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Datsun 2000 Cam Info÷

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Most of you will not be affected by this due to a cam replacement; but may be suffering as a result of geometry problems anyway.

This is not intended to be a thorough discussion of cam geometry; just some things floating around in my head I've told people before about dealing with the U20 cylinder head in emails and while leaning over a car fender. I'm just banging this stuff out on this keyboard; I have little time for these tasks; however much I sometimes enjoy them;-- I'm lucky if I take time to proofread it!

CAM GEOMETRY refers to the relationship (location) between the center of the camshaft and the tips of the valves; and how this changes how the cam, rockers and valves interact together.

All kinds of things can affect it; rockers, cam, length of valve stems and valve lock position of any aftermarket valves have been installed ever, how far valve is sunk into head during previous work, head being surfaced on the top for warpage, HOW it was surfaced, if it was shimmed, HOW it was shimmed, thickness of lashpads; if cam towers were switched... First you have to get cam to turn with two fingers without rockers in place; if it doesn't you have a problem with tower alignment either from head warpage or them just not being installed correctly. On day one they were installed and THEN the towers were drilled for the cam. Even taking the towers off a new head and bolting them back on can cause misalignment enough to snap a cam. This is why Nissan says never remove them; but it is necessary to do so if the top needs to be surfaced. If heads were available and $150 you never would mess with it. I used the singular form of "valve" above instead of valves because EVERY lobe on the cam may require a different thickness lashpad in radical situations or at least be running on a different section of the rocker arm. Altering the height of the center of the cam in relation to the top of the installed valve stem either from milling/valve work/base circle changes to cam etc etc etc can make each lobe run in a different position on its rocker arm. The critical thing to remember is if the cam lobe runs OFF the roadway on the rocker arm; the cam is now TOAST, JUNK, not even good as a prybar since it is a casting. The position the cam is going to run can be determined using "blueing" like for setting differentials. Again; this is the field your head rebuilder will deal with.

When heads warp they can also sink and twist; all of this can be surfaced out and then each valve can be different in its new valve/cam geometry world. A shop that deals with overhead cam engines may be able to take on this task for you. There is more to it than appears on the surface. Even a new cam is not a bolt in unless your head has never been run before; otherwise a new camshaft can present the same problems as a regrind as to geometry problems. Sometimes a reground cam with an altered base circle can work BETTER than a new cam. So many variables. You can always weld up every single journal and recut them to bring back stock base circle; $$$$ or use new camshaft $$$$ :-(

Read up on overhead camshaft base circle info on the internet; I've found some great discussions; the best by far is a long paper that "Racer Brown" wrote that is probably the bible of camshaft design; operation and problems. I don't know if that was really his first name; but it should have been! It is a must read for anyone who ever gives the lumpy stick a second look. Back in time when I thought I knew something about camshafts I started reading his writings and I soon realized I knew NOTHING...NOTHING. And I STILL DON'T comparatively. I've had a few people far more involved with cams than I am admit they too were blown away by his writings.

With a stock cam; even a reground one; base circle problems are at a minimum. This usually comes into play with a big change in the grind over what the cam was designed for. Base circle refers to the diameter of the shaft "under" the lobe; so to measure it on the U20 cams you measure the cam diameter with the lobe pointed straight up.

The only thing we are dealing with here is the camshaft portion of the equation; this would assume your head was correct in all other aspects.

Keep in mind there is a fairly large range of change that a head can take before the cam lobe is in danger of running off the rocker "pad". But just for fun:

For every .002 thousandths a cam's base circle has been reduced below 1.300 or 1.305 maximum; add .0014 to the button thickness to retain the position it was in. This is kind of a crude method as any number of things are probably already affecting the geometry as it is (more than the cam); I am assuming a rough figure of 1.4 for the rocker ratio; but it is a starting point. Different rockers will make different numbers; as will using the old style rockers (which you shouldn't be doing). It is possible to have 8 different thicknesses needed to make all of them the same. Forget about head problems; just the fact that many valve jobs are done with any though to valve height is enough to throw everything off. Still a lot of pushrod machinists out there waiting for the OHC's to go away... ??? Assuming you are striking for a middle of the patch wear pattern; you can also adjust the valve clearance (lash) up or down to get to that position (to check) and start with that change in adjustment as a figure to add on to your original pad to figure a likely thicker pad (or sometimes thinner). I instead usually use a set of feeler gauges I have chopped down to fit in under the rocker arm end to get an idea of what I'd need. You have to just go with what will work unless you have time or a pleasant machinist who doesn't mind taking thicker ones and milling them to different thicknesses!

I've also been in discussions where someone has said just start with half the base circle reduction; but this makes no sense to me because of the rocker arm ratio. Everybody's got an opinion! I've never had it work out that easy when playing with one of these. And of course the rocker arm ratio of "1.4" may be correct for the overall lift; but when discussing it in relation to geometry; the ratio varies as the rocker moves; probably from 1.25-1.75:1? Speaking of Bob Sharp; his engine team had some ideas that seem almost opposed to what so many people accept as how it is. You can watch a couple of hours disappear staring at their manual with a U20 head in front of you to see how it all goes together. I'm just saying once you go away from near stock conditions it gets complicated; and once you want more than average horsepower and torque it gets complicated.

At one time I had a set of springs; I think they were from some fuel pump; that worked perfectly as fake valve springs; I could install them on a head and install the rockers but still easily turn the cam to do preliminary checking while generating enough pressure to be able to check the position pattern of the cam as it traveled over the rocker arms.

Although "correct" is having it run in the middle of the rocker; having it more towards the adjuster side can increase the lift a bit and I think that way the valve timing is advanced a skosh. I'm going off of old memories here; I'd have to have a cylinder head in front of me to verify what I'm mumbling about. I don't let the wipe pattern get within .025 of the end of the available run area; 1/32 would be safer.

Like I have said; the cam is usually the least of the problem; we didn't have that many inquiries about it the past decades; all kinds of nonsense gets spread now with the internet; maybe it is just that. For those of you that are just curious or of the black helicopter club; we are going to begin listing the base circle measurements of cams we have for sale.

There is "right" and there is "wrong"; but I have seen many engines WAY off from being "right" that produce amazing power and have shown to be durable.

Even adjusting the lashpads and checking wipe patterns is not as far as it could go to maximizing power and minimizing friction; that is reached so geometry is adjusted so that the center tip of the rocker arm engages the lashpad center and 1/2 valve lift. Details like this are why engines can make 15-20% more power in the right hands; all other items being unchanged.

If you are going to be having or have rocker arm geometry problems I do not suggest reground rockers. We typically have both new and reground rockers. Reground rockers can sometimes aggravate the problem; sometimes not. Even the NEW rockers can vary in size and shape. The reground rockers are cut the bare minimum to make them serviceable though...and that $400 for new rockers really cuts into the bowling money.

Actually solving cam geometry problems can be a very lengthy task and discussion. The causes are myriad and different head / cam conditions can make it tough! Your cam supplier should be going over this with you. We have enough trouble keeping up with taking care of our own customers to be doing after sale work on someone else's job. A shop that deals with overhead cam engines may be able to take on this task for you. There is a lot to it.


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