Curiosity, what Autolite # ?

3924. Dart 310 heads. I didn't verify, but know they mentioned the wrong plugs were in it previously. They were about .5" too long and the ground electrode had come off on one plug/cylinder.

an autolite 3924 is 3/4" long. what was in there before that was longer? any one know were this plug heat range falls on the chart? hot, cold or middle?

It’s a pretty hot plug from what I’ve read.

no idea what was in it before the shop pulled them during the rebuild.

In NGK speek, it’s a heat range 5.
Most Mercruiser blue motors (500hp, 500efi, 525efi) use a heat range 6 plug. 6 is 1 step colder than 5.

What I found.
See last page (4) here:
https://www.autolite.com/docs/defaul...heat-range.pdf

Autolite 3924 crosses to NGK 7373 FR5 V-Power Spark Plug and NGK 7938 BKR5E V Power Spark Plug

One step colder Autolite 3923 crosses to

NGK 6962 BKR6E V-Power Spark Plug

As a note, the BKR6E cone gapped at .031” . I set to .035” just because :)

Weird mistake to have happened because most aluminum bbc and sbc take 3/4” (.750”) reach gasketed spark plugs.

Ive used quite a few autolite 3923 in aluminum headed marine engines in the 8.8 to 10-1 arena, a 3924 typically too hot

How old is the balancer? They set the timing before doing any hard dyno runs and the ring could have slipped and now reads 40. Get a piston stop and check your timing mark. 3924 is recommended by AFR for Pump Gas, I run them in my motor and they are not too hot based on the ring on the electrode. But my motor runs super cool anyway. You definitely have some distribution issues. Even bank looks a little rich across the board with 4 and 7 really fat. I would try a 1" tapered spacer with a 1" open to try and get better distribution. When I pull my plugs you can't tell one from the other and I'm running a custom 1000CFM 4150 on a Brodix HV2000 intake with both spacers I mentioned.

those 2 black plugs are something wrong wires not on correct cyl? cam firing order swap? black and wet is not firing