Rocker Pedestal Preload Shims

[737flf]

When stripping down my griff engine I discovered that there weren't any shims under the rocker pedestals and I have no reason to believe that they were dismantled previously.

"The Bible" is quite specific about the neccesity for them, does anyone else have any input?

Cheers.

Alistair.

p.s. I've also checked my old 1987 Range Rover and it doesn't have any either.

[chrispitman]

If it is still the original engine, cam, followers then it wont have any. Shimms are usually fitted as part of a new cam install to get an even preload. I belive this is more critical the higher the rev limit.

[clive f]

thats right, TVR never bothered to set the pre load, they say it wasn`t necessary, but if they did then maybe the top end would last a bit longer, when I first fitted new lifters ect with my first cam change I put them straight in to see what the load gap was on the lifters and it was between 2 to 4mm on all of them!! rather than the 20 to 60 thou needed.

[350Matt]

clive f said:

thats right, TVR never bothered to set the pre load, they say it wasn`t necessary,

Thats shocking.....
It doesn't take that long to do

[Pupp]

What exactly are these shims supposed to 'pre-load'? Reason I ask, is they actually lift the rocker in relation to the valve tip and the pushrod so increasing shim thickness (or introducing one where none was before) reduces any preloading effect on the lifter or valve, no? Never quite understood what their purpose was in a hydraulic lifter set up...

Mine does have 'em tho

[GreenV8S]

They preload the lifters.

The lifters expand to take up the slack in the valve train. Ideally you want them to be almost fully expanded in normal use, because if you over-rev the engine the lifters have a tendancy to pump up so that the valve stays open. The more preload the lifter has the more scope it has to pump up. But if the lifter doesn't have enough preload, it can run out of travel when it's trying to adjust to allow for thermal expansion in the engine. So there's quite a narrow window where you really want the preload to be.