Valve timing question

[Einar]

Hi everyone,

I have picked apart one of my cylinders to replace the piston, and I will shortly put everything back together.
(Technical Discussion › BMW Technical Q&A, Primarily R65 › Questions on broken piston)

How do I ensure I don't mess up the valve timing?

I disassembled it with the piston all the way out and valves closed (I think), however I have since rotated the back wheel and have lost track of where I am in the engine cycle.
I am currently waiting on parts.

Best regards,
Einar

[Luca]

You can always figure out valve timing by the lifters/pushrods/rockers if you know the piston position (look through the plug hole or timing hole)

You can't mess up the timing since you aren't messing with the camshaft chain and the engine has a wasted spark ignition system (always fires near TDC)  However, if you are worried about figuring out when TDC on the compression stroke is, it will always be when the piston approaches TDC after the intake valve is open on a downstroke.  The compression stroke will have both valves closed.  The other upstroke (the exhaust stroke) will have the exhaust valve open (and the exhaust pushrod will stick farther out).

Since the engine alternates cylinders when firing, you can also find TDC at compression on the right by finding TDC at exhaust on the left, and vice versa

[Einar]

Luca,

Thank you for your response.

If I understand you correctly, that means I can safely put the bike at TDC, attach the piston, put back the valves, adjust the valve clearance, and fire her up.

Control questions for my understanding:
1. So that means I don't have to worry about installing the valves wrongly and having them touch the piston head?
2. Is the spark plug then also firing on the upstroke when the exhaust valve is open?
3. Would it not be necessary to circulate engine oil through the valves before I start the engine?

Best regards,
Einar

[Barry]

As Luca said you can't get this wrong if the cam chain remains  untouched.  The only thing you have to do is set the valve clearance at the correct TDC.

It's a wasted spark system with both plugs sparking at the same time so you can't get that wrong.

By all means crank the engine with the plugs out and earthed (or +12v feed to the ignition coil disconnected) until you see oil at the rockers.

[Einar]

Thank you for the response Barry. Cam chain untouched, so good to go.

A "Wasted" system because it wastes sparks....I just got that.  

[Barry]

...wastes sparks but saves the need for 2 sets of points so an overall gain in simplicity.

[Luca]

If I understand you correctly, that means I can safely put the bike at TDC, attach the piston, put back the valves, adjust the valve clearance, and fire her up.

TDC on the compression stroke.

[georgesgiralt]

Einar,
A lot of things are implied here.
If I read you correctly, you only removed ONE cylinder. So I do hope you won't do the valve clearance job on this only cylinder.
The process is to remove both valve covers.
Then put the engine at TDC
Check which set of valve has some play (one side should be free and the other one lightly closed)
Set the valve lash on this side.
Turn the crankshaft for ONE turn. The two piston are now at TDC.
If you have done things right, the cylinder just set should have its valves lightly closed and the other one have some play for you to adjust.
Once done, you can spin up the engine with the ignition system lacking the +12V and check for the oil coming at the valves rockers.
Hope this helps.
Oh, I suggest you turn the engine a couple of turns AFTER assembly and before the play adjustment in order for the parts to set properly and all the play to adjust.

[Einar]

Thank you all for your very useful input.

Yes I only removed one cylinder, in order to change the piston, but I am going to remove the other one as well to visually inspect + change pushrod seals.
I will do valve clearance adjustment at the very end, on both sides indeed.

My initial questions were whether I could mess up the assembly and hence the timing sequence on:
1) Compression vs. spark plug firing (answer seems to be no)
2) Wrong opening/closing of valves at TDC (answer seems to be no since I haven't touched the cam chain)

On the note of valve clearance adjustment, I need to identify compression stroke from exhaust stroke, and adjust clearance at TDC on the compression stroke.

Please correct me if any of these assumptions are wrong.

[Barry]

On the note of valve clearance adjustment, I need to identify compression stroke from exhaust stroke, and adjust clearance at TDC on the compression stroke.

Please correct me if any of these assumptions are wrong.

Georges already spelled this out:

Remove the valve cover on the other cylinder, rotate engine until TDC is in the timing window. The other cylinder will now either have both valves closed and you will be able to feel the valve clearances in which case rotate the engine through 360 Deg until TDC mark appears again and then set the clearance on the cylinder you worked on. Alternatively the other cylinder will not have the valves closed in which case there is no need do rotate before setting the clearance on the cylinder you worked on.

[georgesgiralt]

Einar,
If you remove both jugs, and try to figure out which cylinder to adjust, don't bother.
Take the engine at TDC, choose one cylinder and adjust the valve lash on it. Then turn the crankshaft one revolution (360°) to go to TDC again then adjust the other cylinder.
As Airhead use a wasted spark system, it is immaterial which cylinder fires first. But it is important to have one at the end of the compression cycle (the one which will be firing) and the other at the crossing between end of exhaust and beginning to fill. So both valves seized by their rockers.
If the system was not wasting one spark, you should have to check which cylinder will fire and choose this one to adjust the valves lash. Hope this is not confusing for you.

[Einar]

Thanks guys. Great comments and explanations.
I fully understand the way to move forward and will update you once I have made my next steps.
It might be a week or two depending on when my parts arrive.
Cheers