Correct shim sizes-what a difference

Posted: Sun Jun 17, 2018 10:57 am

I haven't been on this forum for a while but I just wanted to share my latest bike story. I haven't checked my valve clearances for a while but my cb900f was running a bit rough and i checked this and that with my carbs etc with no real improvement so i thought it's time to check my valve clearances. All of them were .005 .006 or .007 until I got to the last two and they were .003 so I did the calculations and ordered two new ones which brought the clearance to .006. Had a bit of an issue with the old gasket but once that was sorted out and everything back together I took the bike for a few short rides and it didn't miss a beat. Then there was a bike get together about 50km from my place and I decided to go and had a good time then rode home and the bike didn't miss a beat and I almost can't believe that by adjusting the valve clearance from .003 to .006 with two new shims could make that much difference.

Posted: Sun Jun 17, 2018 11:45 am

If sonicrete is readings this I’m sure he’s smiling.

Posted: Sun Jun 17, 2018 8:17 pm

I go one step further; I have my shims ground by an engineer buddy so that all the valve clearances are exactly .1mm. I do the initial checking over and over to ensure my readings are constant before calculating the odd ball sizes I'm going to need a shim ground down to. Anal? Rolling Eyes

Posted: Sun Jun 17, 2018 9:10 pm

cbx1956 wrote:

I go one step further; I have my shims ground by an engineer buddy so that all the valve clearances are exactly .1mm. I do the initial checking over and over to ensure my readings are constant before calculating the odd ball sizes I'm going to need a shim ground down to. Anal? Rolling Eyes

I do the same but have use of a surface grinder at work. Since new I have always set my 900’s valve clearances so a 0.002” feeler slips in but an 0.003” doesn’t. Almost at 40k miles now and still runs sweet as ever.

Posted: Sun Jun 17, 2018 10:30 pm

IMO, agonizing over precise thin valve lash is probably futile as there is likely more difference in cam lobe height than that, not to mention the huge slop in the (stock) cam bearings that is intentionally different in each bearing to allow for motor flex.

Also various localized temperature differences in a running motor may result in various thermal expansion...

I generally try to keep my valve lashes within .001" of each other, but am pretty sure it doesn't really have too much bearing on what is happening on a hot motor running WOT.

I don't really buy into getting that extra .002" of lift by running the valves real tight. I'd rather they get more seat time for cooling. Many more modern solid lifter motors run much larger clearances. If more lift is all that important, then I suggest bigger cams are in order, as that makes a real noticeable difference in performance.

And BTW, Megacycle 138-30 high lift cams cams specify .005" intake and .006" exhaust clearance, FWIW. They spec .004" for their smaller cams.

Posted: Mon Jun 18, 2018 2:52 am

What Gene said! I try to set mine at the loose end of the spec - they're only ever going to get tighter so keeping them at the loose end helps ensure they're not going to go out of spec between checks.

Posted: Mon Jun 18, 2018 1:23 pm

And then there are those of us who get great satisfaction from just knowing they are all dialed in exactly to what the original specs say. And if that means they have to be maintained more frequently, then great, more opportunities to tinker. We each have our own reasons for loving these old bikes.

Posted: Tue Jun 19, 2018 5:21 pm

swiftnick wrote:

And then there are those of us who get great satisfaction from just knowing they are all dialed in exactly to what the original specs say. And if that means they have to be maintained more frequently, then great, more opportunities to tinker. We each have our own reasons for loving these old bikes.

Amen to that! beerchug