Valve Spring Shim - Bottom Valve Spring Seat

[Paul Sutton]

I have looked and looked and cannot find any references to the correct valve spring shim orientation. The shims fitted in my SH have two different surface profiles. On one side the shim is perfectly flat with sharp outer edges, the other surface is rounded on the edges.

• Which of these surfaces should the larger diameter valve spring contact?
• Does the shim rotate against the aluminum cylinder head, or should it stay locked in one position?

Thank you.

PS. I have checked the manual but the picture quality is very poor.

 

[grizld1]

Paul, believe it or not, this is one of those rare cases where orientation doesn't matter, and no, the shim won't rotate. There's nothing special about the shims; if some day you install a performance cam that calls for performance springs that have to be shimmed to spec, you can use any steel washers with the right OD and thickness. (Now we can look for some smart guy who's never done the job to chime in and dispute that last statement.)

 

[Paul Sutton]

Thank you Grizld1. When I removed the valves one shim was the other way round with the rounded edges up.

 

[peanut]

Paul, believe it or not, this is one of those rare cases where orientation doesn't matter, and no, the shim won't rotate. There's nothing special about the shims; if some day you install a performance cam that calls for performance springs that have to be shimmed to spec, you can use any steel washers with the right OD and thickness. (Now we can look for some smart guy who's never done the job to chime in and dispute that last statement.)

as you say they are simply to protect and prevent wear of the aluminium head from the steel springs and not a precision shim .

If I recall correctly they have an identical ID ,OD and thickness to the first clutch shim that fits against the bearing .

When I rebuilt my engine I had a spare shim left over ...... I had double and triple checked everything so I scratched my head and put it in my bits box.
Turned out the PO had used a valve spring shim to space the clutch basket off the crankcase by an additional 1mm to prevent rubbing.... hence the extra shim in the box . If they are indeed an identical part... whats the betting that a clutch shim costs a lot more than a valve spring shim !

 

[grizld1]

OE valve shims are1.5 mm thick. The thickest clutch washer is 1 mm. thick.

 

[peanut]

thanks for the info I couldn't be bothered to check tbh. It certainly did the job intended.

I guess the PO used whatever he had to hand at the time. .
I got the stripped engine in a box so had no way of knowing an extra shim had been fitted although I suppose I should have guessed when I saw the wear marks cut into the crankcase behind the basket.

 

[grizld1]

One more thing. The valve spring shims are not there simply for wear prevention, although they do accomplish that. Their primary purpose is to apply correct preload to the valve springs. That's why you have specs given for installed valve spring height.

 

[peanut]

yes of course thats a given.

I guess most of us simply follow the Yamaha manual specifications and part numbers and don't give this a second thought beyond checking spring length is within tolerance and we are using the correct valve spring shim.
With such an agricultural engine design there's not much point in us worrying too much about 0.5mm valve spring shim thickness when we never test spring rates and harmonics and cam lobe profiles etc .

Incidentally I read somewhere recently that excessive vibration at a certain rpm in engines can sometimes be due to valve spring harmonics rather than fuel/air mixtures, timing or crank twist which are usually blamed