Valve adjustment question:

[speeder]

This is a generic query, (not related to any particular engine), about valve lash adjustment. I've always been very meticulous about setting VL on engines that I tune and do it "by the book" but this question bothers me:

Most VL specs are @ TDC for each particular cylinder. Obviously, this is not at the furthest point from the lobe for each tappet. Usually, the cam looks like rabbit ears splayed at 10/2 o'clock or such. I've always assumed from this adjustment spec that it does not matter where on the round part of the cam you set clearance as long as you are not on the lobe or the *slight rise lead-up ramp* to the lobe that most cam profiles have.

I just set clearance on my MB diesel and TDC for #1 cylinder according to the crank pulley mark resulted in rabbit ears that were tilted pretty far off symmetrical, IOW, one lobe was pointing almost straight up and the other was pointing almost sideways. There is no problem w/ chain or sprocket, in fact chain was completely un-stretched according to timing marks and car runs great.

I chose to set the clearances as follows: I slowly turned motor by hand and set each individual VC on the engine as the lobe was pointing straight-up away from the tappet. I did not have time to re-check each one @ TDC for each cylinder but I will later. (The valve cover takes a whole 10 minutes to remove on these cars). The measurements should be the same, right?

To further complicate things, MB is one of those manufacturers that gives a hot and cold VC spec on the sticker under the hood, with over/under temperatures. The problem with that is that when I start the hot valve adjustment, the temp is definitely over the *hot minimum*, when I'm doing the last ones--not so much. Also, (and this has always confused me), the hot setting is .05mm larger than the cold spec. If valve clearances get LARGER as an engine heats-up, why isn't every VC spec in the world *half a mouse hair* cold?? Just use the thinnest feeler gauge on the tool setting them cold? I mean, if they get looser as they heat-up, what's the problem? I always thought that it was the other way around.

Anywhoo... I always just set them as the spec says. I recently set them on my older Toyota Land Cruiser and the Haynes manual, (actually a very good HM for this vehicle), says to set them all with (2) different cam positions. IOW, put motor @ TDC #1 and set half the valves and then TDC #5 or 6, (can't remember), and set the remaining ones. This definitely related to my original question about lobe position.

Sorry for the long, neurotic question but I truly agonize over these OCD mechanical issues. Thanks in advance as always...

[island911]

IMO, many people get FAR to anal (OCD) about setting valve lash.

What should alleviate the OCD the most is in having more information - specifically, Know the tolerance on the recommended gap. (the 911 is +/- .002" -that's quite a lot) Also keep in mind, the tolerance, on the cam profile is somewhere around a tenth (or less) of the gap tolerance. (read: you can set them any where off-lobe)

[speeder]

Thanks. One of the confusing things to me is that on some engines it seems like the tolerance of the cam profile is a lot looser than you state, ie. I get different measurements depending on where on the off-lobe part of the cam I'm measuring.

Also, I prefer to set valve lash with plugs, (or glow plugs on a diesel), installed so that the compression completely seats the valve. Sometimes there could be a tiny piece of carbon or something keeping the valve from seating 100%. I'm with you on the tolerance thing and the unimportance of perfect adjustment, (as long as they are not tight), but sometimes it seems like they could be WAY off based on different measurements on different parts of cam. (Off-lobe).

[nota]

older motors with push rods, the rod grows when hot to close clearances
OHC motors donot have the long thin push rod growth problem

[Zeke]

Originally Posted by island911

IMO, many people get FAR to anal (OCD) about setting valve lash.

What should alleviate the OCD the most is in having more information - specifically, Know the tolerance on the recommended gap. (the 911 is +/- .002" -that's quite a lot) Also keep in mind, the tolerance, on the cam profile is somewhere around a tenth (or less) of the gap tolerance. (read: you can set them any where off-lobe)

Originally Posted by speeder

Thanks. One of the confusing things to me is that on some engines it seems like the tolerance of the cam profile is a lot looser than you state, ie. I get different measurements depending on where on the off-lobe part of the cam I'm measuring.
.......................................

I've wondered about the eccentricity of a cam base circle for each valve. There has to be some deflection static.

You want real OCD, set the clearance with a degree wheel maintaining the minimum clearance on the tightest lobe. That is to say each valve opens at the same time in each cylinder's cycle, not adjusted by minimum clearance alone, but by where it begins to move on the ramp.

Then you're hoping the duration is the same from lobe to lobe.

[johnsjmc]

Or set it while running. Rather messy or impossable on a 911 but can be done on many engines. I used to have a steel valve cover with the centre cut out to allow lash adjustment on a small block chev with a solid cam.

[DUK]

The way I've adjusted valves has never let me down.

Adjust the intake valve as the exhaust starts to open. And adjust the exhaust as the intake has just closed. It's easy to figure this out if you do it once or twice and you'll notice that the base circle is almost centered on the valve being adjusted. On the 911 I feel the intake rocker loosen up at the end of it's lift and do the exhaust. Then I move the crank 120* and adjust the next one and so on. Then I feel for the exhaust rocker to tighten up at the start of it's cycle and do the intake. Again, just move the crank 120* and do the rest.