Topic: primary drive chain failure?

[wenger828]

how common are failures with the primary drive chain? i know people on here will probably say replace it while everythings apart, but this is a budget build, and personally, they look perfectly fine to me..

[Prospect]

Which bike?   

I asked my machinist who has been working with cb's for 30 years the same question.  He said not to worry about it and if it looks fine and measures fine then use it.  But, because I wanted everything new in the engine and not have to worry about anything while I'm around riding I replaced it anyway. 

[MCRider]

Only a worry to guys like KOS, MEC, Voxonda, etc.

I've Never seen it happen on a non-track bike incl. big bores.

[mec]

never seen a primary drive chain failure.
but think of the fact, that a lenghtened chain will wear out the drive sproket (crankshaft) faster.

mec

[TwoTired]

They don't normally fail, as in part links.
The 500/550 has no tensioner, when chains wear out, they get bigger under revs and begin to chew on engine cases.

If you told us what bike you were concerned about, we could give better and relevant advise.

[wenger828]

sorry, its for my 78' cb750F.. it looks perfectly fine to me and the roller tensioner as well honestly.. the bike only has like 8k miles on it

[madmtnmotors]

Got 60,000 on my 78' 750F. Others have logged many more, still going strong.

[01Thomas]

I _know_ that a primary drive chain has failed on my CB750 K1. I know this because way back when I opened the engine up in 1984 the damage to the crankcases was evident.
I also _know_  that my bike is on it's third camshaft, because when I saw the damaged crankcase webs I also found a broken mounting lug of the original camshaft in the recesses of the motor.

I don't think my bike had been on a track - it still wore its' 4 into 4 exhaust pipes and they were definitely not the height of fashion in 1983. I also never found any evidence of cosmetic damage like racebikes usually have.

Some history to explain the above: I bought the bike used in 1983. After some 2000 km it seized the camshaft in the left-most cam bearing. Stuck a million kilometers from anywhere, I didn't know why the motor was so tight and so I did the only thing I knew: kicked the kickstarter and kicked again and again for probably 30 minutes until all of a sudden the motor freed up and started and ran, but noisily. In my ignorance (I was 22 years old then) and running against a deadline I put another 1500 km on it before I could open it and examine the damage.

That is when I found the camshaft in two pieces... (I still have them, chromed). Then I thought it a good idea to strip the engine completely to remove all the accumulated home-made grinding paste and that's when I found the damaged webs and the broken camshaft fastening lug inside the crankcase.

[Bailgang]

I also never found any evidence of cosmetic damage like racebikes usually have.

On it's 3rd cam and 2nd primary drive by 1983? That wasn't enough evidence to prove that somebody absolutely thrashed your bike in 1 way shape or form before you got it?

[tlbranth]

The primary chain runs under pretty ideal conditions, no dirt, constant lubrication. If it measures with no stretch, I'd leave it.

[madmtnmotors]

On it's 3rd cam and 2nd primary drive by 1983? That wasn't enough evidence to prove that somebody absolutely thrashed your bike in 1 way shape or form before you got it?

Neglect, such as 5000+ miles between oil changes, running low on oil, etc... qualifies as "thrashing"

[01Thomas]

Certainly it was thrashed - but I doubt that it was raced on a track.

Now it's some 31 years later and still in almost original condition: I fitted a 2nd disc brake in the early nineties and I've recovered the seat and resprayed the bike after a prang - still the original Candy Gold. It shares the daily-driver duties with 5 other bikes.

[Jerry Rxman Griffin aka MuthaF'er]

Gotta say I ran the #$%* outta mine on the street for 30,000+ miles (not that much). Bought it new by the way. Blew it up after it's first 6000 miles. Valve from missed 1 to 2 shift. Next 25,000 miles with a Yoshimura kit and a heavy wrist. My engine rattled. Cam chain probably, primaries probably, clutch no doubt. Was my primary tensioner worn and a weak spring - probably. Were the primary chains stretched - probably. But those chains were in one piece after 35 years abuse. Did I just replace all that stuff - yes. When you spend $7000 on an engine, yes you replace wear stuff regardless. Does it rattle now - NO. Didn't think it'd EVER be that quiet  Aftermarket heavy duty primary chains are available - for a price. I believed in the new stock chains enough I spent the difference on a new cam. Yes, my bike should have 30 more horsepower and I will use those hp but not like/as much as I abused it in it's first 35 years. I'm not worried 

[Bill/BentON Racing]

Used but good,OK, not racing!
Racing, new OEM OK!
Racing,Big H/P,Big Money,H/Duty recommended
As they said...never seen one break,they DO stretch!! Bill

[KRONUS0100]

i also have had 1 fail on my original 76 7foF motor. this was 8 years ago and  i was new to 750 hondas and everyone kept telling me it was cam chain noise.  after a full season of riding it, i finally decided to see if i could adjust it right 1 more time before settiling on a replacement cam chain.  i took it out and really rode the heck out of it and made sure the motor was at hot operating temps, brought it back in drive and put it on center stand in neutral with motor running at idle.  I adjusted it 2 or 3 times as per my clymer manual, and all of a sudden the motor went totally silent.  no noise from clutch basket, no noise from cam chain, and im sitting there thinking hot damm finally got it right. as i was snugging up the tensioner bolt, it threw both primary chains thru the bottom of the case along with the tensioner and chunks of the oil pan.  it was primary chain noise the whole time.  needless to say this is what led to the 1972 K motor being in the bike, which is being replaced now by a very low miles 78 F3 motor.

[MCRider]

Wow! So you're the one!

[MRieck]

I stretched out a set in relatively short order @ close to 100RWHP. They will clank around at idle and bounce off the tensioner (causing jerky movement) occasionally when taking off from a stop. Those are symptoms.

[Eydugstr]

how common are failures with the primary drive chain? i know people on here will probably say replace it while everythings apart, but this is a budget build, and personally, they look perfectly fine to me..

Never seen a primary chain fail, but have seen some very chewed up cases from where tensioners did!

As far as reassembling them goes, make sure to put them back running the original direction, and replace the primary tensioner.  Hondaman covers it in his book, think the Haynes manual did, too.  The tensioner's roller is made from a hard black plastic, which will show wear after 30+years.  Before I removed the chains, I went ahead and tagged & marked them (left/right & direction)