Crankshaft Sprocket: Replace or don't replace?

[Hector]

The camshaft and oil pump sprockets/gears are off but I need to get the crankshaft sprocket off which doesn't look that easy on my m30. Is it worth the trouble? Need some convincing from you guys. The car has 173K and I plan to keep it.

Remember, I got the oil pan off, radiator off, entire front end of engine is off.

[Bill R.]

The camshaft and oil pump sprockets/gears are off but I need to get the crankshaft sprocket off which doesn't look that easy on my m30. Is it worth the trouble? Need some convincing from you guys. The car has 173K and I plan to keep it.

Remember, I got the oil pan off, radiator off, entire front end of engine is off.

[winfred]

ditto

[George M]

yup...change it. Bill R posted a good procedure some time ago to help Jeff N do it which may help if available in the archives or if Jeff would post it.
George

[Hector]

I'll get my butt in gear to start chipping away on this task.

Thanks much.

[Hector]

or something that's holding the crankshaft sprocket in place before I pull the thing out. It's been a few weeks since I 've been up in that part of the engine and don't recall seeing anything that would indicate such a thing.

[Jeff N.]

Bill and others helped my through this a bit ago. Not too hard really but I did have the engine out of my car.

- removing the sprocket. The sprocket's held on via friction fit combined with a woodruff key or slot (can't recall which) to keep it from rotating. I pulled off the sprocket with a sears gear puller centered on the crank nose. Put the puller arms on the teeth of the sprocket until I pulled it forward enough to get the arms behind the sprocket. Tapped on the gear with a ballpeen hammer to help get it moving. The gear itself is very soft metal so expect that you'll basically ruin the current gear by doing this (there's no way back once you start pulling it).

- replacing the gear. This was fun. You'll need an old qt size pot, a quart of synthetic motor oil, a campstove or similar, a candy thermometer and a good set of pliers. Basically, you'll want to heat the oil bath and sprocket to about 300 degrees (Bill- that sound about the right temp?). Grab (carefully as so not to bung up the teeth - it's soft) the sprocket from the oil bath and quickly slide it onto the crankshaft. Hold it in place and wait about 30 seconds for it to cool. That's it.

Cheers!

Jeff

[Hector]

pulling the sprocket out will be easier than installing it. I gotta go back to the car and plan a method of attack to get this thing done. Also, I just can't go banging on the sprocket with a hammer upon installation as I will definitley mess up the gear. How did you slide the sprocket back into position? Did you have something soft like a piece of wood or something over the gear to prevent it from getting all banged up from the impact of the hammer?

[Bill R.]

pulling the sprocket out will be easier than installing it. I gotta go back to the car and plan a method of attack to get this thing done. Also, I just can't go banging on the sprocket with a hammer upon installation as I will definitley mess up the gear. How did you slide the sprocket back into position? Did you have something soft like a piece of wood or something over the gear to prevent it from getting all banged up from the impact of the hammer?

[Hector]

That is too easy and shweet. Now I'm looking forward to installing this. So I was wrong, installation is easier than removal.

Yup, I see now. "Hold it in place and wait about 30 seconds for it to cool" is the give away.

Thanks Bill

[Jeff N.]

New and old cranksprocket gear side by side. You see the spots on the old one where it was damaged during removal from the hammer tapping. Note the wear on the teeth too.

http://home.comcast.net/~jsnord/pics/gears.jpg

Old engine front...

http://home.comcast.net/~jsnord/pics/eng_front.jpg

With new chain, sprockets, pump and guides...

http://home.comcast.net/~jsnord/pics/eng_front_new.jpg

[Jeff N.]

....not sure I'd suggest using newspaper quite this way.. :)


http://home.comcast.net/~jsnord/pics/gear_heat.jpg

[Bill R.]

folgers or maxwell house coffee can with about a quart of oil in the bottom
and one of those 2 burner electric hotplates with the spiral wound heating
elements to provide the heat... Temperature control was fairly simple this way...











....not sure I'd suggest using newspaper quite this way.. :)


http://home.comcast.net/%7Ejsnord/pics/gear_heat.jpg

[Unregistered]

This topic comes up periodically on Click & Clack--The Tappet Bros.: To make the gear fit more loosely to slide it onto the shaft, would it not be better to freeze it? Freezing makes the metal molecules a bit smaller, causing them to contract in on themselves, such that the hole in the gear gets a bit larger, i.e., looser on the shaft.

Last week, while rebuilding an alternator, I found the bearings and sheave would not readily fit on the shaft, until I froze the whole lot in my refrigerator. Then, while still frozen, no problemo.

Now, the crankshaft on an M30 cannot be put in the freezer. But, squirting the part with compressed gas or liquid nitrogen will cool it (latent heat of evaporation), and/or it could be wrapped with a Baggie filled with (dry?) ice.

Comments, anyone?

[Bill R.]

the crank gear and the crankshaft you want to heat the gear to expand it which makes the hole larger and cool the crank, however the crank in the engine assembled has a lot of thermal mass and you'd waste a lot of compressed gas or liquid nitrogen trying to significantly cool the crank down and its uneeded anyway, heating the gear in an oil bath greatly expands it and it slides right on.. Now when pressing bronze valve guides into a aluminum head we used to chill the guides in a nitrogen bath or if the shop didn't have that capability put them in the freezer overnight and then heat the head up in a oven... This helped to avoid splitting the guide boss on the head when pressing the guides in. But its uneeded for crank gears.
forgot to add that chilling the gear shrinks the hole diameter not increases it








This topic comes up periodically on Click & Clack--The Tappet Bros.: To make the gear fit more loosely to slide it onto the shaft, would it not be better to freeze it? Freezing makes the metal molecules a bit smaller, causing them to contract in on themselves, such that the hole in the gear gets a bit larger, i.e., looser on the shaft.

Last week, while rebuilding an alternator, I found the bearings and sheave would not readily fit on the shaft, until I froze the whole lot in my refrigerator. Then, while still frozen, no problemo.

Now, the crankshaft on an M30 cannot be put in the freezer. But, squirting the part with compressed gas or liquid nitrogen will cool it (latent heat of evaporation), and/or it could be wrapped with a Baggie filled with (dry?) ice.

Comments, anyone?

[Hector]

Thanks for the pictures, too. This is definitely compelling evidence why I should change the crankshaft sprocket. Wow! Gear has been gnawed by the chain over time.

We got some hot plates here at work but it looks as though they only go up to 200 C. I gotta check and make sure. Looks like I may have to buy a torch... no problem.

Man, you guys got me "cranked up." Can't wait.

Bill, you are right about the expansion of metal bit you posted. I deal with this sorta thing on a weekly basis.

Unregistered guest: Molecules typically don't get small, unless they are going through some extreme reaction--like fusion.... It's the spacing between molecules that gets small. For what it's worth, compressing a gas will decrease intermolecular distances thus going through a phase change--gas to liquid... Geeze, my nerdy geek side is surfacing again. Need to "supress my emotions." ;o)

Thanks Bill & Jeff for the good stuff.

[Robin-535im]

....not sure I'd suggest using newspaper quite this way.. :)


http://home.comcast.net/~jsnord/pics/gear_heat.jpg

My wife gave it to me after it dropped and chipped the enamel. White Chantel 3 qt.

I used to work in a place where we assembled turbomachinery, and heated press fit was fairly common. We'd put the outer piece in an oven at 400 degrees, the inner piece in a dry-ice bath and wear big welding gloves to fit it together by hand. But if you messed up - what a mess. Hot and cold metal can come to near thermal equlibrium in seconds, and if it wasn't lined up right you practially had to throw it away and start again because once it stuck, it stuck forever.

[Robin-535im]

Like Bill said: Heating a ring with a positive coefficient of thermal expansion makes the inner diameter grow, not shrink :) You can cool the shaft though, but it's not as good as heating the sprocket.

[Unregistered]

OK, lemme get this straight:

1.) A ring, when cooled will shrink in onto itself, making the inside diameter larger, so it will slide more easily onto a shaft of given outside diameter.

2.) A sprocket is a ring.

3.) Nevertheless, heating the sprocket works best, in spite of what item 1 says.

Seems to me, upon chilling, the ring (or anything but water, which expands on freezing, unlike 'most everything else in the world) would contract in onto itself, so its inside diameter would be larger, and its outside diameter would be smaller. The constituent molecules are clueless as to inside, outside, up or down: They just contract into a smaller space. This is essentially what Tom and Ray (the Tappet Bros. on Sat. morning NPR's "Car Talk" show) say. Tom or Ray has a master's degree in physics from MIT and they run a car fixit shop, so they seem like a creditable source.

As this is an extremely handy thing to know, can somebody please micrometer-measure a sprocket, ring (or whatever) at room temp, at heated temp, and at freezing temp, and tell us all the difference(s)?

[Bill R.]

Thanks for the pictures, too. This is definitely compelling evidence why I should change the crankshaft sprocket. Wow! Gear has been gnawed by the chain over time.

We got some hot plates here at work but it looks as though they only go up to 200 C. I gotta check and make sure. Looks like I may have to buy a torch... no problem.

Man, you guys got me "cranked up." Can't wait.

Bill, you are right about the expansion of metal bit you posted. I deal with this sorta thing on a weekly basis.

Unregistered guest: Molecules typically don't get small, unless they are going through some extreme reaction--like fusion.... It's the spacing between molecules that gets small. For what it's worth, compressing a gas will decrease intermolecular distances thus going through a phase change--gas to liquid... Geeze, my nerdy geek side is surfacing again. Need to "supress my emotions." ;o)

Thanks Bill & Jeff for the good stuff.

[Bill R.]

mit..... One had a degree in economics and the other had a degree in humanities... No physics degrees though.... A ring when cooled shrinks ID and OD...






OK, lemme get this straight:

1.) A ring, when cooled will shrink in onto itself, making the inside diameter larger, so it will slide more easily onto a shaft of given outside diameter.

2.) A sprocket is a ring.

3.) Nevertheless, heating the sprocket works best, in spite of what item 1 says.

Seems to me, upon chilling, the ring (or anything but water, which expands on freezing, unlike 'most everything else in the world) would contract in onto itself, so its inside diameter would be larger, and its outside diameter would be smaller. The constituent molecules are clueless as to inside, outside, up or down: They just contract into a smaller space. This is essentially what Tom and Ray (the Tappet Bros. on Sat. morning NPR's "Car Talk" show) say. Tom or Ray has a master's degree in physics from MIT and they run a car fixit shop, so they seem like a creditable source.

As this is an extremely handy thing to know, can somebody please micrometer-measure a sprocket, ring (or whatever) at room temp, at heated temp, and at freezing temp, and tell us all the difference(s)?

[Robin-535im]

Check out this link: http://www.newton.dep.anl.gov/askasci/phy00/phy00741.htm

BTW: I also have a Master's Degree from MIT. In Mechanical Engineering though, not Economics.. :)

[Hector]

who didn't know the difference between a ball point hammer and a claw hammer. Some are just book smart but have no common sense for stuff in everyday life.

About your experiment of measuring the expansion of a material, you can probably do this yourself with Al. Al has a CTE of 24 microns/m C which is one of those materials that loves to expand/contract. Just stick an Al ring in the freezer measure ring DIA w/ ruler in mm, then heat it up to 100 C and measure ring DIA with ruler again. You should see the difference, and should measure a difference of about 2 mm for the ID from 0 to 100 C.

[Hector]

you'll measure about .2 mm (about 8 mils) so you'll probably need a caliper for the measurements.


who didn't know the difference between a ball point hammer and a claw hammer. Some are just book smart but have no common sense for stuff in everyday life.

About your experiment of measuring the expansion of a material, you can probably do this yourself with Al. Al has a CTE of 24 microns/m C which is one of those materials that loves to expand/contract. Just stick an Al ring in the freezer measure ring DIA w/ ruler in mm, then heat it up to 100 C and measure ring DIA with ruler again. You should see the difference, and should measure a difference of about 2 mm for the ID from 0 to 100 C.

[George M]

As Bill says, a ring when cooled shrinks I.D. and O.D. and conversely when heated expands I.D. and O.D. The best way to rationalize why a ring I.D. shrinks versus grows when cooled is...visualize a single chain of atoms around the I.D. of the ring...a diametral chain only one atom thick. When cooled, the distance between the atoms becomes less reducing the I.D./length of the chain comprising the hole. Conversely, when heated, the distance grows increasing the I.D. of the hole. The O.D. of the sprocket is a bit more intuitive but the same principle applies.
On that note, I haven't seen any good porn on this site in a long time. Someone help me out.
George

[Unregistered]

Well, both Tom & Ray attended MIT, but I was mistaken about their degrees, which apparently did not include physics, per Google info. I have emailed my question to their Car Talk website, and hope they answer me from among thousands of questions. We'll see.

My recent empirical experience with pressing a bearing on an alternator shaft demonstrated that freezing works--it went on faily easily, despite difficulty during room-temperature attempts.

Using keywords "steel thermal expansion" on Google found various interesting websites, with info about thermal coefficients of expansion of various materials. For example, one noted that a copper disk with a hole in the middle (sorta like an old music short-play 33 rpm vinyl record or a modern CD) would enlarge the hole when the disk is heated. That jibes with what others are saying here on Bimmer.info. Granted, steel, copper, and everything else have different thermal expansion properties/coefficients, but presumably behave in essentially the same way, albeit at different rates.

Remember when the Challenger space shuttle blew up? That was due to thermal contraction from over-chilling of a large (admittedly rubber) O-ring. The O-ring material got skinnier and its sealing function failed, letting hot gas blow past. The rest is sad history.

It seems conceivable that both heating AND chilling may work to enlarge the doughnut hole. Heating by virtue of making the entire piece grow, pulling the inner diameter outwards in the process, thereby increasing ID. Chilling by reducing the gap between molecules, causing the inner and outer surfaces & diameters to converge slightly, also increasing ID.

Curious, I'm gonna get my kid to work this problem in his HS physics and metals classes--heated, chilled, and ambient measurments of a used bearing ring. Before & after measurments in each temp. scenario. However, the teachers/kids are busy at school's year-end, so this may take awhile.

[632 Regal]

porn is a good thing!

(_o_) (_o_)