#6 piston has a cracked skirt. Not broken off yet but....
So the plan is to just replace all 6 along and call it good. Due to rust issues, a full rebuild isn't a worthwhile expense.

Problem is, this is one of the engines built on Chrysler's worn-out boring machine and every piston is a slightly different size, as marked on the side of the block. I haven't rebuild one of these before with varying piston sizes.

I can get the C-D-E, etc size pistons from the dealer, but I'd prefer to just get a set of "standard" size Sealed Power's and drop them in for economic reasons, provided I don't need machine work.

The differences between piston sizes is so minor I'm assuming it won't be a real issue, but would like other opinions.
Anyone done this? Any problems just using a standard size?

 

The difference in piston size is really very small, so the rings will all be the same. But I would check the ring gap in the cylinder just as a precaution. It's not just the 4.0 that has slightly different piston sizes, most of the Chrysler engines are like that. I was told why at a Chrysler training center, but that was over 20 years ago and I forgot why they're like that.

 

A Beef eater once said "just rip off the horns and wipe it's ****" What u doen Chuck? Cracked. reallly? Ridge ream & hone, rings and rod inserts, Lap the valves with fuel line on a reversible drill. New valve stem seals of course, and an oil pump it you are felling fat. (I might just check it).

Just yacking. I don't build pianos. Guess you know cylinder taper is an issue you can check with a feeler gauge in the ring gap high, and low....

 

Bustedback, I hadn't gotten it that you were an engine guy. Likely you and Radi have more experience than me, but wouldn't cylinder taper be a major concern if you are going with new pistons?

Radi, just knurl the skirts with a glass cutter.....(or maybe not)

 

Is it just me, or has anyone else realized it seems to be #6 that cracks the skirt?

 

Sure seems that way. More heat back there, maybe?

 

Could be.

 

It's a demonic thing. You know...666.

Agreed, I've seen lots of posts mentioning #6 as taking a dive.

 

Check that area of the block after a good run with an IR thermometer. Mine runs considerably hotter back there than elsewhere. I'll put money on the #6 skirt compressing a bit in the heat, then out-slapping the other 5 when it's cold until it finally gives way.

I know the Kenosha plant was upgraded in late '96, the original block line just wore out. I'd guess that led to varying bore sizes (but don't really know for certain). I'm curious now whether post-'96 engines also have different size pistons?

Yeah, cracked. Plain as day. The tap tap started getting louder earlier this summer, to the point I figured I'd better look.
I do have a valve guide knurling tool around here somewhere, maybe I'll use that and a little JB weld
I have the egg-beater style valve lapper, a ridge reamer and a bore gauge, just don't want to spend a lot on this XJ because it's clock is ticking anyway due to undercarriage rust. I can get a set of 6 Sealed Power pistons for about $20/ea. Buying individual sizes from the dealer is probably going to be 5X that. Not sure it's worth it over a 0.001" difference.

 

Chilton's Method:
1. Remove rings from piston.
2. INsert a .001 feeler gauge into the bore.
3. Insert piston top first, into the bore alongside the feeler gauge. Withe the entire piston inserted the piston should not bind against the gauge.
4. Repeat with a .002 feeler gauge and the piston should bind.
5. If the piston binds using the .001 feeler gauge or does not bind using the .002 feeler gauge the piston is not the correct size for the bore. Replace the piston and/or hone the bore to gain proper clearance.

Basically you have from .001-.0019 to play with. If your running a junker I would only replace the number six and call it good. Why bother with the rest of it. Otherwise spend the money and take the mill out for the next one.

 

Agreed, farthest from the water pump. Also Big bird has a point....A guy might just do "rings and rods" without the crack. Maybe just one. Guess you will see what you have at some point.

I found a section of fuel line on the valve stem to a reversible variable speed drill to be pretty nifty for lapping valves.

 

Thx for the info.
After doing a little more digging I find that "C" is the standard size, A-B are undersize and D+ are oversize. I have nothing smaller than "C", so that basically eliminates my original concern that standard size would be too large for some bores.

I tossed around the idea of just replacing #6, but for an extra $100 I can do them all. Head and pan are off either way, it isn't much more work.
Dropping $800+ on a full rebuild and machine work that I'd have to sub out is another story. Sort of a shame because the XJ is gorgeous on the outside, no dings, no visible rust, paint still shines like it did on the showroom floor. Problem is it's on it's second set of floorpans and now the frame rails near the spring mounts and the front spring perches are going. I figure another winter, maybe two and it'll probably be done.

 

"Re-used the rings", Really?

 

Oh heck Don, my wife will tell you I'm cheap. She isn't kidding.
You lap them on a sheet of emery paper to clean them up and slap 'em back in. As long as they still have some spring left and the installed gaps are OK-ish, they'll work fine for a while. I'll probably be excommunicated from the he-man engine builder's club but I drew the line at not replacing parts I could reasonably expect to work another 10K miles or so. Ask me in about a month if I was stupid.
I figured if I broke one then they were too brittle and I'd get a set. None broke.
Time will tell if I've created an oil pump and been penny wise yet pound foolish. I've put about 300 miles on, vacuum is good, no sign of blowby, engine runs smoothly. (and no more clack clack clack....)
I wouldn't do it on an engine I intended to last several years or cared if it used a bit of oil, but this isn't one of those.
I wanted to avoid a rod through the block or exploded piston and keep it running another 10-12K. Period.

Truth is, I should've done the oil pump, main bearings, timing set, valves, seats, seals... it's all worn...but you have to draw the line somewhere lest 'as long as I'm in there...." turns what was intended to be limited repairs into a project larger than what I intended to accomplish.

 

Hottest part of the engine is the back - which is why Chevvy went to a "reverse-flow" cooling system for the next-gen SBC (Gen IV?)

Flowing coolant to the back of the block and bringing it forward results in less of a temperature differential between the front and rear of the block than flowing it front-to-back, since the front also has airflow over it.