How much should I shave my 280z Head

[jamo240]

Hi Gav

 

I will give you an extreme example to try to visualise the concept....

 

Imagine that the base circle of the cam is ground with a radius of 20mm....that is, the base circle is 20mm from the centreline of the cam's axis of rotation. The rocker would then be 20mm + 0.012" away from the centreline of the cam. If the cam lobe was pointing away from the rocker, it would have to rotate quite some way in either direction (depending on the grind) until the shape of the cam ramp exceeded the 0.012" clearance and began to open the valve. Our total lift is a function of the difference in the distance of the lobe point to centreline ( let's say it is 30mm) vs base circle to centreline (in this case 20mm). Therefore total lift = 30 - 20 = 10mm.

 

Now....assume we had a super duper mega throw down race grind, and ground all 20mm from the base circle, directly opposite the pointy bit of the cam lobe. We would essentially have a flat opposite the cam lobe, and the rocker would be very near the centreline of cam rotation, as we would have to raise the rocker by 20mm to retain our 0.012" rocker clearance. Now, if we rotated the cam in either direction, we would readily use up our 0.012" clearance and commence opening the valve... We would also have very high lift, as we have subtracted only from the base circle of the cam, but not the nose. So, total lift = 30 - 0mm = 30mm.

 

That is why grinding from the base circle is necessary to, and results in a bigger cam without altering the profile side of the cam (although both are altered in reality).

 

Hope that helps!

 

Jamo

[luvemfast]

Here's a quick CAD model to show the basic cam lift.

No duration specifics are shown, tangents only used.

[luvemfast]

Quick Google search found this

Maybe you need some tips on how to use it Gav

[PeterAllen]

I haven't been involved with engines for a long long time but one thing I do remember was the heat treatment process which created a hard surface on contact areas. Cam lobes and rocker faces had a very hard surface around 0.003"-0.005" deep. It was called nitriding. I can remember people grinding through that hardened layer when trying to remove wear marks or re-profiling a cam. The re-worked cams and rockers wore out in no time.

 

I'm not sure what manufacturing processes are used these day on new cams but I would be wary on grinding the surface of 40 year old cams unless they can be heat treated again.

[fluegel]

I haven't been involved with engines for a long long time but one thing I do remember was the heat treatment process which created a hard surface on contact areas. Cam lobes and rocker faces had a very hard surface around 0.003"-0.005" deep. It was called nitriding. I can remember people grinding through that hardened layer when trying to remove wear marks or re-profiling a cam. The re-worked cams and rockers wore out in no time.

 

I'm not sure what manufacturing processes are used these day on new cams but I would be wary on grinding the surface of 40 year old cams unless they can be heat treated again.

 

I think the rocker faces are cast iron and not hardened whereas the cams are nitrided so you have the ideal bearing situation of hard against soft. Some people simply regrind rockers with wet and dry paper on a sheet of glass.

 

Re. shaved heads the present head on my 260 engine has had 3mm shaved as a result I have had to fit a twin sprocket tensioner set up. I am going to remove it some time and try the trick of moving the curved guide in closer by filing out the bolt holes and doing the same with the tensioner at the bottom. If that doesn't work I have an E88 head which I will get reconditioned

[d3c0y]

Have you shimmed the cam towers up on it yet?

[gav240z]

Maybe you need some tips on how to use it Gav

 

Thanks... I don't know how to use my Google properly (

) actually I did a couple of searches but didn't find what I wanted to know. That image helps along with this discussion here, thinking I finally see how it works.

http://www.jalopyjournal.com/forum/threads/how-the-hell-does-regrinding-a-cam-work.36558/

 

By making the base radius smaller and moving the rockers upward, we are effectively making the lobe ramp steeper.