Johnson 1958 18 hp

[dietzer]

Anyone know what a stock brand new motor compression would be on this model? Want to buy a old new stock motor, but wanna make sure it’s new still. Not almost new.

[chris-p]

Based on motors with very low hours I have found, I believe the 18s had about 145 psi brand new.

That aside, for every 100 motors I see that are advertised as NOS/Never Used, I would say maybe 5 percent are accurate.

[lloyd]

I have found that the OMC 18’s need to have about 125 PSI cold and dry to run and idle well.

[shane-r13]

My 1959 Evinrude 18hp Golden Jubilee LOW (stored since 1975) hour motor has 125 psi top and 122 psi bottom. So I would say a NOS should be higher.

[chinewalker]

Chances are you will not see anything remotely "accurate" as there is great variation between gauges. Best you can hope for is to have the motor be over 100psi in both cylinders and no more than 10% variance between cylinders. This is why you’re not likely to find published compression readings in any OMC produced shop manuals.

Actually, a brand new motor will likely have LESS compression as the rings are not yet seated in their bores and haven’t gone through any break in period.

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