Topic: So...I've got this motor, right.

[MattFreeman]

I have a 1976 CB750F with 70000 miles (so I've been told). Am I kidding myself in believing that the primary/cam chains and sprockets wont be worn out?

Is there a way to measure wear on the sprockets aside from visually checking for "ramping"? I assume there are length measurements that will tell you if a chain is worn.

Do you plan on changing the crank and sprockets if your chain is worn?

Do these parts typically wear out?

Are the worn sprockets tolerant of a new chain?

I've opened the proverbial can o'worms and am looking for a little hope that I wont have to replace EVERYTHING.

Just to be clear, I'm referring to the chains and sprockets inside the crankcase/bottom end.

TIA
Matt

[Toxic]

Was the engine running when you got it? Do you know anything about the history of the engine?  Have you ever heard it run?  A lot can be determined if you can give it a listen.

If no to all of the above and you have just happen upon an engine with 70K on it you have two choices.
A/ cross your fingers and instal it and hope for the best.  Maybe the engine has already under gone some internal maintenance.

B/ Crack it open and have a look.

But with 70K I would be shocked if all the regular items didn't need attention.  Cam chain, drive chain, cam chain guides, etc. etc.

Which brings us to the "might as well" theory of motorcycle repair, since you are already in there you might as well replace all the high wear items.

[MattFreeman]

Was the engine running when you got it? Do you know anything about the history of the engine?  Have you ever heard it run?  A lot can be determined if you can give it a listen.

But with 70K I would be shocked if all the regular items didn't need attention.  Cam chain, drive chain, cam chain guides, etc. etc.

Don't know much about history...

I guess my question is..... if I do change the cam/primary drive chains (I plan to) do I have to plan on changing the internal sprockets?

What I'm trying to figure is do I need to plan on changing the crank as I assume that the sprockets are permanently attached.

[mick7504]

There is probably no need to be concerned about the crankshaft and primary drive sprockets.
They are usually quite resilient to a reasonable amount of punishment.
A new set of primary chains, tensioner, cam chain, rollers, chain guide and cam sprocket are a worth while investment.
http://www.cyclexchange.net/Engine%20Parts%20Page.htm
http://www.z1enterprises.com/catalog.aspx?pid=CB750F
Take off the cam chain adjustor and clean and lubricate the adjustor shaft.
Clean out the oil pump and check it for excessive wear.
It is the heart of the motor.
A good tune up and it should run hard providing everything else is in reasonable condition.
Clean oil and filters are essential.
Mick


 
 

[mystic_1]

I have yet to see anyone post here that had their crank sprocket teeth worn past service limits.  In fact, you rarely hear about primary chains that are "too worn to use" unless they are actually broken.  Some people have mentioned having one primary chain worn slightly more than the other.  There's a measurement procedure for checking the chains by measuring the distance between the oil sump pan mating surface (on the crankcase) and the primary tensioner arm.  That'll tell you how bad off your chains are.  Check your manual for this procedure.  Odds are that your crank and transmission sprockets are OK unless there's some other damage present.

mystic_1

[MCRider]

There is probably no need to be concerned about the crankshaft and primary drive sprockets.
They are usually quite resilient to a reasonable amount of punishment.
A new set of primary chains, tensioner, cam chain, rollers, chain guide and cam sprocket are a worth while investment.
http://www.cyclexchange.net/Engine%20Parts%20Page.htm
http://www.z1enterprises.com/catalog.aspx?pid=CB750F
Take off the cam chain adjustor and clean and lubricate the adjustor shaft.
Clean out the oil pump and check it for excessive wear.
It is the heart of the motor.
A good tune up and it should run hard providing everything else is in reasonable condition.
Clean oil and filters are essential.
Mick
 

I'd say Mick here has the best answer. I would only add, if you're that far into it, and care to learn about it, you could probably score a low mileage crank somewhere. eBay or such. May have to buy a parts engine to get it. But its not necessary. If you're building a stocker.  If its a hot rod, may want to look around. IMO

And if the engine is running, and there are no other issues, and you enjoy it, I'd continue running it as is. There have been stockers with 125,000 miles on them. Buy another low mileage engine, build it up at your leisure and swap them.

[MattFreeman]

Thanks for the opinions, I appreciate all the info I can get.

The bike ran pretty OK when I got it. Started easy, pulled ok, made a rattle in neutral (I'm thinking it was the cam chain), and had a pretty decent head gasket leak (probably for a while).

The plan, so far, is to do the cylinders and pistons. 811 or 836, 10:25 and heavy duty studs.

I'd really like to have Mreick do my head/valves. And up the cam slightly. And whatever the bottom end needs.

Kinda upgraded stock rather than a hot rod.

If I can get another 30-40K out of the majority of the bottom then I'll think about the rest when it's time to do the cylinders again. I assume that I'll want to do the bearings but if it's not unheard of to get 100K+ outta these then, if everything looks OK, when I get the bottom cracked then I guess I'd like to save a few hundred on the crank, chains and sprockets.

[MCRider]

Thanks for the opinions, I appreciate all the info I can get.

The bike ran pretty OK when I got it. Started easy, pulled ok, made a rattle in neutral (I'm thinking it was the cam chain), and had a pretty decent head gasket leak (probably for a while).

The plan, so far, is to do the cylinders and pistons. 811 or 836, 10:25 and heavy duty studs.

I'd really like to have Mreick do my head/valves. And up the cam slightly. And whatever the bottom end needs.

Kinda upgraded stock rather than a hot rod.

If I can get another 30-40K out of the majority of the bottom then I'll think about the rest when it's time to do the cylinders again. I assume that I'll want to do the bearings but if it's not unheard of to get 100K+ outta these then, if everything looks OK, when I get the bottom cracked then I guess I'd like to save a few hundred on the crank, chains and sprockets.

Sounds like a hot rod to me. Pistons, Headwork, cam. A cam (and usually springs) would beg for a new camchain. A high compression 811 can make really goodpower.  One thing leads to another.  $$

Upgraded stock would be small stock over bore maybe to a true 750cc, decarb & cleanup head, mild valve job, maybe just lap them in, maybe a pipe.

pretty old bottom end for a hot rod, but it will work.

The rattle at neutral is often the clutch basket. Pull in the lever and if the rattle goes away, its typical and can't be fixed.  If not its likely the primary chains and we've talked about that.