Scarred piston on cyl 3, Z24 engine

[Macklen]

Hi.

I'm rebuilding a Z24 engine that I got to replace the one on my truck and upon disassembly I noticed fairly noticeable scratch marks on the side of the piston.

The cylinder does not show quite as bad marks, but there are a couple of scratches on the corresponding spot and they don't pass the fingernail test: it feels.

I was hoping to overhaul this engine -my first- and replace the original on my truck, which does need to be rebuilt as well, but has not been removed. I got this one dirt cheap and decided to start with it, but it seems both will need to be overhauled.

I plan to install the better one and sell the other, if I get both working.

Can I get away with honing the scratched cylinder and installing new rings, or does it need to go to a machine shop?

If the latter, can only the affected cylinder be bored and oversize piston installed, or do I have to do all four?

I'll post pictures ASAP.

[Macklen]

https://www.instagram.com/p/CKXa6_SjcAs/?igshid=1mfj633jhazi5

[datzenmike]

Well that's the thrust side of the piston. Can't see the bore at all. Can you get closer? And a picture showing a bit of the top of piston 3 from the intake side??

 

 

Well if you're going to bore one it's cheaper to do all four. Course you'll need all new over size pistons and rings. The good news is you will have a new engine with hundreds of thousands of miles ahead of it. You could hone it and slap that piston back in with new rings but not worth it to put in your truck unless just something to keep it on the road while actually re-building the one in the truck.  

 

 Yes you could run one over size piston.

[Macklen]

Thanks for the quick reply. I'll post the pics you requested early tomorrow, for sure.

I live in a small town in the mountains in Mexico, a trip to a nearby city to get to a machine shop is not out of the question, but certainly not the most practical, so I'm considering options.

Pics tomorrow, promise.

[datzenmike]

Can you also include a picture of the upper rod bearing shell?

 

Probably hone and ring that one, slip it in to your truck and properly rebuild the other.

 

Make sure that piston rocks easily from side to side on the rod and the oil hole is unobstructed on the bottom.

 

[Charlie69]

5 hours ago, Macklen said:

Hi.

I'm rebuilding a Z24 engine that I got to replace the one on my truck and upon disassembly I noticed fairly noticeable scratch marks on the side of the piston.

The cylinder does not show quite as bad marks, but there are a couple of scratches on the corresponding spot and they don't pass the fingernail test: it feels.

I was hoping to overhaul this engine -my first- and replace the original on my truck, which does need to be rebuilt as well, but has not been removed. I got this one dirt cheap and decided to start with it, but it seems both will need to be overhauled.

I plan to install the better one and sell the other, if I get both working.

Can I get away with honing the scratched cylinder and installing new rings, or does it need to go to a machine shop?

If the latter, can only the affected cylinder be bored and oversize piston installed, or do I have to do all four?

I'll post pictures ASAP.

It will be extra work but I would put this one together with a good used piston new rings and gasket set.  Pull the motor from your truck put this repaired engine in and do a full rebuild to the original engine. once that rebuild is done then sell the other engine.

 

If I was doing this and did not care about a numbers matching engine I would rebuild the one with scratches and put it in and sell the original engine as a used engine.

[Macklen]

Here are more detailed pics of the piston. I cleaned it as best as I could, I can tell it's been heating more than the others from the crust in it.

Also, it doesn't quite rock freely, it feels as if it kinda catches, like the pin is out of round a bit.

The oil hole its unobstructed, the markings on the top are gone.

Pics coming...

 

[Macklen]

https://www.instagram.com/p/CKZc8T5D6xR/?igshid=qgpb2s49ee39

[Macklen]

37 minutes ago, Macklen said:

Here are more detailed pics of the piston. I cleaned it as best as I could, I can tell it's been heating more than the others from the crust in it.

Also, it doesn't quite rock freely, it feels as if it kinda catches, like the pin is out of round a bit.

The oil hole its unobstructed, the markings on the top are gone.

Pics coming...

 

On second thought all pistons feel like that, so maybe it's normal? Like the pin rotation stops a bit before the "ear" thingies on the piston bottom hit the rod.

[datzenmike]

Pick one and rebuild it.

 

The one that's apart you could hone the bad cylinder and smooth the piston with light sand paper. The piston pin is pressed into the rod but the piston should rock side to side and slide along the pin length slightly and freely. WD-40 the piston pin and rock the piston side to side and slide the piston along the rod length to loosen it. Lack of proper oil changes has probably led to this*. You could do this with all the pistons if you like but you will need to then hone and put new rings on them also. THIS IS NOT INTENDED AS A REBUILD.... JUST TO GET IT GOING LONG ENOUGH TO TAKE THE OTHER OUT AND REBUILD PROPERLY. The other pistons may have scored their walls also.

 

OR

 

Take this one and over bore 1mm and replace with 1mm over bore pistons and rings, new main and rod bearings and have basically a new engine. The head can be rebuilt now or later but I would run it as is first and see how well it runs. Maybe rebuilding it isn't necessary. 'Rebuilding' this engine would save taking the old one out, putting this 'repaired' one in while rebuilding the other and then pulling it out to put the rebuilt one back in.

 

 

* can you post a picture of the head? Is it all brown with deposits?

[Macklen]

Sure, pic of head coming.

 

That would be best, since I do need the truck running to get to a machine shop, and I have no problem with doing the whole thing twice.

I can half-ass this one as a temporary solution, do the other properly and then go back on the first one.

Now, does it make sense to throw in new rod bearings now, or wait for the second time around?

Also, when new rod bearings do go in, does the crank need to be messed with? Not for crank bearings, but rod ones.

[datzenmike]

Well pick one. Myself I'd rebuild the scored piston one as it's already out and apart. Cylinders will be bored to new and new pistons and ring anyway. But your call.

 

Change rod and main bearing on the engine you do the full rebuild on.  Good enough as is on the scored piston one.

[Macklen]

https://www.instagram.com/p/CKZvaNDDqwD/?igshid=a92zhn9nwm8w

[Macklen]

I really appreaciate the feedback. I'll think it over and post more pics as I go.

[datzenmike]

Cylinder 3 and 4 was running much better than 1 and 2. You can see the exhaust valve is tan because it got hot enough to burn off deposits. Front cylinders may be burning oil.... another good reason to rebuild this one than put it into your truck and have to put a quart of oil in every week. Valve train is black with sludge and has that brown color from not changing the oil often enough.

[Macklen]

It'll definitely get done, I'm actually excited to do both engines right. It's really a matter of getting a couple rides into town on someone else's truck.

I don't think parts will be an issue, the 720 is pretty much everywhere around here.

[datzenmike]

Is the engine in the truck working??? If it's working use it to take stuff into town.

[Macklen]

Well, it got home and it was parked running. Sudden heavy stuttering and blue smoke out the pipe. I figured a ring blew and I parked it before further damage. I have not inspected it, I took it as a pretext to do what I wanted to do for a while, which is a full rebuild.

It had been losing power the last year.

Been riding a motorcycle since.

Edited January 24, 2021 by Macklen

[datzenmike]

So your truck is not really running then? So you can't use it to take the block in for machining. No matter what you decide, that engine must come out. Take it out and take it apart. Now decide which one you want to rebuild. 

 

A/ Must have a running truck to get back and forth with engine to rebuild it

 

a/ get a set of cheap cast iron standard size rings for one piston. Lightly sand piston smooth. Loosen piston on pin so it moves freely. Hone the bore. Assemble and swap engine into truck and get running. Take truck engine apart and rebuild it using running truck

 

 

 

 

B/ Don't need a running truck have friend that can drive me.

 

b/ take engine out of truck and apart. Decide which one is in best shape and rebuild it.

 

 

[Macklen]

I think A is the way to go.

The one in the truck will be properly done, it's way cleaner too, unless there's damage I haven't seen yet.

I'll continue to update so other folks can use what I learn as well.

This forum rocks.

[Charlie69]

If you are going to bore and install new pistons in your rebuilt engine then use one of the pistons from the engine in your truck for a temporary fix for the engine with the scored piston.

[Stoffregen Motorsports]

On 1/23/2021 at 1:17 PM, Macklen said:

https://www.instagram.com/p/CKZvaNDDqwD/?igshid=a92zhn9nwm8w

There appears to be a lot of pitting in the bore, probably from sitting a long time with water in the cylinders. If the engine ever had a leaky head gasket, that could have been the cause.

 

The scratches on the piston are from overheating. Metal from the cylinder wall actually starts to rub off and adhere to the piston. It is possible to lightly sand that off the piston, but with the condition of the cylinder bore, I wouldn't bother trying to put this engine back together without an overbore and new pistons.