U20 Broken Timing Chain - What to Expect

[sfyks870]

From what I recall, you said the po heard a sound that he thought was a broken timing chain during a run and shut it down to prevent more damage. From the looks of it, I would think its totally plausible what he heard was the bolt coming lose and getting smacked around by the timing set on its way to the lower end. These guys know more than me by a long shot, but I know if it were mine, a lot more tear down and inspection would be in order. It's unfortunate, and possibly unnecessary, but I would much rather do the unnecessary tear down and inspection rather than assume it's fine and grenade an engine.

[jhayden]

From what I recall, you said the po heard a sound that he thought was a broken timing chain during a run and shut it down to prevent more damage. From the looks of it, I would think its totally plausible what he heard was the bolt coming lose and getting smacked around by the timing set on its way to the lower end. These guys know more than me by a long shot, but I know if it were mine, a lot more tear down and inspection would be in order. It's unfortunate, and possibly unnecessary, but I would much rather do the unnecessary tear down and inspection rather than assume it's fine and grenade an engine.

Good suggestion, and for those who may not be aware, the oil pan can be removed easily with the engine still in the car.

[Gregs672000]

I'm a little late to the inspection and most issues have been addressed. The cam is a bit nasty, but you might be able to have a cam grinder re-cut it and save it... cams of any condition are getting harder to find. Isky in CA can look/do it, as can Delta Cam in Tacoma WA. I suspect a bad rocker arm that will likely need replacing, and if the other rockers are OK and two piece (you can see the separate contact pad) Isky and Delta are set up to surface them properly. Otherwise you may be looking at all new rockers which are NOT cheap, and possibly a slightly thicker set of lash caps to make sure the cam lobe rides in the middle of the rocker pad (more or less, slightly off is not a huge deal... running off the end is. We can tell you how to check as you won't be able to just look at it very accurately. You'll probably be fine). Refurbished rockers will also be an option. No matter what, you must surface the rockers unless they are running on the exact same cam lobe (lobe not resurfaced... untouched) or they can damage themselves due to having already worn in a pattern when the cam was broken in (number them with a sharpie any time they come out). The cam and rockers will need to be broken in properly if/once surfaced. Regarding the tensioner, just shim it and leave about 1/8 to 1/4" throw on the shaft so it can move a little. Agreed on the suggestion to pull the pan and see if you can locate the cam locating screw. I note the split washer on one cam bolt is dead. Look through some previous threads (pics?) regarding inspecting the timing gears for wear as they will need to be assessed (or post pics). Plan on replacing the chains at the very least... they stretch over time. Cam cover bolts are lightly hand tightened... you see the result of over tightening them, often to try to stop cover leaks which really requires a new gasket as they get hard. Not sure you will be able to remove the front cover without pulling the head... sometimes, sometimes not... the front cover has locating pins. Oil... 10/40 or 20/50... I've used Valvoline 20/50 VR1 racing for years (summer use only, WA state temps), however will be switching at the next change to a Penzoil Ultra Platinum (I think that's what it's called...) 10/40 after researching. If you pull the pan to look for the missing cam pin, pull a rod bearing cap and see what they look like... easy. The Evil L was not damaged to where I would cut it off, but do inspect it carefully... once shimmed all should remain ok and it is nice to have it as it holds the cam gear and chain in place when pulling the head (mine is cut off... the L, not my head.... ).

Overall, looks to me like you got a pretty good score. I expect it is a somewhat worn but not dead U20. Spend time reading the manual (on line) and the other various posts in the tech wiki on this site... tons of valuable information developed over years of folks working on and loving these cars that covers multiple systems and will save you time, money and grief! BTW, remember you have a later Nissan EI distributor and your engine is "de-smogged" so ignition timing is 17 initial, not 0 as in later cars (should you see this in the manual).

Welcome! We're here to help so do not hesitate to ask ANY questions.

[WHFred]

Thanks for the clarification. It's making sense now.

Just to be clear, the purpose of the evil L seems to be a bracket to mount the cam sprocket to when servicing the cam. Is that right? Seems crazy that something that is not functional for operation of the engine causes such headaches!

I did realize that I cannot take the timing cover off without head removal, so for now we'll leave all that intact and plan on running compression tests this week.

While looking in my factory U20 manual that came with the car, the section on rocker adjustment shows an error - the decimal conversion for .3mm (exhaust) and .2mm (intake) are reversed. Am I correct that exhaust should be set to .0118 and intake to .0079?

IMG_6966.jpg

Next, I pulled the alternator. I noticed that under the alternator connected to the frame is a ground wire. Mine was not connected to the alternator at all. Should it be? I noticed the alternator has a post labeled BATT which is the positive output, but also has a hole on the back labeled E which was unused. I couldn't tell from the manual if the ground went there.

On the part of the wiring harness that comes from the alternator, on the drivers side inner fender, is a single wire coming out of the harness. It's white and has a male spade connector crimped to it, but it's just hanging loose. Any idea what this was/is for?

[david premo]

Alright WHFred,
Let’s go through this one item at a time, with all engines the larger valve clearance when different are exhaust clearances as the exhaust valve needs more time to transfer heat to the cylinder head to prevent the valve from being burnt on the edge. The evil L was designed for one purpose to keep the chains in time when removing the head or cam. The L it totally unnecessary as the chain cannot go anywhere as the Jack-shaft sprocket prevents it from going anywhere, so at worst you pull the chain up and bring it back into contact with the lower sprocket.

With regard to the removal of the timing cover with the cylinder head on it can be done by removing the studs that go into the head with a small pair of pliers and dropping the oil pan which has 4 studs in it. After you remove the timing cover you can inspect the guilds, chains and tensioners to verify that everything is in good working order. Personally I would remove the head as it would be a validation of the condition of everything, the cost of a head gasket would be negligible compared to the piece of mind know what things look like.

Do whatever you want, it’s your car the advice you have gotten is free and merely a recommendation of how to proceed without the risk of damaging the engine. Myself personally I would want to know what all the potential issues are and not just hope for good luck after the fact.

With regard to the alternator the large white wire does go to the batt post on the alternator. The large white wire goes to both the starter and amp meter and is the source of power for the entire electrical system. Be careful, many people on this site will recommend converting to a larger output alternator, not a problem if you install something that will protect your main charging wire in the harness.
Dave

[Gregs672000]

As Dave mentioned, be careful with that white wire and inspect it closely... it goes to the amp meter and if grounded or somehow compromised it can/will use the amp meter as an expensive fuse and create smoke/fire under the dash... not saying this to worry you but to make sure it's correct (check online wiring diagrams... thanks Curtis!). I and several others have installed a 30amp or so fuse or fusible link in order to protect against that. Not sure why your white wire would be disconnected in the first place unless it's been replaced or compromised at some point... look!

Like you, I think I'd do a compression and if possible a leak down test before further disassembly as well... that should give good info. Just putting some compressed air into a sealed cylinder and listening for leaks and where (intake/exhaust/crankcase) will give some idea if things are bad. You sound experienced but if you need some guidance on doing this let us know. The leak down test wouldn't risk cranking the motor over and over, but based only on how things look you're probably fine cranking it and compression would be helpful.

Regarding the timing chain, this is an interference engine. Making sure you're not a tooth off is critical, and while it rarely happens it is possible to have a chain rotate off a tooth, and if the engine moves you can end up installing the cam a tooth off, so keeping the chain up against the lower gear and marking the chain to the cam gear is important, if just for piece of mind. It is highly suggested that you put the engine at TDC (far left mark in the crank with you facing the engine, both lobes for the cam pointing up and off the rockers on #1), mark the chain and cam gear with a sharpie, take a pic of the chain and cam gear before you pull the head... good reference if needed. Really glad you have a manual... look it over and things will start to make more and more sense!

Good stuff... keep digging!

[WHFred]

So I ended up doing a compression test. Not good! A dry test yielded readings for cylinders 1-4 of 55/135/120/85. A wet test (squirt oil into the cylinder) barely changed the readings although its possible I didn't use enough oil? I was worring about hydrolocking and causing more damage.

I don't have a leakdown tester at the moment, but I did use my compression tester hose to connect compressed air at 100psi, with piston at tdc, a wrench on the crank bolt to not allow the engine to turn, throttle wide open, and visual confirmation that cam lobes were pointing up. On the cylinder with 55psi there was no noticable air from the carb or exhaust, but a huge rush of air coming past the rings and into the crankcase and out the oil galleys in the head.

Before I pull the engine and rebuild I decided I would try and get it started. After draining the gas tank, replacing all rubber fuel lines, installing new battery and new battery cables, oil change, rocker adjusting, the engine started right up! Sounded awful, but was running. Sounds like it has a terrible exhaust leak around #4. I adjusted the timing and shot some cleaner in the carbs and it sounds better.

One thing that is odd that someone may help me understand - the oil pressure was super low at idle - like barely registered on the gauge. I put in new 10w40 conventional oil. When I had pulled the oil pan I noticed that the oil pump looked brand new, so maybe PO had this issue and replaced it. What else would cause low oil pressure besides a weak oil pump?

Once I get the brakes and clutch sorted out I'll attempt my first drive in the car. After that it'll be a lot of cutting out rust and welding in new metal, rebuilding the engine, paint, interior, ..., $$$ .

[SOUPY]

WHFred, Sounds like you have gone thru alot of work to get engine running! IMHO I would not start engine again especially for a test drive. Right now everything is holding together, so it seems. Obviously you have a problem on 1 and 4. Could it be a burnt or hole in piston? A severely scored cylinder?
If you continue to run it you risk locking up engine, throwing a rod or in general just doing more damage that is already there. As for oil pressure, somewhat normal at idle, but once you tear engine down I think you will find the culprit! Good luck and keep us updated

[redroadster]

Pull the valve cover and see it oiling on top ,cranking with plugs out ...guage it the best way
Before running again. Some parts stores rent a boroscope too plus HF has some

[Gregs672000]

I'm in agreement that you are risking more damage by running the motor, without much benefit to you. If you end up scoring the cylinder walls you may do damage beyond what can be machined out, then you're looking for a new block etc etc. With compression like that and the air coming past and into the crankcase, you know it's significantly damaged/worn. It's good that there was no detectable air coming past the valves, but that might be over shadowed by the "huge rush of air" into the crankcase and not as noticeable... in other words, they may be bad too but not as noticeable. Given the volume of air you describe I'm concerned we may have had piston to valve contact.

I'm really sorry this has come to pass. We were all hoping for a better result for you. However, parts are available and the engine is not hard to build, and a U20 is a great engine, especially for its time. Yes, Nissan did learn more about OHC engines and made the single chain L-series afterwards, but no L-series cylinder head can flow air like a U20 until you get into full race heads per my old cylinder head developer.

At this point I'd go ahead and pull the head and evaluate it so you can prepare and make some financial decisions. That cam is going to need attention, and at least one rocker. The timing gears are another area of concern, though thanks to our wonderful vendors we have multiple gear options at reasonable prices. I hope you continue forward with this project... we're here to help in any way we can.

P.S Oil pressure is actually around 10lbs at idle but the guage is poor on these cars... should rise up with rpms. My oil pump has seen at least 300K miles (yep!) and works just fine.

[theunz]

I concur with Greg and Soupy. The head is not that hard to pull and it’s almost a sure bet that it’s going to come to that anyway. As an aside, be very conservative if you need to have the head milled as it takes very little to ruin the head by over milling. If the head does need to be milled BE SURE TO RESEARCH AND ASK QUESTIONS HERE before letting the machine shop touch it. As for the oil pump I know a low to no reading at idle is normal but it still makes me uneasy!

[WHFred]

Great advice - thank you all.

I am fortunate to have a number of classic Datsun owners in the immediate area, and I am sure I can lean on them for recommendation on a shop experienced with the U20. I agree, one of the things I love about this motor is it's simplicity.

[Gladisco]

Wow! the timing of this thread is amazing. I just picked up a ‘68 2000 and was as of today picked up by an experienced mechanic for an inspection. I am a short time lurker here and am trying to absorb as much as possible. I am excited and a bit frightened on what he discovers especially after reading this but ready to embark on this journey as well. I will start a new thread and with a proper introduction and won’t hijack this anymore than I already have. Duncan will roll again asap:

[WHFred]

Welcome, Gladisco!

From the pic it looks like it needs nothing on the outside. Hopefully under the hood and the carpets it's just as pristine!

Mine, on the other hand, needs just a tiny bit more work. Yes, those are holes. But, it's just sheet metal.

IMG_7030.jpg

[Gregs672000]

Welcome, Gladisco!

From the pic it looks like it needs nothing on the outside. Hopefully under the hood and the carpets it's just as pristine!

Mine, on the other hand, needs just a tiny bit more work. Yes, those are holes. But, it's just sheet metal.
IMG_7030.jpg

Another new owner just posted his success in using dry ice to remove the undercoating (spread it around, wait a little bit, hit it with a mallet and it shatters off). May be helpful if you decide to patch holes vs complete replace. 38 yrs ago mine was worse and I had limited skills/no experience and no access to replacements, so we welded in patches... never been a problem and the Sems Rust Seal has never failed.