Topic: 77 cb 400f crankcase interchange

[jcoryea1]

Hello all-
  first mistake..I let a friend take the 400f for a ride..when he returned, it had the dreaded camchain noise. coincidence perhaps..anyway , my mechanic scoped the internals. There has been contact with the lower crankcase and/or the "horseshoe" guide tensioner. We have not removed the engine yet to inspect the full extent of damage.
  I will probably need a horseshoe and or a lower case,if anyone on the forum has good ones. David Silver Spares makes a stainless "horseshoe"  There are several cases from '75 CB400F's, on e-bay. However, I read where the earlier cases have issues with the rear motor mount cracking,maybe incompatability for studs,etc. please advise .. I have a 14 k mile bike ,all restored, so it's worth saving.  We did try adjusting the tensioner,before the scope inspection. engine running, but didn't seem to make a difference.  now, with the metal removed, I'm thinking of a rebuild. even if there are no sparkly bits in the oil..I don't want to ruin this engine. having said that, I would be looking for GENUINE Honda parts, not Taiwan with Honda sku  #'s. Any recommendations ?  I'm in northern  NY   Thanks again.
  Let's keep these lovely old bikes running!
  John
  1976 Bonneville 750(12.5 k)
   1980 CX500 cafe (? k,but runs great..restored last winter)
   1977 Cb400f (14k)
    1972 Honda SL125 cafe (@400 miles since resto)
   1972 SL125 dualsport (@800 miles since resto)
   1972 Yamaha JT-2MX (0 miles) restored for my 18 month old grandson)
   1980 Suzuki PE175 (0 miles since resto )
   1981 Gl500 parts bike.
  No more...wife will kill me.

[Kwality]

Hi.  Yes.  If the chain is loose enough to dig into the cases it's also likely to have dug into the pivot arm  - causing it to seize.  This is not uncommon on the 400f.  A lack of periodic tensioning of the camchain means that the loose chain flaps around and digs into the pivot point of the tensioner arm, causing it to seize.  Very loose camchains can also dig/grind into the cases which causes little bits of aluminium to float around your engine in the oil - not ideal by any means. 

To check whether your pivot is seized, loosen the locking bolt, and remove the blanking bolt.  Then, with the engine running, insert a thin screwdriver down the blanking bolt hole.  This brings it into contact with the top of the adjuster.  If the pivot is not seized, then you will feel the adjuster rapidly jumping up and down.  Applying downward pressure on the top of the adjuster translates the pressure via the pivot arm to the slipper, which bows and tensions the chain. You'll be able to tell if the sytem is working by sound:  a loose chain is a noisy high-pitched rattle, so if pressure on the screwdriver lessens the rattle then retighten the locking bolt and you're good to go.

If the pivot is seized then you have decisions to make:  you could try, with the engine off, (gently) tapping on the screwdriver through the blanking hole with a hammer to try to break through the burrs on the pivot arm that have caused the seizure, but then you could well have broken steel burrs floating around your engine.  You could also live with a loose camchain and its associated problems.  Or you could split the motor and refresh the system. David Silver stocks replacement cam chains, slippers and guides.  It's unlikely that you will need a new bottom case, unless the chain has worn all the way through.

Some more detail and part numbers etc here: http://www.cb400f.btinternet.co.uk/cam.htm
Good luck!

[jcoryea1]

Thanks for the info.. ..I really appreciate it..are the 75-77 cases interchangeable?
  Thanks,
  John

[scottly]

The upper and lower crankcase halves are machined as a unit; you can't swap a lower with a different upper, regardless of year.