BIG MISTAKE!

You have just received your first audible warning that you have been neglecting the timing chains and tensioners. It's called the U20 Death Rattle. Pay attention now and your 2000 will continue to give you many more miles of driving fun. Ignore it and you will be spending big bucks on a new engine (insert big smile from parts vendor as he sees the possibility of keeping his kid in college for another year).

The explanation is a little involved, but once you understand it, it makes life so much easier. Plus there are several things you can do to prevent it. Here we go.

U20 Timing Chains
The U20 (2000cc) engine is a development off of the R16 (1600cc) engine. The R16 is a pushrod motor with the cam in the side of the block. The cam also runs the fuel pump, oil pump and distributor. There is a timing chain that runs from the crankshaft to the camshaft. Bolted to the front of the block are a small timing chain guide on the upper side and an oil-pressure-operated chain tensioner on the lower side. Oil enters the tensioner from the main oil gallery and pushes the "foot" against the timing chain, keeping the slack out of it. It also oils the chain. Simple and reliable.

When Nissan made the U20, they based it on the R16. They added an overhead-cam cylinder head and simpy added another chain and set of gears to run the cam. They took all the lobes off the R16 camshaft and christened it a "jackshaft." It still runs the fuel pump, oil pump and distributor. But this made things complicated up front. Now we have a total of four gears, two chains, two chain guides and two tensioners to deal with. Here's what it looks like all assembled. To make matters worse, the upper tensioner is as far from the oil pump as it is possible to get, so pressure builds there last. See this diagram of the lubrication system.

The "Evil L"
At this point Nissan made a decision that would spark heated debate in the roadster community for years to come. They added a bracket to the upper timing chain guide that would support the cam gear and upper timing chain so you could remove the cam or even the entire head without losing the chain down the front cover. The guide is bolted to the front of the block, and the bracket angles out underneath the cam gear and then goes up in front of it, forming an "L." Simply unbolt the cam gear, run a 6mm bolt through the hole in the bracket and screw it into the threaded hole in the center of the cam gear. You can even use the screw that acts as the locating dowel for the cam. Great solution! This is what it looks like.

So what's wrong with it? Well, when the chain gets old and stretches, it gets too much slack in it. And when the upper tensioner doesn't have enough pressure to remove the slack, the chain can swing inward and make contact with the lower part of the bracket. That's what makes the buzzing sound. Given enough time, it will cut notches into the bracket. This is bad for a couple of reasons: it puts metal shavings into the oil, and it accelerates the wear on the chain. Not good. Here's a particularly bad example compared to a new one.

Disaster!
But there is an even worse disaster waiting to happen. After enough abuse, the bracket can actually fracture and break off from the rest of the guide. You now have a major chunk of steel flailing around inside the front cover! If you can't quite form an image of what that implies, drop some nuts and bolts into your food processor and push "Liquefy." Many things can happen at this point, and none of them are good. Choose one or more from the following list:

• The bracket causes the timing chain to skip a tooth, putting the cam out of time with the crank. The pistons come up and do a tapdance on the valves, bending several and putting big notches in the pistons.
• The timing chain breaks. The cam stops but the crank doesn't, and the pistons come up and do a tapdance on the valves, etc.
• The bracket gets flung through the top of the cam cover, making a huge mess and letting all the oil escape (That's where the weld marks on the cam cover come from).
• In addition to breaking, the chain makes an escape through the cam cover (see previous selection).
• The bracket drops down into the timing gears and jams them, causing the Woodruff keys to shear. The Woodruff keys are the little half-moon-shaped pieces of metal that hold the timing gears onto the jackshaft and crankshaft. The cam stops, the crank doesn't, etc. Starting to get the picture?

If you are VERY, VERY lucky, the bracket may drop down the front cover and jam in such a way that it doesn't touch anything. The first roadster motor I ever took apart had this happen. I've never seen it again in 30+ years of playing with these cars.

For obvious reasons, this bracket has been dubbed the "Evil L" by many people. Some people cut the bracket off when building an engine rather than risk disaster. But I think that's just treating the symptom. Plus the bracket is pretty handy.

What's really going on
The most obvious problem is that there is too much slack in the upper chain. This comes from normal wear and hard use. Timing chains in these cars were only designed to last about 60,000 miles, which is the same life expectancy as timing belts in modern cars. However, there are several other factors involved and several things you can do to extend the life of the chain and prevent the self-destruct sequence.

First, if the cylinder head has been milled (resurfaced), this puts more slack into the chain by moving the cam closer to the jackshaft. Most experienced roadster owners know to shim the cam towers, and that's a good start. But it isn't everything.

Second, the later roadsters had an aluminum tube in the center of the oil filter mount that kept the oil filter full when the engine was shut off. Without this tube, all the oil drains back into the crankcase, since the filter basically mounts upside-down (easy to get at but a bad design decision). Since the oil pump has to fill up the filter before pressure starts to build in the system, and since the upper tensioner is so far away, this accelerates the wear on the tensioner and lets the chain run slack on startup. This is why the rattle appears at startup and then goes away.

Third, there is an often-overlooked bolt that secures the upper timing chain guide to the cylinder head. You can see where it goes in this view. Since the guide is unsupported above the surface of the block except for this bolt, leaving the bolt out can cause the guide (and bracket) to be more susceptible to vibration and impacts. Most people discover this bolt when they try to remove the head and it won't budge. They've removed all the head bolts and maybe even remembered the two studs at the very front of the head. After tugging, swearing, hitting it with a mallet, etc., they discover this one little bolt. Then they forget to put it back in afterwards. Sometimes milling the head will drop the bolt hole down low enough that the bolt hole in the guide doesn't line up any more, so they leave the bolt off. It doesn't look like it does much anyway, right?

Fourth, as the chain wears and stretches, the tensioner comes farther out of its bore under normal conditions. This means that it will allow more slop in the chain when it is fully retracted or when the oil pressure drops off.

So what can you do about it?
The obvious stuff is to keep up on the oil changes (oil and filter every 3,000 miles religiously) and plan on a major teardown to replace the chains, guides and tensioners every 60,000 miles. With the price of those components climbing, I know that hurts, but lunching the whole engine will hurt a lot more. Plus you don't get to plan when to do it if the engine blows. Beyond that, there are several things you can do.

• Make sure the anti-drainback tube is in the oil filter mount. The vendors can supply you with these if yours is missing. It has to be a good tight fit for it to work. Some people epoxy them in place.
• Inspect things frequently. It's only two bolts to pull the cam cover. The first inspection point should be the side of the L bracket where the chain might contact it. Get in there with a flashlight and look for the telltale contact marks. They'll be on the underside of the bracket, so look carefully.
• The second inspection point is to check the slack in the chain. You do this by putting the car in gear (2nd works well for this) and then rolling the car backwards by hand to put all the tension on the opposite side of the chain. Maybe I should explain that a bit. When the engine is running the tension is on the "down" side of the chain. That's the side away from the tensioner. By rotating it backwards, this will push the tensioner foot back into its bore and make the "down" side go slack. Now push against the chain and see if you can make it touch the bracket.
• If the chain is touching the bracket, you can shim the tensioner to remove some of the slack. This takes a little prep work because there is a thin paper gasket behind the upper tensioner that will have to be replaced. Use a gasket sealer like Copper Coat; DO NOT use RTV on this or you will plug up the oil passage in the tensioner and make things much worse! Some of the vendors supply tensioner shims or you can make your own from washers. If you go the do-it-yourself route you will have to grind off one side of the washers to make them clear the plate on the back of the tensioner. Here's what a shimmed tensioner looks like.
  
• Make sure the little bolt that I talked about earlier is present.

So wrap a big wire-wrap around the tensioner and snug it down tight to keep the spring compressed, then use the tail of the wire-wrap to lower the tensioner into place. Once the bolts are snugged down, cut the wire-wrap and pull it out. I use the same technique to remove it so stuff doesn't fly all over from the spring tension.

Another interesting alternative
Brian Hollands of SEROC (SouthEast Roadster Owners Club) has devised a customized L-bracket that is removeable! He calls it the "Friendly L." If you are really concerned about having the bracket in your engine at all, you might want to check this out. Read all about it here.