Topic: seeking advice re cam-chain tensioner... 75 cb400f

[joe camarda]

I have a 75 cb400f that I am doing a mild-cafe build on. The engine is out of the frame and knowledgeable friends tell me that I should replace the cam-chain tensioner as that is considered a weak point on this bike.

So my questions:

Is there an upgraded tensioner available?
Should I just replace it with a new OEM part (even assuming it is still available)?
Is there a modification which would reduce the likelihood of the part failing?

FWIW the engine has approx 20K miles on it.

TIA for advice and comments.

[Frostyboy]

I realise that the 400 tensioner is nothing at all like in my 550, however reading posts revealed by the search function up the top of this page, it seems that similar to the 550 all hell can break loose if too much tension is applied to the adjusting bolt or locknut & damaging the thread.
Be gentle with the pressure you apply to the bolt & locknut. Understand that similar to the 550 the adjuster looks after itself, more pressure on the bolt doesn't increase the tension on the chain. It simply holds the tensioner in the right spot after the spring takes up the slack.

Do you have the manual to follow?   http://www.mediafire.com/?mnglmnn1qmd

[camelman]

I echo prior comment on the tensioner bolt.

You'll need new slippers and a new chain. The old chain has stretched which will affect power and possibly may break.

[Frostyboy]

You'll need new slippers and a new chain. The old chain has stretched which will affect power and possibly may break.

Like I said, I don't have a 400, but camelman, are you saying that a 400 has to be pulled down every 20k for a cam chain?

Joe, is the chain noisy even after adjustment?

[HondanutRider]

I believe that the problem with the 350 Four and the 400 Four (which are essentially similar motors except for the cylinder size) is that the spring for the cam chain tensioner is barely adequate to take up the proper tension for adjustment.  It means that many times the chain is too loose, even after following the original "standard" method of adjusting the tension.  If you do some research, then you will find a method recommended to remove the upper plugging hole, above this adjusting spring, in front of the motor and above the slackening bolt with locknut.  By inserting, say a screwdriver into this hole you can press down on the spring mechanism to add more tension to the cam chain when adjusting it.  You have to be careful and not add too much tension to the mechanism for the chain - just enough to quiet its running.  You also can replace the bolt in the plugging hole with a longer one, fashioned to allow it to provide the additional pressure on the mechanism while you adjust the tension, rather than trying to fiddle with a screwdriver stuffed in the hole.

Also, as has been previously warned, don't over-tighten those locking bolts or they can strip their threads in the cylinder.

[camelman]

Well written hondanutrider.

To the original poster, I was just being brief since I hate typing on my iPhone and my Internet is down. If you are NOT planning on pulling the engine apart, then just replacing the slippers is a good idea since they will be worn by now. The chain will last many more miles, but it will have stretched since it was originally installed. that stretch will affect camshaft timing as compared to crankshaft rotation giving a little delay in opening the valves. The engine will run fine, but it will lose a tad bit of power.

If you decide to pull the engine apart to investigate it, then replacing the chain is a no brainer. Alternatively, you could leave the engine together and replace the camchain with a master link style chain.

So, the question is what all you have planned for the engine. If you want to keep engine work really simple, then just replace the slippers. If you feel like replacing the chain, then you can go with a masterlink style chain without disassembly of the engine. If you decide to get into the engine (you would only need to pull the valve cover, pull the camshaft, pull the clutch components and oil pump off, pull the top case bolts, flip it over and pull the bottom case bolts) then you can replace the chain with an endless chain. Be forewarned, reassembly is a little tricky. That would also give you a chance to replace all the engine seals and check first gear to see if the first gear bush is worn.

Any of those options is fine. I know my way around these engines now, so replacing the chain with an endless style chain (considered to be stronger) would only take me a couple hours. If you aren't familiar with engines, then it would likely take you a whole weekend.

Regarding the first gear bush, the center will wear and the gear will become slightly loose on the shaft. This can be fixed easily by a machinist which will make shifting much smoother.

Again, your original plan of just replacing the slippers is fine too. Make sure to check the slippers for wear first in case you do not need to change them. Info on that should be in any cb400f repair manual.

[lucky]

I have a 75 cb400f that I am doing a mild-cafe build on. The engine is out of the frame and knowledgeable friends tell me that I should replace the cam-chain tensioner as that is considered a weak point on this bike.

So my questions:

Is there an upgraded tensioner available?
Should I just replace it with a new OEM part (even assuming it is still available)?
Is there a modification which would reduce the likelihood of the part failing?

FWIW the engine has approx 20K miles on it.

TIA for advice and comments.

It is not just the mileage. It is the number of years the part has been in the engine.
The rubber rollers get old. Some owners have decided to just keep using the old cam chain and later found many little pieces of rubber in the oil. Those little pieces of the cam chain rollers can plug up the oil galleries.

The CB750 is different though because it has a dry sump oil system. Only part of the rubber bits can go through the oil pump screen to the engine. On a non dry sump engine the rubber bits can go through the oil filter and clog it up and other oil galleries in the cams and crankshaft oil holes as well as the cylinders.

Think about the AGE of those rubber rollers.

[HondanutRider]

Contrary to what Lucky says in his post, I don't believe that the 350/400 motors have rubber rollers for the chain tensioners.  What they use instead are some sliders or slippers that Camelman mentions.  I think they are made of a hard nylon-like material, made to flex so that they provide tension as they rub against the chain.  They do wear and when they need to be replaced.  I don't think that they "chunk off" into pieces with age, as the rubber rollers would tend to do.

[jessezm]

That's right, there are no rubber rollers in the 400f, but there is a rubber coated guide/slipper and tensioner that can get chewed to some extent and generally worn by a loose chain.

A lot of great info above and I don't want to replicate it, but one thing I would add that should go into your decision on whether or not to tear down the motor is if the tensioner arm itself is seized.    This happens when the chain doesn't get adjusted or the adjusting spring doesn't do its job and you end up with a loose chain.  The chain slaps around in the cam chain tunnel and literally chews into the hinge of the tensioner arm, effectively seizing it into place and preventing future adjustment.  You can test this by loosening the locknut on the front of the engine, pulling the cam cover, pulling out the tensioner, and pushing down on the arm (which pushes up on the bottom of the tensioner) with a long screwdriver or something.  You should feel it give with good resistance and spring back.  If you push and it doesn't give, or it gives after much cajoling but doesn't spring back, you can bet that it's seized and will need to be replaced (full tear down).  There are a few boutique outfits that provide a nice replacement for the arm, which is not available from Honda (TTR400 and Phil Denton Engineering to name two)

If, however, there is free movement and good tension in the arm, I would do as Camelman suggested and replace the tensioner and guide (which are still available), as well as the chain, which is easy to replace from above with a split link by attaching the new to the old chain and turning the crank.

Nuff said?