Topic: CB550 white smoke after top end rebuild

[bryanj]

No more difficult than any twin i worked on and way easier than Norton or BSA! Just remember to be super clean on rebuild.

[JB Stellies]

..........but quite different to a single cylinder two stroke

[JB Stellies]

The engine is apart again and the cause of the white smoke was cracked valve stem guides. Luckily I have a spare engine and the head looks OK, crossing fingers. Thanks to all who offered advice.

[alacrity]

The engine is apart again and the cause of the white smoke was cracked valve stem guides. Luckily I have a spare engine and the head looks OK, crossing fingers. Thanks to all who offered advice.

Hey there.  Pro tip:  before you put this all back together, knock out the valve guides and put New OEM honda seals in there (little Viton o-rings) and also only use honda oem valve STEM seals (the structured ones with the little springs).  I have had to re-do top ends way too soon because of crappy aftermarket seals that harden and leak.  The result is always white smoke, most noticeable on start up.

[bryanj]

Sorry dissagree there and would NEVER remove guides unless new was being fitted as they never go back exactly the same so a valve job is then needed. Never ever had a "leak" from the guide external O ring but many broken stem seals.

READ THE MANUAL about refitting valve cover then go back and READ IT AGAIN until you fully understand what can happen if you do not hold the cam followers up.

Not doing that is probably why your original guides were cracked!

[calj737]

The engine is apart again and the cause of the white smoke was cracked valve stem guides. Luckily I have a spare engine and the head looks OK, crossing fingers. Thanks to all who offered advice.

Hey there.  Pro tip:  before you put this all back together, knock out the valve guides and put New OEM honda seals in there (little Viton o-rings) and also only use honda oem valve STEM seals (the structured ones with the little springs).  I have had to re-do top ends way too soon because of crappy aftermarket seals that harden and leak.  The result is always white smoke, most noticeable on start up.

I think you’re a bit confused. The guides do not need to be removed to replace the seals. If a guide is removed, as bryanj says, the valve seats must be re-cut. This should not be considered a routine maintenance procedure, only a repair process.

[alacrity]

Hi Cal!  I am often confused and even more often confusing. But in this case?  Once it is determined the smoke is coming from oil leaking into the combustion chamber, there are really only three ways for that to happen: Blow past the rings and/or leaky valve STEM seals and/or leaky valve GUIDE seals.  If there was no need for valve guide seals, would Honda have installed them? If they never wear out/need replacing, would they stock and sell new ones?  This is why on every top end I do, I pull the guides and mic them (replace as needed - stock part 12023-465-405 or dynoman sells good aftermarket brass ones, for example) and ALWAYS replace all 8 of the stem and all 8 of the guide seals. You might get lucky and have the valve guides not leak. But why not freshen those components as well while the thing is apart? Why count on really old o-rings and possibly loose/bad fitting/messed up guides (maybe someone replaced a bent valve 20 years ago and didn't replace the now marred guide? etc). Of course all this is about oil control issues -- not combustion/performance issues.

The engine is apart again and the cause of the white smoke was cracked valve stem guides. Luckily I have a spare engine and the head looks OK, crossing fingers. Thanks to all who offered advice.

Hey there.  Pro tip:  before you put this all back together, knock out the valve guides and put New OEM honda seals in there (little Viton o-rings) and also only use honda oem valve STEM seals (the structured ones with the little springs).  I have had to re-do top ends way too soon because of crappy aftermarket seals that harden and leak.  The result is always white smoke, most noticeable on start up.

I think you’re a bit confused. The guides do not need to be removed to replace the seals. If a guide is removed, as bryanj says, the valve seats must be re-cut. This should not be considered a routine maintenance procedure, only a repair process.

[calj737]

Your post said: “knock out the guides and replace the seals.... as it’s something I routinely do”. You make no mention of recurring seats, and as I said, replacing seals does not necessitate guide removal.

If a guide is worn, replace it (as I said). If it’s not worn, replacing it is superfluous and costly. The OP had a cracked guide if I recall now, certainly cause to replace that one (and any other in need). But replacement for the sake of replacement is foolish and zero benefit.

[bryanj]

In 40+years this is the only time i have ever heard advocated removing guides to replace an O ring.
Every time you take a guide out you loose metal from the head and have to recut the seat so loose metal from there as well as recessing the valve head. Replace O ring when guide needs replacing yes but again i have only ever seen guides worn out on 750F2/3 and damaged by incorrect assembly on 500/550 apart from totally destroyed engines after missuse.