Question about Top Dead Center

[bad Chad]

Why does it matter, when checking valve lash, if it's the compression stork or intake?

When I set valves, I find top dead center, and then I wiggle each rocker arm, if they both have free play, I assume I'm good to go to check, am I missing something?

[Dirk_S]

It’s all about the relationship between the cam lobes. The only time both valves will have free play will be TDC on the compression stroke. The cam lobes push the pushrods up into the rockers, then the rockers push down the valves. The intake’s cam lobe will already be past its apex and back to its base circle, but the exhaust has yet to begin its ascent, thus also on its base circle. Both pushrods are at their most free.

At TDC on the exhaust stroke, the intake valve is already opening before the exhaust valve has closed, and so there will be no point on that TDC where both valves are at their full looseness. Does that make sense?

Diagram of the anatomy of a cam lobe for reference:




…and a photo of a camshaft from an older Guzzi small block:

[Perazzimx14]

TDC on the compression stroke all the valves intake and exhaust on that cylinder are fully closed and the cam is not contacting any of the valve stems allow you to measure the clearance AKA space between the cam lobe and valve stem.

[moto]

Why does it matter, when checking valve lash, if it's the compression stork or intake?

When I set valves, I find top dead center, and then I wiggle each rocker arm, if they both have free play, I assume I'm good to go to check, am I missing something?

It matters quite a bit whether you are on the compression stroke, but your method of wiggling the rocker arms is a good way of determining that that is where you are. Your method can only fail if the clearances have been reduced to zero. In short, you're good to go. Don't worry. You do you.

[Kev m]

Why does it matter, when checking valve lash, if it's the compression stork or intake?

When I set valves, I find top dead center, and then I wiggle each rocker arm, if they both have free play, I assume I'm good to go to check, am I missing something?

The Readers Digest answer is: on TDC exhaust there's a good chance the exhaust valve is still open or preloaded and or the same can be said for the intake. Even if it's just preloaded since the lifter is on the ramp of the lobe you aren't at the point of maximum play, which is exactly what you're trying to check.

It's almost certain that if both have play you are at TDC compression and NOT exhaust.

[DaGootz]

Why does it matter, when checking valve lash, if it's the compression stork or intake?

When I set valves, I find top dead center, and then I wiggle each rocker arm, if they both have free play, I assume I'm good to go to check, am I missing something?

This is a good video for you..... https://youtu.be/vg0x4_mQd9M?si=BvGP7z_hI-8b8CM-

[aklawok]

 The ancient gods of camshaft design have commanded that the one point where all tappet valves on a cylinder   MUST be fully closed is TDC compression. This is the point where the demons of fire go boom and need to be contained. After which the lobe of the cam may increase slightly in diameter decreasing the gap to help prevent valve slap. Fast closing and fast opening of the valves gives us the the blessings of the angel Performance.
Just humor! 😄

[Bulldog9]

As others have said, on the compression stroke. I rotate till the intake valve opens then closes, then bring motor to TDC.

[Huzo]

Jeeez, why look for an alternative ?
Just bring the darn thing to TDC on the compression stroke and set the buggers.

[DaGootz]

Jeeez, why look for an alternative ?
Just bring the darn thing to TDC on the compression stroke and set the buggers.

[Dirk_S]

Jeeez, why look for an alternative ?
Just bring the darn thing to TDC on the compression stroke and set the buggers.

Doesn’t read to me like bad Chad is seeking alternatives, but rather looking to understand the ‘why.’ I’m a person who has always asked ‘why’ and ‘how’ since I could speak, to the disappointment of my grandparents who raised me (“I don’t know, Dirk. Why do you always have to ask questions??” — a flustered Marlin “Pap” Shearer).

If we don’t seek answers, we don’t learn good.

[bad Chad]

So after all this, I believe I have confirmed if I am able to wiggle both lifters on either TDC, the valve lash can be accurately checked.  If Im unable to wiggle both lifters, it would behove me to make sure I'm checking TDC on the compression stroke.

[Bulldog9]

So after all this, I believe I have confirmed if I am able to wiggle both lifters on either TDC, the valve lash can be accurately checked.  If Im unable to wiggle both lifters, it would behove me to make sure I'm checking TDC on the compression stroke.

Yes and no. It all depends if the valves were set correctly. This motor has two TDCs.

This is why it is best to set the valves at the TDC just AFTER the intake valves open and close in the rotation.

[guzzisteve]

If you feel a puff out of spark plug hole you are on correct stroke. Pretty simple!! Wondering when you'd be back. Breakfast this weekend!!

[Dirk_S]

If your tappets have moved, then you might not have that telltale sign of both being ‘wiggly’. Best to assess by the mechanical process, which additionally helps retain the ‘why’.

[Huzo]

Tell ya’ what…
Stick your finger in the plug hole and as you feel it trying to blow it out, that there is your top dead centre approaching on the correct stroke.
Here’s the thing.
If both valves were fully closed at the top of the exhaust and beginning of the intake stroke/s, then that would mean there was not enough time to empty out the exhaust and the inlet would be opening a bit too late to effectively start the intake process.
There is a time at TDC on the exhaust/inlet strokes, where neither valves are fully closed at the same time, there fore there is load on both cam lobes..
ie… No wiggliness…
This is called valve overlap. It does not occur at the top of the compression stroke…
Next time you’re staring into your valves, slowly bring the piston to TDC with the exhaust valve non wiggly, that is when the exhaust is being emptied, (valve open).
You will note that the exhaust valve is still closing as the inlet is beginning to open, no good for clearance checking…
One more revolution and you’re on the top of compression stroke, set your clearances at TDC on that stroke…

[faffi]

It is quite amazing how excited we can become over something as simple as how to adjust valves 

[Huzo]

It is quite amazing how excited we can become over something as simple as how to adjust valves 

How can you detect my level of excitement from over there ?

[Kev m]

So after all this, I believe I have confirmed if I am able to wiggle both lifters on either TDC, the valve lash can be accurately checked.  If Im unable to wiggle both lifters, it would behove me to make sure I'm checking TDC on the compression stroke.

Delete the word "either" and this post is correct.

If you are at TDC the only way that both valves could wiggle are if you are on compression OR one/both valves are so grossly out of spec that you would have noise and/or running problems and then it would behove you to confirm proper position.

If you are at TDC and you can't wiggle one or both, correct, double check that you are on compression. If you are, one or more is too tight. If you're not, then as Parliament said "Get Down On It".

Your post was almost completely correct.

[Dirk_S]

…If you're not, then as Parliament said "Get Down On It".

…and as Dragnet said, “Missed it by that much!”

[JoeW]

Chad, I understand your frustration. Unlike the older Guzzis, there is no easy way to turn the motor over. You can assume that if you have some lash that the valve is closed but, you can't be sure if the follower isn't just on the edge of the opening or closing ramp. If it is, the clearance will be too big.  I recommend getting the rear wheel off the ground, put the bike in gear and rotate the rear wheel. Use a drinking straw down the spark plug hole until  the piston is up towards the top. It does not have to be dead on TDC, just close on the compression stroke. 

[Kev m]

…and as Dragnet said, “Missed it by that much!”

Ha ha, I was thinking about that earlier when it came on the radio and I remembered I'd credited the wrong artist but then couldn't remember what thread it was in to fix it....

[bad Chad]

Thanks bros.  Steve, thanks for the spam plug puff reminder, yep off to breakfast Sat!

Most always a good idea to, "Get down on it!"

Joe, yes, thanks, I do wish Guzzi would have left an access plug like in the old days, and it was so nice to just put a socket on the alt nut and turn the dam motor over, rotating via the drivetrain is a bit of a pain in the ass.

Also, is it still recommend to use Corn oil in your rear end?

[guzzisteve]

They make stands for swingarm, then your hands get dirty.

[faffi]

This is the Volvo way, which you can basically use with any number of cylinders by adjusting to how many pots the engine has. No looking for TDC

https://www.youtube.com/watch?v=vg4bbJ2Mpj4