Trying to set timing on a street hemi. Has thick plug wires so I cannot get a flash from my timing light. Car has a msd so I am using a clamp on sears/penske vintage light.

I don't have a stock wire to substitute, it has 90* boots at the distributor end and I don't want to pierce the jacket of the new wires.

I am not sure if the insulation thickness is the problem or if the clamp is not closed tightly enough to register

I tested the light on another vehicle with regular diameter wires and it is working.

I was thinking of cutting the plastic on the clamp larger but it looks like the magnets are 100% around.

Any ideas for a simple fix otherwise I will try to get 1 stock wire to substitute.

Put a gum-band around the end of the clamp on the timing light to squeeze it tighter after you clamp it on the wire. This works for me.

Try opening up the spark plug gap to .045 or larger if it is smaller than that now
The coil will make the voltage needed to jump the easiest gap, at idle it is very low voltage usually

An inductive pickup works on current, not voltage... once the arc at the plug is initiated, the gap resistance drops to a very low value. (Swamped by the resistor plug or resistor wire if used). Then the energy in the coil is delivered over time. (Yes, there's more to it but this is simplified as much as possible)

Most likely the problem is (as the OP noted) the clamp probably isn't closing all the way. Even a very small air gap of thousandths in a transformer core causes most of the flux to appear across the gap, so the secondary (the pickup winding) will not see much signal to trigger the light. Make sure the clamp is completely closed and that the core ends are free of debris

I had a spare set of old wires on a old cap. Cap was drilled to check for indexing with #1 for the crank trigger. Pop it on, do what i needed to do, pop it off. Easy.