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rebuilt engine - now knocking noise

Discussion in 'Technical and Troubleshooting Torque' started by damo03, May 8, 2009.

  1. So I did a top end rebuild on my old girl (1981 CB400 Twin). Gave her new rings, pistons and got valves re-ground and valve seats reground as well as new stem seals.

    Put her together and upon firing up there is a knocking noise that is like a tappet noise but louder and slightly deeper. I have rechecked valve clearances but it's not that. Engine revs freely.

    I'm certain that the camshaft, timing, pistons are all correct orientation and alignment. The noise is NOT present If i crank the engine without spark. I gave all the valves a massive clearance and ran engine but I couldn't tell if the noise was gone as the tappet noise was too loud.

    It's mystified me. All I can think of is that when I got the valves ground the machinist asked me if i had valve shims or a screw adjustment as he had to grind a lot of the valves away, and thus the valve stem protrudes alot further. As I have screw adjustment I can't see why this may cause a problem, although perhaps someone my know if it would?

    Any helps or hints would be appreciated. As i already said, the engine runs fine apart from this noise, and although it appears to be up the top, as I have never had any noises to compare I am unsure if it is coming from the barrel / pistons / rings.



  2. H'mmmm :? Something's trying to get out...

    Turn the engine over by hand with the rocker cover off (use the nut under the right hand cover) and see if you can narrow down the area the noise is coming from.

    Not sure what "without spark" means. If you mean with the plugs out, then make sure you have the correct spark plugs fitted, some long nose ones may touch the piston crowns, esp if you have after market ones which may have a different crown height/shape. Hope you put the pistons in the right way round, from memory these Hondas have three valve heads and a offset s/plug, so maybe the plug is touching the piston crown if the cut-outs are in the other side. Just wild guessing as I haven't worked on one of these for about 20yrs....

    Look at your valves - Maybe one is sticking open and is "kissing" the piston. Or the length is too long and they are not closing fully (ie. you need new seats).

    Also check all your cam caps are tightened up. Can't think of anything else for now.
  3. cam chain tensioner?
  4. Yes, mine did that too. Twice on two different engines. The second time just after a top end rebuild.

    The crank seized a couple of hundred miles later on both occasions.

    Sorry :( .
  5. Thanks for the reply's people. I pulled the head off and found that the pistons I was sold are not correct! The pistons I was supplied do not have a two little indentations in the crown (as the old pistons do). This is where the piston is hitting the head. For the second time in 12 months I have been sold the incorrect parts for a motorbike engine.

    I ground the head a few mm where it was hitting (which is as much grinding of the head all I was prepared to do) and put it back together, but the flaming piston still hits when the engine is running.

    What I suspect is that there are 3 different series of my engine and the pistons are for a different serial number of the same engine. So now, I need to work out whether to grind a couple of indentations in the piston crown as I don't want to grind the head any more.

    Thanks alot bloody hopeless retailers! there's 2 full day's of work down the bloody drain. I'm off to have a beer. The bike can wait for another week. (back in the bloody cage for another week) :(
  6. Get the pistons "fly cut"
  7. yeah, give a fly a little file and send him thru the plug hole and tell him where to start, it takes a while but if he knows his s h i t he will buzz thru it...........
  8. nothing worse than being sold the wrong parts..

    other than when they're in you engine already..

  9. That sucks mate...
    but the standard reply from the supplier would be:
    " It is the responsibility of the installer to ensure that all components supplied are correct for the application"
    Sorry :cry:
  10. That seems perfectly reasonable to me, since he did his own rebuild. A quick comparison of the old and new pistons would have shown the difference immediately. No sympathy here mate.

    I certainly would not have ground the head, since putting metal back on is much harder, and if it didn't work, which it hasn't, you have now changed the compression ratio and the firing chamber volume for the correct pistons. If you grind the pistons, you will weaken them, and possibly end up burning a hole through them in the thinner section. That would be very bad.

    Didn't you turn the engine over by hand once it was fully reassembled and check that everything moved freely? While the knocking would have been less obvious, it should have been detectable.

    I would have, and would now, pull the pistons out, and get the correct ones. You know that is what you need to do. You may get something back on the incorrect pistons, but not if you grind them, or if the battering they have been receiving has resulted in damage.
  11. I've been mulling over whether to continue to be tactful or whether to give you the advice I wish someone had given me when I was pouring money into an old CB400 twin. Always supposing I'd taken it of course :) . But I can't hold out any longer.

    Make it work as well as possible without spending any more money or serious effort, sell it for whatever you can get, in cash, then move cities and leave no forwarding address. Then do whatever you have to to afford something - anything - else and never, ever, buy a Honda again.

    It may seem harsh, but your psyche and your wallet will thank you in the end :wink: .
  12. :?

    ....... :popcorn:
  13. I'm no mechanic... Just mucked around with old 1980s cars and not too much into the engine but why didn't you compare the new and old pistons before paying for them?

    If I were to try and rebuild an engine i'd be making sure all the bits looked the same.