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Cam Rebuilding, Swapping, Buildup, & Restoration

 

Cam Design

A hydralic flat tappet cam's lobes are ground with a slight taper which spins the lifter and pushes the cam rearward. Also consider the rearward thrust of the cam produced by driving the distributor gear, which also drives the oil pump. The back of the cam gear then contacts the block preventing the whole assembly from going any further back. OTOH a roller cam has perfectly straight lobes. Of course to keep the cam from wandering around you must ues a thrust plate or cam button.

Excessive cam end play causes erratic timing and causes/is caused by (in a self reinforcing loop)timing chain stretch. Using a cam button does not cause any damage, and in fact stabilizes timing at high rpm. I can't think of any reason why a device that eliminates cam end play would cause premature wear on the cam. Even the timing cover shows no effect from the cam button, since the bronze button is the sacrificial (softer) component.

Mondello makes a bronze spacer that installs behind the cam gear (between the gear and the block) for use in setting up your cam properly. This is usually used with a bronze cam button.

[ Thanks to Walter, Scott Mullen, Glenn Connors for this information ]

Cam Nomenclature

Lobe centers are the apex of the lift for the intake and exhaust lobes. Lobe seperation is the number of degrees that seperates the two lobe centers. Overlap is the number of degrees that the cam holds both intake and exhaust open at the same moment. Cams are a very simple concept, but are extremely complicated in use.

Probably the best place to learn about these terms is from cam cataloges. Most have detailed information with pictures.

[ Thanks to Danny, Walter, Mark Prince for this information ]

Replacing the Cam

Make sure you spring the extra $35 for a new set of lifters (and $20 more for the lifter extractor tool). If the lifters are stuck in the block, don't just replace the cam. Even assuming that you could do it, you wouldn't want to; the lifters develop a wear pattern with the individual lobe of a camshaft, and will cause excessive wear on a new cam's lobes.

If you were going to try replacing just the cam, I suppose you could unbolt the rockers, take out the pushrods, and pull the lifters up in their bores enough to clear the cam journals. Once you are at that point, however, you're only an inch away from pulling the lifter out of the bore anyway. Besides, it'd be much less than fun if one of the lifters decided to take a dive into the oil pan when you had the cam out.

[ Thanks to Bob Barry for this information ]

Picking a Cam

All Olds passenger-car V-8's (except the '83-'84 H/O and '86-'87 442's) after about 1973 ran the same mild cam grind: .400" lift/~260° duration. Actually, some 260's and 307's may have had an even milder cam.

The H/O-442 cam in the 307's was a bit more radical: ~.430" lift, I recall. Of course, if you've got a roller-cam, so you can't just use the grind (and especially not the actual cam) from a late-70's 350.

Some helpful cam specs:
250 HP 350: .400" intake and exhaust lift -- 250° intake duration -- 264° exhaust duration -- 111° lobe centers.
310 HP 350: .400" intake and exhaust lift -- 258° intake duration -- 272° exhaust duration -- 111° lobe centers.
325 HP 350: .474" intake and exhaust lift -- 308° intake duration -- 308° exhaust duration -- 113° lobe centers.

[ Thanks to Bob Barry, others for this information ]

Degreeing the Cam

Yes, normally you just line up the dots on the sprockets when installing a cam. Unfortunately, many emissions V-8's reduced emissions (and power) by retarding the camshaft; sometimes the dowel on the camshaft would be a few degrees retarded, sometimes the dowel hole in the cam sprocket. When you add in production tolerances, it's quite possible that your cam will be *nowhere* near where it should be, even though you've matched up the dots.

This is one thing Mondello warns about in his tech manual, and that I've seen myself; even name-brand sprocket sets, with the dots matched, will retard the cam.

Now sometimes you want the cam to be retarded, sometimes you want it advanced, but most of the time you just want it installed "straight up". With a degree wheel attached to the front of the crank, you measure where certain camshaft events (max lift, opening .050", etc) occur in relation to TDC and BDC of the #1 piston; these are measured in degrees on the crank.

You can adjust the cam timing by using an offset bushing in the sprocket dowel hole, or using one of those multi-keyway timing sprockets.

And yes, if you could trust the dots, you wouldn't need to degree the cam unless you wanted to advance or retard it for some reason; I'd say that even on a stock Olds rebuild, you'd want to do it just to make sure the cam is where it should be.

[ Thanks to Bob Barry for this information ]
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