Cylinder head removal - don't do what I did.

[MattW]

Using Steptoe's excellent howto I've just removed my left hand head in order to change the gasket.

However when bringing the cylinder to TDC, I neglected to check that it was TDC on the compression stroke rather than exhaust. This matters because on compression, inlet and exhaust valves are fully closed and there is a couple of degrees freeplay in the camshaft which aids refitting the cam sprocket onto it's keyway.

On the exhaust stroke, TDC actually has the inlet and exhaust valves cracked open slightly (exhaust just about to close, inlet just opening) which means that the camshaft is locked solid by the valve spring tension and there is no freeplay.

The way to check that you've got it right is to make sure that the inlet valves are closing as you approach TDC (or bring it to TDC and see if you can wiggle the rockers as if you are going to check valve clearances - if they feel solid, turn the engine another revolution and try again).

I of course didn't do any of this and only realised when I got the head off and noticed both valves open slightly. I'm probably going to have an interesting time trying to refit the cam sprocket

 

[Colban]

Can you not simply refit the head,without bolting it down then fit the spocket and simply turn the engine 360 degrees with the front crankshaft nut.
Or support the driven sprocket while you turn the motor.
Mind you,that sounds a bit too easy and may simply compound the original error.

 

[MattW]

The problem will be lack of 'adjustment wiggle' that the lash in the cam on compression allows - will make it tricky to get the keyway in the camshaft lined up perfectly with the corresponding key on the camshaft sprocket.

I've been thinking is through (it's been a slow morning) and I think the way forward will be to remove the plastic front cover off the engine and get a spanner on the crankshaft. This should allow me to tweak the sprocket round as required to get the key lined up (I'd guess that trying to achieve the tiny movement through turning the back wheel with the bike in gear would be a nightmare)

I think.....

 

[Trullion]

I always do that to adjust the valves, it only takes a minute or so to remove the cover. 16mm ring spanner on the nut allows much more precise control IMHO. Try not to turn the crank anti-clockwise or the camchain might come off or jam at the other end. On the LHS the cutaway should be vertical, with the arrows horizontal.

 

[MattW]

I always do that to adjust the valves, it only takes a minute or so to remove the cover. 16mm ring spanner on the nut allows much more precise control IMHO. Try not to turn the crank anti-clockwise or the camchain might come off or jam at the other end. On the LHS the cutaway should be vertical, with the arrows horizontal.

Thanks Trullion,

I guess I might have to tweak it anticlockwise slightly, but it'd be a tiny amount. Sorry to be thick, but what cutaway and arrows are you referring to (only had my GS a week).

 

[DrFarkoff]

If you slackened the camchain tensioner you should have enough free play?

 

[MattW]

If you slackened the camchain tensioner you should have enough free play?

I did slacken the tensioner - perhaps I'm worrying about nothing but Steptoe makes reference to it being a fiddle and needing the freeplay you get with the camshaft at compression TDC....

 

[northengit]

As he man said... remove the cam tensioner/s....

Did you remove or slacken the tensioners.... removal gives that bit of free space to line the sprocket to the cutaway.

 

[MattW]

Did you remove or slacken the tensioners.... removal gives that bit of free space to line the sprocket to the cutaway.

I removed the left tensioner as far as I could (couldn't actually withdraw it completely due to it being tight for space on top of the cylinder).

I guess I'll find out when I decide what to do about the studs and summon the courage to bolt it all back together

Thanks for the advice everyone.

 

[Steptoe]

just replace it the same as you removed it.

 

[MattW]

just replace it the same as you removed it.

That's my plan

cheers
Matt

 

[Slowdown]

just replace it the same as you removed it.

Shurley the reverse of how you removed it ???

 

[MattW]

Shurley the reverse of how you removed it ???

Good point, but stop calling me Shirley

 

[Steptoe]

Shurley the reverse of how you removed it ???

If you say so.