I'm baffled - camshafts and clearances

[BrianC]

Below is a copy of a PM I sent to Tony.Mon, but I thought I would also put it out on a more general distribution in an attempt to rid myself of todays frustrations !!!!

Hello tony

As you know im trying to fix the front head on my vtr.

Some time ago i bought a front head from you and i have been assembling it back today. The camshafts and caps were off as delivered, but all the buckets were in their positions so i was presuming not disturbed.

I have put the camshafts in the right positions and fitted the inlet shaft and caps which was fine with correct gaps but when i tried to fit the exhaust camshaft the shaft seemed to tighten up as i wound the cap bolts down. Even before it was tightened fully there seems to be no gap between the bottom of the cams and the bucket, and when tightened to the torque the camshaft wont turn at all. So i took the shaft out again and removed the buckets , removed the shims, then replaced the buckets minus shims. Fitted the camshaft again and it rotates freely, but the gap between the bucket and shaft is now only about 1.2 mm.

So im completely confused !!!!!

Going back to the original head. I think i had already said that there looked to have been a slight clash between one inlet valve and the piston, with a very slight witness mark in the carbon on the crown. All the valve gaps bar that one were fine and that one was onky 0.20 so just outside spec.

Your thoughts on this one?

I hope you can help and thank you in advance

Brian

[popkat]

Fit the Exhaust cam first. Gently pull the cam chain taught (without rotating the crank) as you put the cam down, mark the chain and sprocket with tippex so you can see it doesn't move. make sure the crank doesn't move when tightening the cam down. If marks are still aligned when cam is bolted down then go on and fit the inlet cam.



.

[BrianC]

Thanks pop

maybe I confused you

The head is off the bike and as i said the head from tony just cant get a valve clearence with the existing shims

Brian

[tony.mon]

Please don't assume that the caps have the correct shims under them.
Pull the caps off and have a look at the shims, measure them and post it up.

It's often the case that when you assemble a head on the bench and set perfect clearances, then fit the head, the clearances are a bit off.
I assume it's because the head distorts slightly when properly torqued down.

[BrianC]

When i said i couldnt get a valve clearence what i meant was zero or less clearence.

Are you saying i should be setting the gaps with the head torqued down?

The other thing i need to establish if the exhaust valve on the original head is damaged.

The valve looks to be seated ok, but as i said the valve clearence, all beit with the head on the bench is 0.20, so just outside spec

Brian

[VTRDark]

You can get approximate clearances with the head off but these will change once the head is torqued down. Also install the CCT too when doing the final check as this puts tension on the chain which I believe pulls on the cams a little changing the gap too. Fiddly bloody job and you may find that tightening or loosening a cam cap bolt ever so slightly will change the gap enough to put it within spec. Most of all it comes down to feel and experience. You want the feeler gauge to be able to slip through the gap without snagging and bending but not so loose that it slides in and out like butter. The gap should just bite the feeler if that makes sense.

I like to get it so it snags and then loosen ever so slightly so it slides in and out with a bite. It may only require an 1/8th of a turn either way on the cam cap bolt above that valve to get there. The cam cap bolts don't need to be spot on torque, actually they are better off slightly less if old used bolts or you may risk snapping one. Yes they do snap easily so go gently and have trust in your feel for it. We should all know when a bolt is getting to that point of snapping so don't push your luck trying to get cam cap bolts on the exact torque according the manual. It's not fun trying to pick a snapped thread out of the head LOL Been there, done that trying to stick to the exact torque on used bolts. Just make sure you use some threadlock on them.