More L valve adjustment for sunken valve.

[datzenmike]

OK a member has contacted me about options for getting a good valve clearance adjustment on his L head. The valve has worn or eroded the seat to the point where the adjustment is bottomed out and the rocker pivot is as far down as it can go and clearance is only about 0.004" almost 0.010" short of what's required.

 

Leaving it this way will certainly burn it if an exhaust and or leak compression. An improperly sealed intake can fail to contain the explosion inside the combustion chamber and the flame can backfire through the carb.

 

OK in a perfect world we would just take the head off and have it rebuilt. But we are Datsun owners, besides problem solving and keeping our vintage junk going with bailing wire and chewing gum.... we are cheap. Easier to just get a used head and just keep on truckin'.

 

So, if you are in this position and are waiting for a used head to show up and there's nothing to loose.....  here's something I tried this afternoon just to see if it would work. A way to add more adjustment to a bottomed out adjuster....

 

 

I removed the rocker pivot bolt with the locking nut. It's at it's highest position with the pivot all the way down. My 24mm wrench is on the threaded sleeve that the pivot screws down into

 

 

Here is everything that is inside the head. Rocker pivot, locking nut and sleeve that holds everything into the head. That little lip just above the bottom nut is for the clip that the mousetrap spring clips into. More on that later.

 

 

 

I used a second lock nut to jam the first on the pivot bolt so I could comfortably hold it against the grinder. The locking nut is 0.320". I ground it down to 0.215" or roughly down to 2/3 original thickness.

 

 

 

Here is the stock lip that holds the mousetrap spring clip

 

 

 

Yes, I ground down the top of the sleeve as well. I didn't want to loose that lip. It would have been about 2mm, it's now half that at least

 

 

 

I bottomed the rocker pivot on each side of the modified one to get a measurement. The shortened one is 0.110" over 1/10 inch. 

 

 

Now I don't know how much extra valve lash this will give but probably plenty. I checked and the pivot did not bottom out even 1/10" deeper into the head.

[racerx]

Great write up and thanks for taking the time to demonstrate with pics and write ups.  What if I replace the adjustment nut with a nut that is 2/3 the size of the stock one, will that work?

[datzenmike]

Sure. The rocker ratio is 1.48 to 1 at the cam. Not sure how this applies to the pivot ball end but 1/10" at least

[Doctor510]

Sure. The rocker ratio is 1.48 to 1 at the cam. Not sure how this applies to the pivot ball end but 1/10" at least

Please remember that this is a temp fix.  Works for a while, then the cam nose and the rocker arm contact pad start to disintegrate and ruin the cam and/or the rocker.  Save your bucks and remove the head, get a good valve job and replace the brass seats.  Cheers

[datzenmike]

Agreed it is not a fix. It's a way to get a few more miles out of it when you don't have the luxury of a valve job or another head. Not going to need 1/10" probably a mm would do. Also not sure which way the cam wipe pattern would change or if it is even significant. The cam wipe position is more affected by the increase in lift. Lowering the pivot bolt should move it towards the valve closing side of the lobe... again, if at all. 

[KoHeartsGPA]

I like this :thumbup:

[racerx]

Kingrat, the cylinder that has a sinking valve is #4, should i make sure it is TDC bfor taking out the bolt and also when i install it back w slimmer adjustment nut

[Doctor510]

Kingrat, the cylinder that has a sinking valve is #4, should i make sure it is TDC bfor taking out the bolt and also when i install it back w slimmer adjustment nut

Make sure that the sick valve's cam lobe is straight up and that the piston is NOT at TDC because you will bend the valve when you depress the valve spring to remove and install the rocker.  Don't learn the hard way like I did 35 years ago.

[datzenmike]

Yes. If the intake lobe is pointing straight up at 12 o'clock your piston should be near 108 degrees which is well over half way down the cylinder and descending.

 

 

Looking back probably just a mm or two would have been enough. 1mm @ rocker ratio of 1.48 would be almost 0.060" of valve lash. Oh well that valve can sink well over another 1/8"

[racerx]

Good thing i ask. Second question, if i unscrew the pivot bolt from head and press on the valve spring will it cause the valve to drop. Thx for everyones patience.

[datzenmike]

The valve cannot fall into the cylinder just by pressing down on it. The rocker arm pushes down on it 50 times a second at 6,000 RPMs.

[racerx]

Ok, will proceed tomorrow. Once again thank you.

[racerx]

Update....took out pivot bolt and installed grinded down 17 mm nut. Then adjusted valve to .008, intake, made knocki g sound so readjusted to .004 and knocking sound not as loud. I have a spare head for L18 and ponder next move to either rebuild the whole engine or just install head. Compression on other 3 cyl about 125. What do you guys think? Btw, thanks for all of the patience and help, especially from datzenmike.

[datzenmike]

Valve lash should be 0.010". Setting 0.008 is slightly tight, 0.004 is really tight.

Make sure cam lobe is pointing straight up when setting the lash and the engine is hot..

125 compression is low and at minimum acceptable, but as long as all are within 10% of each other it's a bare pass. Engine should be hot for a compression test. If you re-test and add a few squirts of oil into the cylinder and this raises the compression then the rings are most of the cause. If oil does not raise the compression the valves may need lapping.

[scooter]

Another way to achieve this would be to use a thinner lash pad. That would keep the rocker in a more normal pivot angle.

 

Ah good ol' brass valve seats.

[datzenmike]

...y..e..s..... but this is easier for the do it yourself, at home mechanic, with no money and lots of time. In keeping with home made maybe sand the lash pad down? To get 0.010 at the cam you'd need 0.015 at the pad. The only problem is that this is permanent. Thinning the pivot post lock nut still allows the head to be re-built and put right.  Any change to the pivot post or lash pad changes the cam wipe position also. Probably not enough to matter.