Buying a compression tester

[dynamic]

I went to the local parts store to get a compression tester. All the models they have were metric only with adapters for 12mm, 14mm & 18mm threads. My motors are both 1971 Merc's. Will any one of these old $25 metric compression testers from the parts store work or do I need a special SAE one? Thanks.

 

[WillyBWright]

Re: Buying a compression tester

Your sparkplugs have 14mm threads.

You need the kind with a rubber hose to be able to get a reading from the bottom cylinder.