What is the size?
 

Crazy title isn't it but this is what I'm talking about
I just ordered a engine compression tester from Amazon and the fittings are in metric, I'm going to test my sons 250 Yamaha for compression.
Are the spark plug threads in metric? What size do I use? It also comes with a universal rubber cone shape fitting that should work when you hold it tight to the spark plug threads but I would feel more comfortable using the screw in adapters .
Thanks for any info.

Take out one of the spark plugs. Compare the threads on the plug to the adaptors. One of them should be identical. That's the adaptor to use. (Lay the adaptor threads next to the plug threads so they touch. The threads of the proper adaptor should mesh perfectly with the spark plug threads.)

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14 mm thread, 26.5 reach. If youve got the more popular compression tester, its the biggest adapter thats also the longest

Thanks Destroyer and spareparts. I am just surprised it's a metric size , I will post what the compression is when I get it. I hope it's not very bad news but I will see.
Just curious. Are all spark plug threads metric?

yes, most are metric, including domestic vehicles

Hey spare , how do I check for spark? I'm going to do a compression test on Sunday but was wondering I also want to see if all the plugs are firing, there are no wires only coils.

you have to use a test stand specifically for checking coils. You could probably rig something up with a spark carb and a ground wire. If its missing from spark , you should be able to tell by the way its idling.

Thanks spare, but honestly I have no idea what you mean.
But can I pull out the coil,attach the spark plug and turn over the engine and see if it's got a spark? I would stand by the plug while my son cranks the engine
What do you think?
I just what to make sure he is getting spark to all the cylinders, but I have a feeling the compression test is not going to have a good out come.