Guest Rolling Parts Posted July 27, 2009 Share Posted July 27, 2009 Background: I'm refreshing a low mileage (80K) engine from a 1979 280ZX parts car. It was all original but I wanted to go through the engine to inspect it, de-gunk it, and put in new gaskets before dropping it into a 240Z. It was very low mileage and looks like it sat for the last 10 years. The Funny thing: I just re-installed the crankshaft and dropped in the pistons. When rotating the crank to feel if there was any binding, I noticed that #6 piston did not go fully to the top of the block when #1 was at TDC. The other piston pairs (2&5 and 3&4) all rose fully flush with the top of the block. Inspection: I took #6 back out to make sure that I just did not leave out a bearing insert. Everything looked fine and I reinstalled the piston. It again did not go flush with the top of the block when #1 was at TDC. Question: Is there any reason that one rod or piston would be short by a few mm? I'd swear that this engine had no signs of ever being opened up before and all the dished pistons were in remarkably good shape because of the low mileage and then sitting so long due to an EFI problem. What the heck? Anyone ever seen something like this? Quote Link to comment Share on other sites More sharing options...
MONGO510 Posted July 27, 2009 Share Posted July 27, 2009 broken or twisted crank, or non standard part. John Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 27, 2009 Share Posted July 27, 2009 You mentioned a few mm, is that the amount of difference between 1 and 6? Quote Link to comment Share on other sites More sharing options...
Xnke Posted July 27, 2009 Share Posted July 27, 2009 bent rod, wrong rod, twisted crank, broken crank, ect. Is it 3mm wrong? that's the difference from a L24 rod and and l28 rod. If it's less, I'd say the rod is bent. Quote Link to comment Share on other sites More sharing options...
josh817 Posted July 27, 2009 Share Posted July 27, 2009 May help to use a scale too. I think Datsun balanced their rods fairly close so if you have an oddball like #6 then maybe its like Xnke said and its a miss matched part. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 27, 2009 Share Posted July 27, 2009 Few mm would be a rod from another engine, as stated. Or a piston with the wrong pin height. In either case, it's 'wrong component in engine'... Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 27, 2009 Share Posted July 27, 2009 Few mm would be a rod from another engine, as stated. Or a piston with the wrong pin height. In either case, it's 'wrong component in engine'... That's why I had to write in and hope that you responded. This engine was so original in condition that it's got me kinda bugged. When it cleaned up all the dished pistons they all looked identical with even discolorations and carbon buildup, had the original weight stampings, and the rods and pins all looked unstressed and rotated freely. The crank also had no signs of stress wear at the bearings. Everything was laid out in order, cleaned, and then assembled back in the same order. Why only ONE piston does not reach deck height while the crank still rotates freely does not seem to fit a "twisted" crank scenario. I'll take out #6 and #1 this evening and do an inspection and measure the heights. If they are identical, then it has to be the crank. This was supposed to be one of those "easy" engine refreshes on an engine I pulled from a low mileage unmolested automatic ZX. Nothing in its condition suggested it'd been opened up or ever serviced. All the old gaskets looked right for what I've seen in in other 30 year old blocks. That's why I'm scratching my head because nothing suggested a reason for finding something oddball like this! I'll write back after looking at the rod part numbers and doing a closer inspection of "what's wrong with this picture". Quote Link to comment Share on other sites More sharing options...
jt1 Posted July 27, 2009 Share Posted July 27, 2009 While it's still assembled, it's pretty easy to check the stroke on # 6, it should be the same as the others. If the crank is twisted, the piston would still likely come to TDC, just not at the same time as #1. If the stroke is correct, then check wrist pin height and rod length when you take it apart. jt Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 27, 2009 Share Posted July 27, 2009 Thanks. I forgot to mention that #1 and #6 rotate in unison. TDC is TDC on both. That's why I thought that I screwed up and forgot the upper bearing when I installed #6 piston. #6 was just lower than all the rest by about what the thickness of the bearing shim would be. When I pulled #6, it inspected fine and both bearing inserts were correct. I don't have accurate gauges so the easy thing for me this evening is to make sure the rods at least have the same part numbers and that they stand up to the same height. I would have done this before reassmbly if there was even one indication that this engine had been opened before or there had been signs of uneven wear on anything... Quote Link to comment Share on other sites More sharing options...
jt1 Posted July 27, 2009 Share Posted July 27, 2009 That pretty much rules out a twisted crank. You can visually compare the rods and pistons, stand the pistons on top and compare the wrist pin holes, then stand the rods on the bottom and compare wrist pin holes again. Is the crank standard or has it been turned? A possibility is somebody spun a bearing on #6, ruined the rod, turned the crank and grabbed a 240 rod for a quick fix. A bent rod on #6 is also a possibility. A quick way to check the stroke on #6 is put the #1 rod & piston in #6, and see if it comes all the way up. jt Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 27, 2009 Share Posted July 27, 2009 I did not have the crank turned & from the looks of the engine the head was never even removed before. That's what's got me baffled, I'd swear that this engine was a time capsule ZX engine from it's crusty fuel rail down to the oil pan gasket. That was my excuse not to measure everything, just to do a "quicky refresh" before dropping it into the 240Z and slapping on the carbs to keep the car on the road while I spent a lot of time building a real new engine for it this winter. I never expected to find such a surprise when opening this thing up! Nice idea about swapping piston holes and see what happens. EDIT: I just picked up a 6" Brown & Sharpe dial caliper from a friend to use this evening. I'll now be able to do an accurate measurement of the rod, the piston, and the crank journal. Should be interesting to see which bit was wrong. The "why" it's that way in an apparently unopened vintage engine might take a bit longer... Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted July 28, 2009 Share Posted July 28, 2009 Could you take pictures of this anomaly? I am curious how much of a difference there is. Thanks. Quote Link to comment Share on other sites More sharing options...
jasper Posted July 28, 2009 Share Posted July 28, 2009 I have seen a couple of engines aspirate water into the intake. This resulted in a bent/distorted rod. No outward signs of damage were visible. Had a slight miss on start up. Ran pretty good otherwise. Poorly executed/planned cold air intakes can cause this. I have also seen customers who think their cars are amphibious!!! Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 28, 2009 Share Posted July 28, 2009 OK, The mystery of why #6 was not TDC at the top of the block was that #6 rod had a very slight "twist" to it so the measurement between the crank journal and the piston pin was shorter by just under 2 mm. All other measurements with the dial caliper were OK. All rod part numbers were identical too. I never knew that there could be a bent rod without any other "signs" of damage to the head, the top of the piston, crank, bearings, or to any other adjacent pistons. All the internals of the engine matched the description of just a low mileage unmolested ZX engine that was ripe for using as a transplant. Just HOW that much force was applied to one rod w/o so much as leaving a trace is beyond what I can come up with. Well, I did not want to get deep into this because it was supposed to be a throwdown engine used for a relatively short time. Thanks for all the great suggestions guys! Now it's looking for a single rod OR dumping the whole set and finding some used decent flattops... Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 28, 2009 Share Posted July 28, 2009 Holy CRAP! A 2mm height difference from a bent rod is hellacious! I was scrapping Superior Rods with 0.002" twist in them...and that was on a 10" bore, 10 1/2" stroke engine (think rod the size of your leg!) Amazing what they will take and just keep on truckin! I WANT PHOTOS! I'm sure a lot of other people are curious to see this one as well. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 28, 2009 Share Posted July 28, 2009 Yea, I was a dumbazz and just assumed that all the parts in the donor L28 NA with low mileage would be OK after seeing the condition of everything else was just as it should be for the time/mileage and overall condition. I was not really taking this refresh seriously because it looked like a textbook "time capsule" ZX engine. That's why I had such a WtF moment after we rotated the internals after finishing up the bearing assemblies and demonstrated the "piston pairs" in the L6. OK, Worst part is that I'm coaching my son through all of this because he's 17 and he wanted to build an engine. He's doing all the assembly work while I was in charge of "QC" by handing him all the parts to install(oops). I'd forgotten that I have another used L28 short block stashed under another bench. I tripped over it when rummaging around for something else last night. Time to harvest (and this time measure) a rod/piston assembly and then drop that one into the #6 hole. We should be back on track to get it all running on the test stand and then dropped into his 240 before real school starts... Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted July 28, 2009 Share Posted July 28, 2009 Holy CRAP!A 2mm height difference from a bent rod is hellacious! I was scrapping Superior Rods with 0.002" twist in them...and that was on a 10" bore, 10 1/2" stroke engine (think rod the size of your leg!) Amazing what they will take and just keep on truckin! I WANT PHOTOS! I'm sure a lot of other people are curious to see this one as well. I second the request for photos. I want to add them to my "basketball bat" collection (like the cam that was completely wiped out, but there's still a pair of 'ears' or 'tabs' from the original grind that are perfectly fine.) Quote Link to comment Share on other sites More sharing options...
jt1 Posted July 28, 2009 Share Posted July 28, 2009 It almost certainly hydrolocked either by water or fuel, or maybe oil. Bad injector, leaking head gasket, or getting water into the intake. Glad you found the problem and have a fix. jt Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 28, 2009 Share Posted July 28, 2009 I'll take a few pictures this evening; then I'll take it to work tomorrow and let a few motor-head engineers that I know have a look. Maybe they can see some pattern in it that would explain it. I like the hydrolock idea except original transmission was an automatic so I was thinking that would rule out some kind of push-start with a locked-up cylinder scenario? Surely the starter in a ZX can't impart enough energy to twist a rod? I hope I can set the lighting right to get few good high contrast pictures. Mind you, I looked right passed it when cleaning the pistons so it's not going to be an "oh the humanity" picture of steel shards and mass destruction. Thanks again guys for all the suggestions! Quote Link to comment Share on other sites More sharing options...
jt1 Posted July 28, 2009 Share Posted July 28, 2009 Usually it's a combo of the starter plus one or two other cylinders firing, then the inertia of the flywheel bends the rod. jt Quote Link to comment Share on other sites More sharing options...
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