Mystery Z24 Engine Rebuild - Too Much vs Not Enough?

[DGTT]

Howdy!

 

I've been lurking on this forum for a while now, but figured I should finally join and ask for some advice.

 

I have an '84 720 4x4 that, for a brief period of time, had more moving parts in cylinder #1 than the good folks at Nissan originally intended for there to be. That engine still runs well enough to get me to work (~30 miles each way over a mountain pass), but it now burns a quart of oil every 100 miles. I borrowed a friend's bore scope and cylinder #1 has so many scores and gouges in the cylinder wall that it isn't worth it for me to try to save. 

 

Not to worry - I have a spare engine! The problem is that I know absolutely nothing about it. It came in the bed of an '85 720 4x4 that I bought as a parts truck. I have no idea how many miles are on it, if the head gasket is good, if it will even run, etc. I'm guessing it's an '85 engine because it has a carburetor on it and a different distributor wiring than my '84, but I really don't know what I'm talking about here. The seller assured me that it was 'good to go', but considering that it was just sitting in a corner of his shop collecting lots of dust and the engine in the parts truck had a blown head gasket, I'm not sure I believe him.

 

I'd like to swap this 'good' engine into my '84, but if I'm going to do that, it seems prudent to do some basic maintenance with the engine on a stand BEFORE I put it in my truck and try to run it. Based on my experience with my '84, I'm going to replace the front main seal, water pump gasket, and valve cover gasket because those seem to be common places for leaks, plus new spark plugs, emissions system delete, and the weber carb I already have. Beyond that, I'm not sure what is a 'good idea' to replace and what is overkill. I don't really want to rebuild the whole engine, but is a new head gasket a good idea? Then I need new head bolts/studs right?  I don't want to go too far down the rabbit hole, but I kinda need my truck to be reliable and leak-free, so it is easy to justify some work now so I don't have to fix issues down the line. Any input as to what is worth doing and what isn't would be greatly appreciated!

 

TLDR: I have a carbureted Z24 engine on a stand in my garage that I know absolutely nothing about but I was told it is 'good to go'. Before I put it in my truck, I'm looking for advice on what is worth doing to the engine before I swap it in to make sure it is reliable and leak-free.

[NC85ST]

With it out of the truck and on a stand, I personally, would pull the head. A new head gasket is not a bad idea. It also lets you have a good look at the pistons and bores. 

[datzenmike]

I wouldn't change the water pump gasket if not leaking. I would change the front timing cover crankshaft seal and the rear crankshaft seal though. Specially the rear as to change it later the engine would have to come back out to do.

 

I would do the oil pan gasket because it's probably covered with oil anyway and hard to tell if it might be leaking or not. Just tightening the bolts leads to over tightening them and wrecking the seal.

 

While the pan is off I would consider replacing the rod and main bearings with new ones. The main bearings can be done one at a time without crankshaft removal by rotating the top shell around 180 and lifting it out, installation is the reverse. If one of the bearings is bad or the crankshaft journal scored now's the time to find out. 

 

Head gaskets tend to blow every 100k on the Z24 unless regularly re-torqued. This is an unknown and may even be why it's out and in the back. The owner may just be telling you what he was told. Before you start, know that the timing chain tensioner will fall out when the cam sprocket is removed so find out how to secure it. If it falls out the only way to get it back in is removing the timing chain cover and that means the crankshaft pulley, distributor, oil pump and spindle, water pump and timing cover have to come off. It's a few hours of extra work and some gaskets that are not needed to right this.

 

Don't lay the head with the valves facing down. Set it on 2x4 blocks.

 

If the exhaust manifold is on, take it off to lighten it. The head can be lifted off with the intake on. This saves replacing that gasket but more, it avoids stripping one of the mounting bolts. Now's the time to make a block off plate to cover the EGR valve hole. Also the time to remove the exhaust tube from the exhaust manifold from around the back of the engine and seal the hole.

 

While head is off replace the valve seals.

 

Clean the mating surfaces and get or borrow a very good straight edge and lay across the head and try to fit a 0.004" feeler gauge under it. If it does, have the head machined.

 

Head bolts on the Z24 can be reused if not damaged.

 

 

 

 

[Madkaw]

Mike has great advice . As far as main bearings - if they look like normal wear when you pop the cap - move on . 
Head gasket for sure since that will get you a good look at the bores. 
We know these engines - like a lot of Datsun/Nissan engines will run forever - even when not properly maintained . 
Leakdown check before doing anything will tell you a lot also 

[bottomwatcher]

If you have an extra trans laying around you could bolt it up then you have a place to mount the starter and you could run it in a test stand. I have a cut off front half of a trans just for this purpose. It also allows you to run a compression test. The only thing I would add would be if in the budget a fresh water pump. For some reason I have never had a water pump last very long on an engine that has set for a long time.

[datzenmike]

Water pumps are expensive because the clutch fan is integral with the pump. You can't replace just one.

 

Just beware of the state of today's replacement 'OEM' parts. Most are exactly what you pay for and are absolute crap. If you have an original I'd leave it. If replacing get an original from Japan. If it has a stamped impeller don't use it.

 

Always use distilled water 50/50 with a good quality antifreeze.

[ElliotV]

On 12/23/2024 at 2:09 PM, datzenmike said:

I wouldn't change the water pump gasket if not leaking. I would change the front timing cover crankshaft seal and the rear crankshaft seal though. Specially the rear as to change it later the engine would have to come back out to do.

 

I would do the oil pan gasket because it's probably covered with oil anyway and hard to tell if it might be leaking or not. Just tightening the bolts leads to over tightening them and wrecking the seal.

 

While the pan is off I would consider replacing the rod and main bearings with new ones. The main bearings can be done one at a time without crankshaft removal by rotating the top shell around 180 and lifting it out, installation is the reverse. If one of the bearings is bad or the crankshaft journal scored now's the time to find out. 

 

Head gaskets tend to blow every 100k on the Z24 unless regularly re-torqued. This is an unknown and may even be why it's out and in the back. The owner may just be telling you what he was told. Before you start, know that the timing chain tensioner will fall out when the cam sprocket is removed so find out how to secure it. If it falls out the only way to get it back in is removing the timing chain cover and that means the crankshaft pulley, distributor, oil pump and spindle, water pump and timing cover have to come off. It's a few hours of extra work and some gaskets that are not needed to right this.

 

Don't lay the head with the valves facing down. Set it on 2x4 blocks.

 

If the exhaust manifold is on, take it off to lighten it. The head can be lifted off with the intake on. This saves replacing that gasket but more, it avoids stripping one of the mounting bolts. Now's the time to make a block off plate to cover the EGR valve hole. Also the time to remove the exhaust tube from the exhaust manifold from around the back of the engine and seal the hole.

 

While head is off replace the valve seals.

 

Clean the mating surfaces and get or borrow a very good straight edge and lay across the head and try to fit a 0.004" feeler gauge under it. If it does, have the head machined.

 

Head bolts on the Z24 can be reused if not damaged.

 

 

 

 

mike on the topic of head bolts being re usable that may be true as I know many have done it but they are in fact torque to yield as I discovered when my new ones came in the mail which I was quite surpised by but nissan still recommends retorqing them and even my 84 service manual mentions this

[ElliotV]

I would at least do the head if you could becuase that would give you a good idea of how that engine looks and even the valve cover can give you a idea of how the timing chain looks which can tell you if its either been replaced or is a low miles engine. For examply when I had mine off I could tell the timing chain and gear set was fairly fresh considering my odometer said 165k but the chain, cam gear, and what I could see of the guides looked great.  I just did the head gasket on my truck and you have to make your own tool which can be done with a belt sander and a 1x2 and some effort and trial and error. I ordered two wedges for the timing chain and made a hybrid of the two since one is too short to reach the tensioner and the other is too thick for the z series engines.   Too thick and too short. Also dropping the oil pan and the gasket is a good idea becuase it is hard to get to that when its in the engine.

[datzenmike]

That is very interesting, have never heard this. I thought the KA the first with TTY. I checked and the Z24 has a different bolt than the previous Z22 and the '84-'86 Z20 option. I assumed this was because the Z24 block is taller in the head bolt area. Also the Z24 is the only Z series engine with the problem of blowing the head gaskets and requiring bolt re-torque. If these are TTY bolts maybe they stretch over time and loose their clamping force? They are brought to the point of deformation, but if they did re-torquing would just stretch them more?

[ElliotV]

2 hours ago, datzenmike said:

That is very interesting, have never heard this. I thought the KA the first with TTY. I checked and the Z24 has a different bolt than the previous Z22 and the '84-'86 Z20 option. I assumed this was because the Z24 block is taller in the head bolt area. Also the Z24 is the only Z series engine with the problem of blowing the head gaskets and requiring bolt re-torque. If these are TTY bolts maybe they stretch over time and loose their clamping force? They are brought to the point of deformation, but if they did re-torquing would just stretch them more?

Im assuming since they are a early tty design they may be different than modern tty bolts so re-tightning them might re-clamp them correctly if they are unlike the modern tty bolts. I always assumed the headgasket issue came from the updated taller deck but maybe it was just the introduction of tty bolts? If so is the z22 and z20 bolts the same length and thread and a possible substitute?  

[datzenmike]

Anything is possible. I thought the bolts were necked down slightly and thinner than the L series head bolts. It is only the Z24 that has the clamping problem and then only every 100k or so. That's a lot of heat cycles. I've talked to people that have removed Z24 heads and found the odd bolt less than fully tight. I doubt the bolts unscrew so maybe the gasket compresses?

[ElliotV]

2 hours ago, datzenmike said:

Anything is possible. I thought the bolts were necked down slightly and thinner than the L series head bolts. It is only the Z24 that has the clamping problem and then only every 100k or so. That's a lot of heat cycles. I've talked to people that have removed Z24 heads and found the odd bolt less than fully tight. I doubt the bolts unscrew so maybe the gasket compresses?

either it compresses more which I have a hard time believing or the bolts literally stretch from so much pressure and so many heat cycles. When mine blew it was when the motor got turned off and then I came and started it again and it blew. Could be that it hit its threshold for heat cycles and stretching and once it got compression again it blew the gasket out.  

[datzenmike]

When head gaskets fail it's the end result of a long process where the gasket has been failing for some time.

 

Bolts can be stretched only so far and when released they return to normal. If you exceed this stretch the deformation is permanent and it stays stretched.  Now TTY bolts are tightened to a set amount and then turned a set angle to 'stretch' them. Now, I'm just saying that none of this is mentioned in the 720 FSM nor is it stated that they are TTY or that they need to be replaced.

 

I don't have a FSM for the later KA engines that do have TTY bolts to see if this is mentioned or not. 

[DGTT]

Wow! Thank you all for the advice! Hadn't even thought about the rear crankshaft seal...

 

I guess my project for this evening is ordering parts 🫠