I found a Johnson 10 hp. 1957 at the junk yard yesterday, to my surprise found out that middlesection and the gearcase are the same as used on the 18 hp. model including the rear boltthat holds the case to the middlesection.When did JOHNSON switch to the smaller gearcase which they used 1961-63?
Hi Bolaget, That would be 1958... They took the 5.5/7.5 unit and added a heavier driveshaft and pinion gear. It worked okay, but didn't hold up to the abuse that the 15/18-derived unit could... If you do find a good 4-bolt 10hp unit from 1958-up, be sure to check all seals, bearings and clutch dog before using. If you can keep the water out and the clutch dog & bearings are in good shape, you should be alright...- Scott
Thank you Scott!As a matter of fact I own one 10 from 1963which seems to be OK!However, it took me by surprise when I found outthat even the piston and and conrods are different on the 1957 model (much lighter)!Why on Earth did they make the modification, problems with the earlier parts???
Hi Boleget, With the powerhead items they improved the motor over time. They added heftier bearings at the wrist pin end of the con rod, went to a smaller flywheel, improved oil scavenging, etc. You'll find a lot of the '50s 10s with "ovaled" wrist pin bearings as they would wear at that point, leading to knocking and eventually failure. The later rods had a wider bearing to improve distribution of the forces. The lighter flywheel put less strain on the crankshaft. With the lower unit, they were looking for economy. They lightened the motor a bit by going to the smaller lower unit and made it cheaper to build. It reduced the potential longevity of the motor as you'll find a lot of those 10s (and the later 9.5s) that chewed up the units. They work fine in normal operation, but they can't take a ton of abuse (which is more like "real" world "normal" operation). A little ding in the prop to throw things out of whack might not bother the bigger unit as much as it does in the smaller unit, etc.- Scott
Thank again Scott,I had a closer look at the engines and and found out that the flywheel is heavier on the 1957 model, even the connecting rods+ the pins are smaller!I was just thinking the weight difference in thepiston-conrods must have an impact on the ballancing of the engine?Think I am going to use those later parts on my 1957!