OMC 18-20-25

[MOTION LINE]

I was just wondering about the differenes on 18-20-25 OMCs year 69, someone told me that the piston timing is different on a 25, will this make it<br />less fuel efficent or will the lowend torque suffer?<br />Any differences in the running, smoth or rough?

 

[Chinewalker]

Re: OMC 18-20-25

Hi Bolaget,<br /> From what I understand - the only difference between the 18 and 20 is a SLIGHT opening up of the reed stops on the 20. Whether that translated into any real-world difference in performance, I don't know.<br /> The 25 is a bit different as it uses different pistons and a different cylinder head. The 25 is fairly tightly strung as it's squeezing 25 horses from a 22-cubic inch cross-flow style motor which is about the high end of things in a service motor application. It has to make a lot of revs to make the horses, topping out over 5500 RPM and they've been known to break crankshafts. The 18/20 make their power at 4500 revs which is a much better speed to keep them together longer. <br /> In my opinion the 18/20 is a better motor as it's not as strung out. About as bulletproof a motor as has ever been made, but like anything they need their share of maintenance to remain that way.<br /><br />- Scott

 

[Dhadley]

Re: OMC 18-20-25

The 18 and 20 are identical. The 25 hp pistons are different. The pins in the 25 piston are lower. The heads should stay with which piston youre using. <br /><br />Trust me about the heads.

 

[MOTION LINE]

Re: OMC 18-20-25

Thanks Guys for the reply, seems to me it is better to stay with the 18-20, full throttle is seldom used and the 25 hp mileage is problably poor as well as the torque around 3500-4000 which is the most used RPM range, just courious to know and now I understand why they switched to the bigger block in the first place!<br />I was thinking I could use the bigger reeds that came from a later 30 hp (bigger cc.)

 

[G DANE]

Re: OMC 18-20-25

The 25 cranckshaft is different too, it will have riveted counterweights on sides, as I understood to compensate for the heavier pistons, and reduce cranckcase volume. Actually, I think you can use the later reedplate, I always thought about that, the shape is different, but the gasket surfaces will fit as I remember. Try it and let us know.

 

[gatorred]

Re: OMC 18-20-25

mine exp with older omc point's and condencers and coils,can cause not fire right or run set to 20 when apart, they all run good but trolling was my thing,slow was what i tuned for <br /><br />put at lest 50 to 1 oil in gas i run more like 40 to 1, happy boating