Shim under or shim over??

Greetings,
Lucked into a set of NOS in the box, Cam Motion Z4 cams. The data sheet states that stock lifters are acceptable. The lift spec is .420; duration 262; stock base circle. I have noted some other manufacturers with very similar spec offerings that require a shim under style valve train. How close to the outer limit would stock lifters be @ .420? Should I just pony up for a shim under set-up and be done with it? I don't put any more or less stress on the bike than the average duffer:evil:

I have been running an Engle .410/300 cam for years
with stock shims,Ape valve springs.But if you shift at
higher than 10 thou,you will want the underbucket shims.

420 lift requires shim on bottom.

Jay

Thanks all...
I was already leaning towards the shim under and now it's for sure. The cam data sheet just didn't seem right. I wonder why the maker would print that it was OK?
Regards

So I just was on the A.P.E. website KZZone.com checking out shim under tappets. Right there in the text of the description it states that shim under tappets are required for lifts greater than (0.420). Ape Jay you replied that my .420 cams needs shim under and I have no reason to doubt your experience. I have read many of your posts and you seem to be 'in the know'. Is .420 the absolute ragged edge for shim on top? I am still going with shim under...I am just curious now:silly: :silly:

Put it together with shim on top. Then turn the engine over with a wrench and the valve cover off. Watch it lift the shim up a bit on one side as the lobe goes around. Then you decide if you want to run it that way.

Does this cam have the standard 1.110" base circle?

Jay

Post edited by: APE Jay, at: 2007/05/04 17:12

Standard base circle yes. As I stated earlier, I will be using shim under regardless. No offence was intended. Clearly you can see that the catalog description could cause confusion? If not, oh well.

hes just telling you what to do so you can see yourself why its a bad idea to run big cams and shim on top. it might lift one enough to spit it sideways:pinch: :sick:

The shim problem is due to how far from center the cam lobe sticks out. If the base circle stays standard, all the additional lift id added away from center line.

Some cams use smaller base circles as the lift increases to keep the tip of the lobe from going farther out on the bucket. Our 435s for example have a 1.080" base circle, so the nose of the lobe is no farther from centerline than a 420 with a stock base circle.

Why is this the issue? If you look at the stock shim on top bucket, you will see a radial groove running around the outside edge where the shim goes. When the shim is in place, if the lobe tip extends out to the edge of the shim, it will try to push the shim down in this groove. This causes the other-side of the shim to lift up. If it lifts up enough, it is out and "spit" right through the front of the head.

We have found that our K410 is the most lobe height you can have with a standard base circle and not have it try and push the shim down in the groove.

Jay

Thanks...I understand what you are saying. Sorry for any confusion:) I'll be ordering up a new valve train over the next two paychecks:pinch: