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I broke my newly rebuilt motor


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1985 720 4x4 ST with Z24. I thought I could get away with a cheap cheater rebuild and shot myself in the foot I think. (No surprises there, right?) Here is what happened. I have new pistons, rings, crank and main bearings, DIY honed the block, coated everything as necessary with assembly lube, new valve stem seals, then poured some 50w over the valve train before starting it. Rods were dial bore checked, and were fine, nothing else was. After the rebuild, I ran the motor for a minute or so and never saw oil pressure. We messed with a few things and did it again. Still no oil pressure. Got a new pump, got oil pressure. By this time the motor had 4 to 6 minutes on it most likely. Tuned the new Weber carb, dialed in the timing, motor was purring like a kitten and revved up great. Oil pressure was holding close to 45 psi. Motor had 20 to 25 minutes at this point.


Decided to take it for a small test drive. Pulled out of the garage slowly and turned into a cul-de-sac got up to maybe 15 mph, then clutched in and let it idle down to flip a u-turn in the cul-de-sac. The motor started stumbling for a few seconds then stopped completely. Would not start. Tried push starting it, but just screeched the tires. Motor appeared to be locked up. Put it in 4th gear and then pushed it backwards to run the crank in reverse and the motor popped free. Got it started again but it ran terribly and made wierd noises. Got pissed and quite for the day.


Two days later, decided to run the motor out of frustrated curiousity. Motor started fine. Purred for about 8 minutes, then started stumbling for a few seconds and stopped. Started again, ran for 3 minutes or so, then stopped and would not crank.


Two days later, same story. If I rev it when it just starts stumbling, it will stay running, but terribly. otherwise it just dies and frequently won't start for a good while.


Never at any time is oil pressure low. Oil pressure does take nearly ten seconds to really begin showing on the gauge however.


Any thoughts on possible problems? Why will it run perfect, and I do mean perfect, for near 10 minutes and then crap out and be totally done?

Thanks for any help in advance.

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Yes, that might be the cause. Not the cheap rebuild kit, but the lack of measuring things.



Rods were dial bore checked ... nothing else was


When you fit new pistons and bearings, those should be checked for correct clearance dimensions, or on occasion they will be too tight and seize when they get hot.

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The gauges don't follow the oil pressure in real time. Next time you get in and turn the key on, notice how long it takes the gas gauge to read full. It's nothing to worry about.


Seems like it seizes when it gets warmed up... 6-10 min. Ive spun a rod bearing and it will squeal like a pig at low speed. A loose rod and or bearing will knock loudly when revved.


Any chance you have an oversize set of rod or main bearings? Got the old boxes???

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I did not measure crank clearances. The packaging all says standard size but I guess it could be mis packaged. I did measure the ring gap and if anything they are a touch too big on the gap. there really is not much in the way of odd noises even when it starts stuttering. I will try again and listen for anything. Guess I get to tear the bottom end apart again.

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Ok, I took apart the bottom end and found one rod bearing that was showing wear. when I clearanced it, it showed somewhere around 8 to 10 thousandths. The book claims 10 to 22 thousandths is safe, so I am definately on the tight side of things. Perhaps even just tight enough to cause the problem I have been seeing. I clearanced all the other bearings and they are nearly middle of spec. I bought a new bearing and installed it and it is also now nearly middle of spec. It clearanced at about 15 thou. I'm hoping I just had a bad off shelf bearing and have corrected that. Now I just need to put the engine back in the truck and see what happens. Wish me luck.

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...it showed somewhere around 8 to 10 thousandths. The book claims 10 to 22 thousandths is safe...


I think you might be reading the book or the gauge wrong. A stock crank in a stock motor should have between .0018 and .003 inches. That reads 1.8 thousandths and 3 thousandths. Could you be confusing the SAE measurement for Metric?


Well it sounds like you're on the right track. I would run to my local machine shop and ask them if you can have one of their old crank polishing emery cloths. Run that on the crank journal to clean it up. Also make sure the rod cap was not on backwards.


Just out of curiosity, did this motor run before? and are you using the same rods and crank that it came with? All pieces of the puzzle, process of elimination.

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The only problem with what I said is that I am retarded. Safe clearance, according the manual I am using, is 0.0010 and 0.0022. My rod bearing clearance was showing around 0.0008 and 0.0010. I put in a new bearing there and clearanced it and it showed a consistent 0.0015, or 1.5 thousandths. All the other clearances check out correctly. I recently re-installed the motor, but am now pulling once again after only running it for a couple of min. The motor seems fine, but I am pretty sure that I installed the clutch plate backward (which pisses me off, noob mistake, just got in a hurry) and now need to pull it and flip it. Hopefully I will be driving the truck by sunday night.

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That would make our trucks an exception to a common rule then. Many clutches can be assembled backwards and that prevents the clutch from ever disengaging completely. There are even a couple of troubleshooting guides from major clutch manufacturers that list reversed plate as a cause of being unable to shift in to gear. That is my problem. I cannot shift in to gear with the truck running. I can shift fine with it off and when I start it with the clutch in I have to keep a foot on the brake or the truck will roll slowly even with the clutch in. And if I clutch out, it grabs. I have replaced and correctly bled the clutch hydraulics (which were working fine when my engine started siezing anyways). The only other possibility is something I did with the clutch kit. Hydraulics are good, trans is good, so the problem has to lie between them. All this functioned last time I put the motor in and nothing was messed with except the clutch kit while the motor was out. That leaves only user error on some part of the install and since the clutch plate is the only part that can really be installed wrong, and doing so is known to cause the problem I am having, I have to try and assume that that is in fact what I did.

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" There are even a couple of troubleshooting guides from major clutch manufacturers that list reversed plate as a cause of being unable to shift in to gear"... yes for some clutches other than Nissan ones. I'm saying I think you can't put a Nissan one on backwards. I'll check tomorrow on the old ones I have out back but the spline on the disc sticks out on one side and has to face the transmission. It it was protruding to the front it would hit the flywheel and not flat against it... the PP wouldn't bolt down over it.



Did you change the release bearing collar? A collar for a 200mm clutch will not fully disengage when used on a 225mm truck clutch. Also you may have bled it but it could still have air in the system.

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I understand what you are saying. I had this motor in the truck and the clutch functioned perfectly, then the motor seized. I pulled the motor, fixed it and put it back in. clutch will not release now. No clutch parts were changed. What other problems might I have created using matching parts that have already been proven to function?

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