Topic: valve clearance - what's the philosophy?

[flatlander]

here are some clearance specs for cams used in the 550. the first two are stock honda cams, and the web and mega are mild performance cams.
apart from the two stock cams, they all come with different recommendations for valve clearance. megacycle even changed their spec over the years, while the grind stayed the same!

i'm a bit puzzled by this. any ideas why these differences?
i know, it's safe to just go with the manufacturer's spec and that different grinds have different needs. but especially the web and mega are not that different. does anybody know the theory behind this?

CB550
IN 0.05mm/0.002in
EX 0.08mm/0.003in

CB650
IN 0.05mm/0.002in
EX 0.08mm/0.003in

webcam 358a
IN 0.10mm/0.004in
EX 0.10mm/0.004in

megacycle 12600 (new spec)
IN 0.13mm/0.005in
EX 0.15mm/0.006in

megacycle 12600 (old spec)
IN 0.10mm/0.004in
EX 0.13mm/0.005in

[PeWe]

I have also had thoughts about this since some cams have much more clearance. 
- The valve will have more time fully seated, more cooling time?
- Valve can withstand a hunting piston hitting it? That must be EX and high rpm.
- Rocker arm/cam will get better lubrication?

The cam I have in now has 0.3mm. DP315 an RC profile
I timed it first with 0.1mm in, 0.15mm EX as many cams have. The cam times became rather hot and plenty of overlap. Numbers more like "normal" hot cams with 0.3mm clearance.

Below with 0 lash and running lash 0.3mm. 0 lash= I tightened adj screw until dial indicator moved which will remove play in valve train.  Cam holders, rocker arms, shafts have all play due to their clearances for oil + wear that affect the numbers too.
(0.15mm almost 0.05" will be something between these numbers)
[0 lash] @0.040"lift    Lift:9,35/9,33mm   Dur: 279/279,5°   IN:34/65°  EX:65/34,5°   LC: 105,5/105,25°   Overlap:68,5°

[0.3mm lash] @0.040"lift   Lift: 8,95/9mm        Dur:265/266°   IN:28,5/56,5°   EX:60/26°     LC: 104/107°    Overlap: 54,5°

This time I finally understood why the Pros here time cam with running lash.

I attach a cam table from Action Fours with different clearances. I have their SS-1 cam I set 0.1/0.15mm when it should have more.

Lets hope the pros will chime in with their huge experiences.

[Bill/BentON Racing]

No pro here, but I always degree with the clearance I'm running, think Mike Rieck does also. Total valve lift you have to allow for clearance. We used to run the V4's @ .002 for more lift etc, stock was .004 to .006 I think.

[flatlander]

i agree with the degreeing at running clearance.
what's up with using a little more, or a little less clearance with the same or similar cams, though?
do different manufacturers simply have different preferences or is there some science behind it?

[MRieck]

No pro here, but I always degree with the clearance I'm running, think Mike Rieck does also. Total valve lift you have to allow for clearance. We used to run the V4's @ .002 for more lift etc, stock was .004 to .006 I think.

Yes....I do. You want the engine to resemble actual running conditions. Also hard to find absolute zero lash especially with shim/bucket arrangements

[scottly]

The owner's manual that came with my 1953 BSA had two different clearance values; the looser setting was for lower speeds where torque was required, and the tighter setting was for high RPM and maximum HP.

[kmb69]

No pro here, but I always degree with the clearance I'm running, think Mike Rieck does also. Total valve lift you have to allow for clearance. We used to run the V4's @ .002 for more lift etc, stock was .004 to .006 I think.

Yes....I do. You want the engine to resemble actual running conditions. Also hard to find absolute zero lash especially with shim/bucket arrangements

The gizmo Brent uses on his DOHC is pretty slick for those shim/bucket arrangements.

[flatlander]

The owner's manual that came with my 1953 BSA had two different clearance values; the looser setting was for lower speeds where torque was required, and the tighter setting was for high RPM and maximum HP.

now that is interesting. makes me wonder why megacycle increased their recommended setting over the years. was it the same reason, or did they fear burning an (exhaust) valve?
then webcam recommends the same for in and ex which is different from the rest... strange

[Captain]

No pro here, but I always degree with the clearance I'm running, think Mike Rieck does also. Total valve lift you have to allow for clearance. We used to run the V4's @ .002 for more lift etc, stock was .004 to .006 I think.

Yes....I do. You want the engine to resemble actual running conditions. Also hard to find absolute zero lash especially with shim/bucket arrangements

The gizmo Brent uses on his DOHC is pretty slick for those shim/bucket arrangements.

A bit tricker now.....Dial gauge is fixed to the unit for total accuracy