C2: '66 L36 Measuring cam lift & compression test

[Dan Pepper]

I'm working on tracking down a persistant miss and have checked/rebuilt all possible ignition settings/parts. Still have the miss - althought I have noticed that #4 hole is fouling its spark plug. Now to check the valve train.

I've read a few posts on taking cam lift measurements and compression testing, and have a couple questions:

When measuring cam lift with a dial indicator (have the indicator and magnetic base ready to go), is the measurement calculated directly from the rocker surface above the valve stem? Should the measurement match the cam lift specs exactly?

Secondly, what is the reason for removing all of the plugs for a compression test? I was too late in learning about this step in the procedure and did the test by removing one plug at a time - and taking the reading on a cold engine (all holes read 170lbs within a few lbs. +-)

[Edward M.]

Re: C2: '66 L36 Measuring cam lift & compression t

Your dial indicator will be OK to read at any location since you are doing a comparison as long as it is from the same location at each position for each lobe.
Taking out all tha plugs just allows the engine to spin over faster and with less draw on the battery as draw is directly proportional to load.
If your cylinders are reading the same pressure then your lobes should be OK.
Check your firing order and ignition wiring routing, shielding and try it at night outside. You can see a short in the dark much easier.
Replacing components doesn't fix problems all the time. The quality of some components is nixed right off the get-go.

[Robert M.]

Re: C2: '66 L36 Measuring cam lift & compression t

Dan
I had a similar miss on my 67 L36. After checking all the items you referred to,a broken valve spring was found to be the cause.

[Clem Z.]

a broken valve spring will

cause a low speed miss as i went thru this with my new 65 396 driving it home to PA from st louis factory.

[Duke W.]

Re: C2: '66 L36 Measuring cam lift & compression t

Set up the indicator plunger as close to inline with the pushrod as you can. Reading at the vavle doesn't tell you much because the rocker arm is not really 1.7:1, and the actual ratio varies through the cycle. By measuring at the pushrod you can compare your readings with the lobe lift spec in the shop manual, and look for consistency across all readings. If you can dig up a second dial indiacator you can measure both valve and cam lift and actually compute the rocker ratio. For a SB it starts out at 1.37:1 and peaks at 1.44:1 at max lift for OEM cams. That's a lot different than 1.5:1, espcially if you want precision measurments. Nobody has ever posted actual measured rocker ratio for big blocks than I'm aware of.

I'm not sure about hydraulic lifters. If they bleed down your readings could be affected. Maybe Clem knows. I've only checks lobe and valvee lift with mechanical lifters.

Duke