The V8 Owners Forum -

As a temporary measure I have fitted the tubular pushrods that I had but have only managed to obtain a preload of approx 35 thou and this is using 48 thou of shims under the rocker posts,which is by no means perfect

I am using a cam with .488" of lift so want to machine the rocker post by 50 thou if possible and need to calculate how much shorter the pushrods need to be so I can order up a set made to measure to schieve a pre-load
of 20 thou.

I know the 1.6/1 rocker ratio comes into the equation but can't remember quite how it's done. I measured the existing pushrods and they are 8.050"

So if any of you technically brilliant guys can give me some guidance I would be most grateful.

Kevin

Well looking at the rocker as a lever:

If the valve end is fixed, then lowering the rocker pedestal by 1thou will require the pushrod to be shortened by 1.625thou to give the same preload.

If the pushrod end is fixed, then lowering the rocker pedestal by 1thou will require the valvestem to be shortened by 2.6thou.

You've got two variables I think (things you can change) - rocker pedestals & pushrods.

If you remove the 48thou shims from the pedestals then you'll need to remove 78thou from the pushrods to keep the same preload.

Then if you take 50thou from the pedestals, then you'll need to take another 81.25thou from the pushrod to keep the same preload.

And finally you want to reduce the preload by 15thou, so you need to take off another 15thou from the pushrod.

Adding it all up gives a reduction in length of 174.25thou, giving a new pushrod length of 7.87575inches.

Sound reasonable??

Chris.

Chris,

Thanks and sorry to be tardy responding, still trying to get my head round it.

My thinking is that the rocker when raised 10thou at the pushrod imparts a deflection of 16thou on the valve tip so isn't that the other way round to what your suggesting? I think i need to do a diagram and see how it works.

If I'm correct if I remove 50 thou from the posts snd remove 48thou of shims ie 98thou i need to divide that by 1.6 (rocker ratio) which will indicate a shortening of the push rod by 61.25thou the remaining 15thou of preload is not affected by the rocker ratio and therefore is a further reduction of pushrod length of 15thou as you suggested, so pushrods should be 8.050 - 76.25thou = 7.975".

Now, which one of us is right?

Kevin.

I think I'm right, but I left my diagrams at work.

Needs a 3rd opinion!

Chris.

CastleMGBV8 wrote:Chris,

If I'm correct if I remove 50 thou from the posts snd remove 48thou of shims ie 98thou i need to divide that by 1.6 (rocker ratio) which will indicate a shortening of the push rod by 61.25thou

It's all to do with similar triangles:


If a=98thou, then similar triangles give you b (which is the amount to be taken from the pushrod) as 98*2.6/1.6 = 159.25 reduction. Add on the 15thou gives 174.25 reduction.

You need to think of the valve stem as fixed.

Make any sense?

Chris.

Chris,

Still trying to get my head round this.

Should have some time tomorrow and will give Ian Richardson of Wildcat a ring, they do various length pushrods so a good reason to ask his advice, if anyone should know, he should.

Will let you know how I get on and own up if you were right.

Kevin.

Chris,

This was bugging me so did some simple geometry,

Drew a line with one end sitting on the fixed point of the valve stem with the pivot point of the rocker to reflect the 1.6/1 ratio.

Drew second line same length as first with the pivot point lowered by a 100 thou and then measured the distance between the two lines and it gives 160 thou therefore 1.6/1

So I was completely arse about face and you were almost correct

Thanks for your input.

Kevin