20mm Nut for Primary Chain Sprocket on Crank: How Tight?

cfloren · Post by **cfloren** » Mon Dec 07, 2009 6:47 pm

I've searched all through the engine forum and cannot find any mention of "how tight" to tighten the 20mm "four-prong" special nut on the left side of the crankshaft. This is the nut that has the lock-tab washer, and retains the spring washer, oil filter sprocket and primary chain sprocket.

I know there are two preferred methods for removal/installation:
1) Chisel
2) Modified socket

Also I know the spring washer needs to be installed with the high middle outwards: http://www.honda305.com/forums/viewtopic.php?t=2738

My question is: how far do you guys go when tightening? Do you tighten this nut until the spring washer is fully compressed? Is there a torque spec for it? Just for a ballpark feel...I wouldn't necessarily use a torque wrench to tighten it.

Thanks in advance.

Goodysnap · Post by **Goodysnap** » Mon Dec 07, 2009 9:05 pm

I tightened mine finger tight then took it to the next notch to lock the tab in the nut. It was plenty tight after 1/4 turn to the next notch. At twenty MM I dont think you would be able to over torque it. But loose would be bad. Make sure to take the time to make sure the washer is in place correctly and it tightens down the oil filter drive gear. If crooked the gear can spin even though the nut is tight. I found mine this way when I took it apart.

JJJake · Post by **JJJake** » Mon Dec 07, 2009 10:00 pm

I just searched through the Honda 1961-68 C72/77/CS/CB Service Manual and didn't find any torque specs. Sorry.

cfloren · Post by **cfloren** » Tue Dec 08, 2009 11:41 am

Goodysnap: you mentioned exactly the reason I'm wondering about this. I have heard several accounts (now including yours) of the oil filter sprocket spinning free on the crank, as a result of this nut loosening. It seems to me it would self-tighten, if anything, since the crank rotates counterclockwise as viewed from the L side of the engine. Although I suppose rapid deceleration (like chopping the throttle suddenly) could cause gradual loosening of the nut.

JJJake: thanks for looking. I searched through the Honda service manual too and the installation section wasn't any help. It says something like "After tightening the nut perfectly, apply more torque until the tab aligns with washer, without ever loosening the nut."

Now I just need to know what they mean by "perfectly!"

Goodysnap · Post by **Goodysnap** » Tue Dec 08, 2009 12:07 pm

I'm assuming that mine was assembled incorrectly at some point. Reason being the lock tab was still in place and the nut was tight(took a bit to loosen it). The positioning of the washer is the problem area here(i think). If its not centered on the the shaft facing the correct way, the washer tightens down on the shaft splines instead of the drive gear, causing the gear to spin freely. I would not think that loosening would be an issue, however , a dab of threadlocker I'm sure would not hurt a thing.

jensen · Post by **jensen** » Wed Dec 09, 2009 7:20 am

Hi,

You're right, there's no value given for the torque in any CB72, C72, CB77 and C77 documentation, but there's no need to because the torque for a given crank nut (Honda) is always around 100 Nm.

Following the standard iso, it is around 350 Nm (for M20*1.0, steel quality between 6.9 and 8.8), but thats way to high for this crank because it's a built-up one, and not necessary either.

For U.S, U.K and other non metric and non ISO countries :

1 Nm = 0,738 ft-lbs, so 100 Nm will be 73,8 ft-lbs

Jensen

cfloren · Post by **cfloren** » Wed Dec 09, 2009 12:02 pm

Thanks guys! I think my questions are answered now.

Goody: I'll use some threadlocker as a safety measure, and I'll be sure everything is aligned as much as possible.

Jensen: Is 100 N-m a standard torque value for this type of nut on similar Honda cranks? Is that just a "known" value and not published anywhere?

Also thanks for the conversion to our wonderful Imperial system. As a mechanical engineer in the US auto industry I'm constantly converting to metric (SI) and back to Imperial, but most people (even most engineers) in the US still think in Imperial units only. We are a stubborn people!