Checking C1 097 cam indexing

[Rod K.]

Assembling my '57 283 270HP short block using an NOS 3736097 cam and checking to insure it is installed indexed to the crank correctly. I'm using a NAPA double roller timing set with the optional 2* retard and advance positions, installed straight up.

The only timing specs I can find are from Duke in a Mar 2011 thread (for one) showing a POML of 108.5*, so I'm checking lifter height .050" below max lift on the #1 intake lobe and find I have a 113.5* POML (assuming POML and "centerline of lobe" are synonymous) which means the cam is apparently advanced 5*.

This seems like a lot of error in the cam or timing gears, so am I missing something? I've checked and rechecked the degree wheel and TDC and the .050 below max lift points and my math.

Is my assumption that POML=CL of the lobe correct for symmetriical lobes? Any suggestions or recommendations before I just retard the cam?

Thanks for any help.

[Duke W.]

Re: Checking C1 097 cam indexing

The POML is the same as the "centerline" only if the lobe is symmetrical. In the case of the Duntov cam it is. No other OE cams that I am aware of post-50s are. That's why I use the more precise "point of maximum lift" rather than the vague "centerline" hot rod lingo.

You should check the indexing in the area of maximum velocity, which is about .100-150" above the base circle on the Duntov. Try again.

For asymmetrical lobes you have to be careful that you are not measuring in realm of asymmetry or your measurements are worthless. For typical OE cams the asymmetry on the closing flank doesn't start until at least .165" above the base circle so .175-200" above the base circle is usually a good place to measure since you are in the symmetric range and velocity is near peak. At .050" below POML, velocity is getting pretty low, so there is less lift per degree of rotation than farther down, and your measurements will have more error than if you measure near the point of peak velocity.

You should measure the indexing at least three times and if the data is fairly close and averages within a degree or two of spec, it's okay.

Some guys obsess over being a degree or two off, but that's within production tolerance and a degree or two is not going to make any difference. There are much better things to worry about.

Duke

[Rod K.]

Re: Checking C1 097 cam indexing

Thanks, Duke. I'll recheck at your suggested lift points. I wondered if my .050 below max lift might not be the right place to check, also wondered if the lifter bores might be offset from the cam CL a little which might affect the measurement.

I wouldn't have been concerned about a couple of degrees, but 5 seemed a bit much for normal tolerances.

Thanks again for your help and all the incredible info you provide.

[Duke W.]

Re: Checking C1 097 cam indexing

Measuring at the lifter is the most accurate, and measuring at the upper tip of the pushrod if the manifold and heads are installed should give essentially the same measurments.

BTW, if your new POML measurement is off more than a couple of degrees, try a different inlet lobe.

Duke

[Rod K.]

Re: Checking C1 097 cam indexing

Just got back to this. Rechecked at .125" lifter lift and came out with 108* inlet lobe CL, within the tolerance of my setup. Thanks for the insight into checking points. Now I can sleep tonight. Thanks again.

[Clem Z.]

Re: Checking C1 097 cam indexing

you need this type that fits into the lifter bore for more accurate readings. has a part for checking both flat tappet and roller cams. http://www.summitracing.com/parts/PRO-66838/?rtype=10 can not be used with the head on

[Duke W.]

Re: Checking C1 097 cam indexing

Just got back to this. Rechecked at .125" lifter lift and came out with 108* inlet lobe CL, within the tolerance of my setup. Thanks for the insight into checking points. Now I can sleep tonight. Thanks again.

That's well within combined production tolerance/measurement error. I don't usually recommend even checking camshaft indexing if a GM, Federal Mogul, or Dana Corp. OE cam is installed with a standard non-adjustable timing set, but it's a good idea if you use an adjustable timing set to alter the design indexing.

Duke