I have a 98 Mariner 200 2.5 and have isolated a problem down to no spark or head gaskets. The top port spark plug looks like brand new and I am going to first do a spark test and go from there. I bought a tester that the plug wire goes over and then the alligator clip clips onto the block for grounding. Does anyone know where to clip it on the block (thinking its aluminum and not a great conductor)?

2 after a compression test and if it's getting spark, could anyone tell me any info they think will help in changing a head gasket? I know it's 30 psi on the torque wrench...is there any cement or grease used or just compression...what's a good method of cleaning old gasket...should it just come off after all bolts are removed?
I'm a total nube at this but have some mechanical expirience.
Thanks for your help,
Joe Reen

Here are my results.
cold comp test
top to bottom port 94, 101, 103 psi
top to bottom strbrd 93, 102, 104 psi
head gasket?
out of ideas.
all six cylndrs have spark...top port plug is cleaner than rest but shows some sign of ignition.
???
Joe