotta decide how I'm gonna fix it (replace one piston, junkyard motor, full rebuild, etc).

red4dr

im worried i have the same problem in my #1, have to drop pan to confirm. subscribing to see how you work through it. im torn between full rebuild vs single cylinder.

Cheebs

Had the same problem. In the process of replacing my #5 piston

Cheebs

Pulled the head but left the intakes on and just slid the piston out. Was surprisingly easy

5-90

I think a tebuchet might not have the range - big-**** catapult, perhaps? Orbital Gauss cannon?

I've torn down a few RENIX engines, and I haven't seen any "busted kilts." (I'm a Scot as well...)

Considering the number and array of engines I've dug in to, a busted skirt is uncommon enough that it should be investigated as to cause.

The causes of broken skirts, I've noted:
- Improper slug-to-bore fit (too loose.) This allows the piston to rock in the bore ("tilted kilt"?) and stress the metal. Oops!
- Broken ring. Can get lodged in the bore wall and gouge the piston, creating a weak area. Then, the piston skirt probably snaps in short order.
- Defective piston (improperly cast.) A gas pocket ("void") in the casting, not noticed from outside after grinding. That gouge may have been a weak point, but this is worse!
- Defective piston (improperly made.) This would be something like the piston ground too small, barrel grind not being right (excessive rocking,) eccentric grind not being right (causing "rock" or "slap.")
- Engine defect (bore too large.) Allows excess rocking or slapping until something fails.
- Engine defect (bore too short.) For some reason, you don't have the full length of the engine bore that you should have - this results in the piston rocking heavily at the bottom. Either the block was miscast, or part of the lower bore (below the cooling jacket) has fractured. Results as above, just different location.

In 35 years or so, I've seen about five busted skirts (no two on the same brand of engine...) 1 was actually a piston problem (found a void on initial post-mortem of the engine. When dissected, found five or six more - same piston); three due to machinist (recently-machined and -overhauled block, bore found to be some 0.040" over stated size! All three the same machinist who had just set up shop, and who soon was doing something else - restaurant, I think,) and one due to a section of the lower bore snapping out - I think that's all of them.

And I'd done a BUNCH of AMC I6 & V8!

belvedere

Finally got around to pulling the head. Cyl 6 doesn't seem to have any damage. Would I be an idiot to hone the cyls and put in a new set of std pistons? Since it's just a winter beater, it's probably not going to see a lot of miles. Also, does anyone know if the aftermarket pistons have an improved design that is less prone to cracking? I appreciate any advice. I've rebuilt engines before, but never done a half-*** rebuild like this.

DFlintstone

Short of having the correct micrometers and such, A guy can get an idea of cylinder taper by fitting a ring in the cylinder up high and checking the ring end gap with a feeler gauge, then moving it down and checking again.

I'd try google for specs there...IIRC the # are similar to rod clearance. (1-2 better than 3-4). Taper over 5 or 6 is not good.

If it looks cool and hones OK, I myself might give it a go. I might go with flat stones on that one. (as opposed to the "bottle brush"). Just thinking if there is an issue, the flat stone might tell you, where the ***** might not.

cruiser54

Go for it. I took a Renix 4.0 that had a cracked block, used the rotating mass of it in another standard HO block which i miced and honed using new rings , put an HO head on it and it runs like a mad dog. Quiet as can be. Used the cam and lifters from another Renix engine but kept the lifters matched to the cam lobes.

torque062

I've been in the same boat before. Mic'd and honed and a set of rings with a new cam (now realizing it is too much cam for the relatively low compression) and it runs absolutely great.

I went cheap for a reason cause a stroker build is in the future and didn't wanna spend much getting the block machined when I will be doing it again for the stroker.