mk1-forum.net

Hello,

I was just wondering if someone can tell me how I can figure out how much extra lift 1.5 roller tip rockers actually add to valve lift after your cam lift.

I have a 286 scatter cam, and it's valve lift is 9.84mm inlet/ and 10.15 exhaust. According to Kents website. I'm having some spacers made as its a none standard head (thicker head and longer valves and where the rockers sit isnt standard either) and I'm trying to figure out where my rockers need to be to give the lift it should be given from the cam and rockers.

In my head (and I may be wrong, so please correct me if need be!) is if I measure how much of a drop (lift) the valve goes down with a dial gauge from a fixed point on the valve/or cap on rotation of the cam to find the full lift of the cam plus 1.5 rockers lift I should be able to determin how much of a spacer I need between my head and rocker posts (when I have the full lift from the two added together) I can work bit by bit to ascertain the spacer requirement!

Cheers Chris

I don't think that you would be able to calculate an
answer for the final result, even if you had a standard setup....

The only maths that you can really guess at is that the standard
rocker ratio is about 1.25:1 and you are using 1.5:1.

1.5 divide 1.25 = 1.2, ie 20% extra lift
I stand to be corrected...

You say that the head has been machined, and the valves
are non-standard, so it's anyone's guess where things
should be. Then there is the base circle size on the cam,
pushrod length, cam machining accuracy and any many other
variables to contend with.

Really, it's suck-it-and-see...

The method is to get a few sets of spacer shims made,
say, each set maybe 40 thou thicker than the previous.
You could even use plain washers, as long as the 3/8"
ones and the 5/16" ones are the same thickness you
should have no problems. You might even be able to use
3/8" size washers on the 5/16" studs if you aim them up
carefully. It's only for measuring purposes...

Use your dial gauge to check the lift at each thickness
increment, then just choose whichever gives the best reading.

If adding thickness only makes things worse, you possibly
need to think about shortening the pillars...

Once you've worked out which was best, get some proper
spacer shims made up.

DON'T FORGET THE OIL HOLE!!!

When you "gap the rockers", what you are actually doing is setting
the slack of the cam follower on the base circle of the cam. With
the increased ratio, you need to compensate for this by using a
larger rocker to valve tip clearance.

Ian

Some good advice there from ianh1968.

I would like to add that you when you reached what you think is right, check that the rocker tips don't ride over the edge of the valve stems, usually, they will be right, not always, but as always, check everything, assume nothing.

Thank you Ian, thats very useful

Thank you gents, for your kind words...

With many of the roller rocker assemblies you buy, the front to back
position of the shaft in relation to stud centres is different. This is in
an attempt to do exactly as Spider says: the point here is to make
sure that the roller/pad actuation area is somewhere close to where
it should be.

One more point to mention is that you should check that you can still
get enough adjustment from the adjuster screws with whatever you
end up with...

I had a setup a while back where I could not get any valve clearance
at all, even with the screws wound right in - the geometry was
completely wrong.


Ian