Maybe i am missing something obvious. when you have a brand new engine you just crank it up and follow the regular break in procedure. When you install a new cam you MUST get it up to 2000 rpm's immediately.

what is the difference?

 

Anytime you install a new flat tappet cam and lifters, the eng will need to be held at a "high idle" for 15-20 mins upon start-up. Doesn't matter if the rest of the eng was rebuilt or not.

If it's a roller cam, there's no need to hold eng at a high idle at start-up.

 

i have never read where you have to do that after a rebuild in any haynes or chilton manual . they all say crank it , set timing, check for leaks etc.

 

Any time you put in new rings on a new cylinder surface you have to re-seat the rings. Likewise with any new flat tappet cam/lifter set.

It's usually about 20 minutes, between 2k - 3k RPMs (varying) for rings or cam. Or both at the same time on a brand new build.

 

The Haynes and Chilton's manuals, and even the Helm manual for that matter, usually refer to procedures for stock or stock replacement parts. Aftermarket parts can have different requirements.

 

i've heard you keep the RPM 2k-3k rpm for 20 mins to break in the cam, then change the oil. Then don't drive over 4k rpm for 500 miles to seat the rings, change the oil, and you're set.

 

http://www.compcams.com/Technical/In.../Files/145.pdf

See steps 12 & 13

 

I've never heard of this period. Very good to know as I'm building a new motor. It says remove the inner valve spring, this wouldn't hurt the outter spring at all with the amount of lift on the cam would it? Just curious...

 

No, removing the inner won't do anything to the outer; it's not like each takes half the lift, or something like that. All that removing the inner will do, is lighten the overall load on the valve train. Of course, the engine's high-RPM capability will be severely destroyed, because the valve won't be controlled near as well by the spring; but most of us wouldn't be breaking in our motors at 6000 RPM anyway.

Break-in without the inner is universal practice in forms of racing that require flat tappet cams by their rules; Nextel cup for example.

Adding something with lots of zinc for break-in is also a VERY GOOD IDEA. As the formulation of off-the-shelf motor oil eliminates heavy metals, flat-tappet cams are more and more vulnerable to the forms of wear that these additives are there to protect against. GM EOS is a very good, easily obtainable additive for this purpose.

Some motor oils still contain the additives in question, particularly ones intended for use in diesels. Rotella T, Delvac, and Delo 400 are often mentioned as ones to use.

 

Also: they don't mention it in the Comp write-up; but there are other reasons for the flat-tappet cam break-in procedure.

The lifter bottom is not "flat"; and the cam isn't either. Rather, the lifter face is concave (bulged outward, with a radius of curvature of around 6 feet); and the cam lobe is at an angle to the axis of the cam, such that the rearward edge of the lobe, is larger than the frontward edge. This is part of what holds a flat-tappet cam in the block, the other part being the distributor gear/oil pump load. Each of those things draws the cam rearwards.

The outermost part of the surface of the lifter is the only place that it touches the cam lobe; and it touches on the rearward-most part of the lobe. This point of contact on the outer edge of the lifter, makes it want to rotate in its bore. This rotation is what keeps them alive; they are not supposed to slide on each other.

At intial startup, the lifters MUST begin to rotate in their bores within a very short time. If they do not, the lifter and cam lobe WILL both be wiped out. For this reason, a good engine builder will always check to make sure that the lifters fit freely in their bores. Often a small hone, like a brake cylinder hone, is used to clean up the bores to allow free movement.

If the motor is allowed to idle for a long time, or even if it takes too much cranking to initially start it up (very common for novice engine builders... look at all the posts on here from people who can't drop a distributor in and reach in the window and fire up the motor), the slow movement of the cam may not be enough to get the lifter started spinning. If this happens, the cam will dig a groove in the bottom of the lifter very rapidly, and that will keep it from EVER turning, and destruction will follow in short order.

When the factories were still using flat-tappet cams, they did this part of break-in at the end of the production line; they fired up the cars, let them fast-idle for a few minutes while checking for leaks and so forth, and that was enough with the weak stock springs. The more spring pressure there is, the more important this is; as higher spring pressures tends to make the lifter less willing to rotate.

 

The lazy ramps on stock cams also seem to be much more forgiving than the aggressive lobe profiles on the latest generation of flat tappet cams.