Flexplate / Flywheel Question...

Okay, how tight am I supposed to torque this thing? Gonna replace it soon...

Also, I can't seem to find it in the FSM...where is it?

Sequoia

Sequoia,

Lets start at the beginning. The 4.0 engine has six cyldiners each of which have a piston which is conected to the crank shaft by a connecting rod. The linear motion of the piston is converted into rotation motion of the crank shaft via the connecting rod.

With 6 cylinders the cylinders fire in a sequential order. These pulses occur every 120 degrees of crankshaft rotation.

It would be hard to drive any vehicle that had pulses like that so years ago they came up with a way to store this rotation energy and end up the flywheel. It is a disk of metal with had rotational mass. It stores and releases energy and gives us a smooth running engine.

On our engines the flywheel is attached to the crankshaft at the rear of the engine. The flywheel has teeth on it which are used by the starter to turn the engine. These teeth are also used by the crankshaft position sensor to let the computer know where the crankshaft is in it's rotation.

I think you will have to pull the transmission away from the engine to gain access to the flywheel bolts. I have never done this job but it is my guess. If you have a manual transmission then you will have the clutch out of the way.

I found a web site which shows it nicely: http://auto.howstuffworks.com/clutch2.htm

Also I think they make a took which you can use to hold the edge of the fly wheel using the teeth so then you can torque the bolts to the right value.

I am not sure if you have a auto or manual transmissoin. In a manual you have a clutch, in an auto you have a torque converter attached to the flywheel.

HTH

Martin

105 ft lbs. for service set. service recheck 95-115 ft. lbs.

I found it in my mot. I-6 service manual Part number 8980 010 422. Page 13

Martin is correct on that tool to grab the starter ring gear. It holds the flywheel (or flex plate) while you torque the bolts. Works great, but if the pan is off you can just wedge a long pry bar into the crank and do it that way.

I'm not sure if they make them for the 4.0L but I have been reading up on 'inertia rings', some on the bmja swear by them on the 4 bangers saying that it increases low end torque [heavier flywheel, more inertia, less stalling at slow rpm]. Hey, as long as you have it apart it might be something to look at and spend money on

Heavy flywheel

I don't know about the enertia wheel but I run a heavy flywheel which does the much same thing (thanks to Tom Houston 'old_man'). It makes it almost impossible to stall out while lugging but don't try to drag race with it. However I run a 4 banger so can't tell how it would do on a 6.

I'll tell you what I recall about flywheels from my Dynamics of Machinery class I took for my BS in Mechanical Engineering. I do not know where the term flex plate came from, I only know them as flywheels.

Flywheels work on mass and distance from the center of rotation.

You can have 2 different model flywheels which have the same diameter and weight (mass) but the one has a higher inertia rating because most of the mass is at the outer diameter.

A low interia flywheel has the ability to change speeds quickly which can be important but if you let out the clutch wrong you can kill the engine easily.

A high interia flywheel does not change speed as quickly which can cause a lag in the change of engine RPMs. This would give you the ability to totally mess up letting out the clutch and still not kill the engine.

It is a trade off on how you use the vehicle. I know with helicopters you want the main rotor to be at constant speed. A high intertia rotor works well and you have less rotor speed droop than with a low intertia rotor.

I'd stay with the stock flywheel - mass and interia - unless you really know what you are doing.

Flex plate is used with automatics. It's not a heavy mass like a flywheel on a manual tranny engine -- just a piece of sheet metal with a ring gear welded around the perimeter. I assume it's called a flex plate because compared with a flywheel it is flexible.

Eagle is correct here. MANUAL transmissions have TRUE FLYWHEELS, where the mass of the flywheel is used to maintain rotational inertia when engaging the clutch.

In AUTOMATIC transmissions, they use a "FLEXPLATE", which is a thin (approx. 3/16" thick) metal disk that connects the torque converter to the back of the crankshaft. They don't need the large rotational mass for two reasons:

1. The shock loads imposed by a stick (like when you dump the clutch!) are not present in an automatic.
2. The torque converter itself has quite a bit of rotational inertia (ever pick one of those up, especially when it's full of OIL?), and hence adding more on the flywheel is unnecessary.

This is my practical side rationalizing against my training from my 30 yr old BSME.

Okay, I did learn something, two names for the same thing. I knew someone had invented the term "flex plate" and knew it was an alternate phase for fly wheel but never cared enough to find out what the difference was.

It sounds like someone in the auto industry got tired of saying manaul fly wheel or automatic fly wheel so they came up with the new word flex plate. Or it could be short for flexable plate which allowed for different engine/transmission combinations with a common flywheel.

When you boil it down to brass tacks there is a flywheel attached the crankshaft. The engineers did the work to figure out what mass was required to have the necessary crankshaft acceleration rates.

I dare to say if you did not have a fly wheel you would not be able to use any gasoline engine due to the inability to control engine RPM.

Anyway now Sequoia knows where the beast is located.

Martin,

You are correct, in that a flywheel makes an IC engine run (more) smoothly. By the way, have you ever seen a flywheel on an older engine, before they began to have integral counterweights on the throws of the crankshaft? They are absolutely MASSIVE. That was the way they dealt with the vibrations imparted by the conn. rod/piston forces on the crankshaft. The one I picked up out of a Model "A" Ford must have weighed close to 100 lbs., and that was a 200 CID four banger!!

Yea and take a look at those on the old John Deere two cylinders(360 degrees between power pulses). They are HUGE. Those things sound downright angry when you through a load on them and the wheels start jerking like crazy. A flywheel con only do so much.

Jeff,

I have not see the fly wheel out of an old Ford but have seen them for large John Deere tractors. I agree they come in all sizes, just like people. I have seen on those TNN shows guys installing low interia flywheels on those rice rockets to allow them to spool up quicker. I think Henry Ford was doing the opposite to keep people from getting into trouble.

I really don't care the size & mass of the flywheel attached to my engine, I just want the vehicle to work. I let the design engineers worry about that one.

We are lucky to have a straight 6 engine available for the XJ and most of us have that engine. The firing order cancels out 1st and 2nd order vibrations. The straight 6 is the only 4 cycle engine that has this characteristic.

An inline 4, V6, V8, or straight 8 do not have internal vibrationn cancelation of a straight 6. This is why we see counter rotating balance shafts in some of those engines.

We are fortuneate to have a vehicle that was well though out. Starting in 1987 AMC rolled out the 4.0 XJ with the bullet proof AW4 and a good drivetrain.

Hopefully my repaired and repainted 88 XJ will be out of the body shop, it will be nice to have it back again! Just hate how long insurance companies take when it their client who hit you.

Martin

Well, thanks for the info, just one more question...

Anybody know where in the FSM it talks about this? That's where I was really confused! Coulnd't find it in there...odd!

Anyway,the plan is to do this possibly next weekend when I"m doing my head gasket. Gonna pull the trans (dreading that) and the pan (rear main, probably change the oil filter out and bearings too). Speaking of which...must get parts...guess that would help! Hopefully the fools at Autozone give me the right parts....

Sucks...doinig all this because of one bad Autozone starter (it gnawed off a few teeth...grr!

Sequoia

Going by memory here, and this was the '88 FSM. I believe the torque for the flywheel to crank bolts was in the engine section, and the torque for the torque converter to flex plate was in the auto tranny section.

I am fairly certain I didn't find everything in one place. That would have been far too easy.

Ya know what, EVERY TIME I have gone to autozone they either don't have the parts in stock, give me the wrong part or it just does not fit. There is no excuse for not having jeep parts in stock, Oh I'm not talking about speciality parts, I'm talking about 'stock' items like shocks, steering stablizers, caps and rotors, and stuff. Geeze, how many years did they use the BBD's, how stupid is it not to have ONE in stock and at least ONE rebuild kit but nooooo, special order. On the other hand they they enough chrome bolt-on equipment to decorate every neon in Pennsylvania and New Jersey in one store..
I needed a fuel pump for the YJ this weekend, sat AZ said 'tues', I ordered it at national Autoparts sat at 7pm, they had it sunday at 1pm. started asking about some other stuff like shocks, steering stablizers, the counter guy finally asked if this was a test and pulled up the whole YJ inventory, geeze, they had both a long and short block in the back as well as a tranny [727] transfer case and could get a new front diff overnite.
4 bbd rebuild kits and 2 bbd's in stock.