Crankshaft Sprocket: Replace or don't replace?

[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.



Old engine front...



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

[Jeff N.]

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

[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..

[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..

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.