t100s rebore

[stevethon]

I'm looking for advice as to whether or not my bike is requiring a rebore.It's a 1969 t100s.
When started,the bike smokes from the left hand side but gradually burns everything off.The top end is Knocking but the tappets are set correctly.It's difficult for me to say if the bike is losing compression as I've had nothing to compare it against,but I'd say it's not too bad.
I'm not sure if new rings would solve the problem,or would it be advisable to go the whole hog (rebore,new pistons and rings).Money isn't the issue,it's just a question as to whether or not I'd be wasting my time.
The cylinder head will be coming off this weekend for reconditioning work.Is there anything in particular I should look out for that may point to the problem?

[twolitre]

If the bike smokes after standing and then clears, it sounds as though you have a worn valve guide or guides.  When you strip it, insert the valve from the 'wrong' end of the valve guide so that it sits in the guide in the normal stem working area.  If sideways movement is perceptible the valve guide or stem is worn enough to allow oil to drain from the rocker box.
  Pistons should have no more than perceptible sideways movement at the top dead centre position at right angles to the crank axis.  If there is little or no movement, re-ringing will probably be okay.  Anything more than that will require the use of an internal micrometer or dial gauge to measure wear and is best left to the experts.  If there is a marked wear ridge, re-ringing, unless 'ridge-dodger' rings are fitted, can produce knocking and/or broken rings.

Don't make the mistake so many people do of connecting oil consumption with compression.  The piston carries compression rings which can survive or fail before the oil control rings (also fitted) and vice versa.  Oil control rings often become clogged with carbon after high mileages or when oil changes have been ignored.

This is a fairly complex subject and if in doubt consult a specialist and let him view the parts.

[stevethon]

Cheers for that.It seems to tie in with what someone else told me.I'll let you know how I get on.

[Rex]

If the smoke clears quickly, there's probably no problem.
As long as it starts and runs, IIAB,DFI...as they say

[twolitre]

So smoke means 'it ain't broke' do it?

[Searchguru]

Hi stevethon,
Does your bike sit on a side stand or on a main stand? I suspect that if it is on a side stand, to the left, then any oil up in the head will drain over to the left. If, as 'twolitre' says, the valve guides are worn then some of that oil will find its way into the combustion chamber. I would see what needs doing with the head first as I am guessing that the rings are not the cause of this oil getting burnt. The top end noise may simply be excessive end float in your rockers. If they are fine pull the barrels off and check the small ends and big ends. You may be opening a can of worms but saving yourself time and money in the long run. For the sake of some gaskets you will get some peace of mind. Good luck

[twolitre]

The bike leaning to the left on the side-stand is a good point.  If you do tend to leave it on the side stand it could mean you should check the valve to guide clearance very carefully on the other side as well.  After all they will have had the same time to wear.

I did not mean to imply that you have piston/ring trouble, which I doubt from your symptoms, but you asked what to look out for so I mentioned checking piston slap at t.d.c. while you have the head off.

Regards, Jim.

[stevethon]

Thanks guys,
The bike has been on the side stand lately.I did consider that so since the last time it was used it's been on the centre stand.
Fired it up and the smoke is still there but definately not so bad.I'll see what's revealed when I get the head off.
Cheers.