OMC 4 hp pistons

[mercuk]

I have and old 1970's 4hp Johnson that I bought that had been badly maintained and as a result sufered a scored piston resulting in low compression and lack of power....
Anyway from what I remember everything else was fine as regards to ignition, carb and gearbox etc so the only thing to do is swap the block out or replace the damaged piston. I know the bores could have scoring and or suffered damged but hopefully not to bad so I can just give it a quick hone out,,

I have found some GOOD second hand pistons for this little twin cylinder 4hp that seem to be up to the job.. Now my question for all you OMC experts out there is any significant piston design changes with these engines or not?!!..

All pistons mentioned come from CROSS-FLOW SCANVENGED engines so have the high domed/shaped pistons....

Are they all interchangable?!!! I.E should all the older OMC twin cyl 4hp engines share the same pistons and rings and have same bore/stroke etc,,, Many thanks in advance for any replies...

 

[JB]

Re: OMC 4 hp pistons

First, you need to determine the model number of your engine, then compare part numbers of the pistons with other model numbers. I suspect some are interchangeable, others not.

 

[mercuk]

Re: OMC 4 hp pistons

Right o.k
model number: 4.BR75A
Serial number B18210 Made in Belgium.

The chap selling the second hand pistons has no age/serial number. The only thing he said he states:

"These pistons fit on a 21 mm shaft , with shells. Con rods are good with shells. Rings complete and Ok."

Some OMC Guru out there must know if its going to work or not..... Thanks.

 

[Chinewalker]

Re: OMC 4 hp pistons

Only changes I am aware of on the 4hp models are later ones used needle bearing on the wrist pin, where earlier ones (up to late 1970s) used bronze bearing. Not sure if the wrist pin diameter changed...

 

[sutor623]

Re: OMC 4 hp pistons

Mercuk I have a broken down 1969 JohhnyRude 4hp at the house. Id be glad to check into anything if it would help.

 

[mercuk]

Re: OMC 4 hp pistons

Wow thanks for the help guys really appreciate it...
Sutor, If you don't mind, could you tell me if the big and little ends use needle bearings or shells please??!! That would be great help to me.. The chap said that the pistons and con rods he has are running shells...
I haven't actually broken into my block yet because its been sat at the back of the garage for nearlly ten years when I got the hump with it for letting me down..... It was my first EVER outboard I bought and left me stranded for several hours......

Anyways over the years I have learnt a great deal with outboards and believe I have the confidence and comptency to take this one on adn get it running again.

My thinking was that if I get lucky, I should be able to just swap the pistons over and use my original crank and con rods...... I hope....

Thanks again to all that replied, Marc.

 

[mercuk]

Re: OMC 4 hp pistons

Only changes I am aware of on the 4hp models are later ones used needle bearing on the wrist pin, where earlier ones (up to late 1970s) used bronze bearing. Not sure if the wrist pin diameter changed...

Chimewalker, from what your saying I should (hopefully) be all right then... I know for a fact the replacement secondhand pistons are running shells (bronze bearings) so presumably my old OMC should be the same......I think.... Couldn't find out definate year for mine... but go a feeling its early-mid 70's.....

Thanks for the input, I appreciate it....

 

[nwcove]

Re: OMC 4 hp pistons

Johnson Crankshaft & Piston Parts for 1975 4hp 4R75A Outboard Motor

 

[mercuk]

Re: OMC 4 hp pistons



I decided to be the pistons anyway they were so cheap, I was worth a chance.

Anyway, thanks to everyone that commented in an effort to help me out... Best regards, Marc.

 

[mercuk]

Re: OMC 4 hp pistons

Only changes I am aware of on the 4hp models are later ones used needle bearing on the wrist pin, where earlier ones (up to late 1970s) used bronze bearing. Not sure if the wrist pin diameter changed...

Just an quick update so that it may help others.......
Well you were correct Chinewalker.....
Unfortunately, it worked to my disadvantage and the replacement pistons were the needle bearing versions and my original ones were bronze..... They were not interchangable......Wrist pin diameters are different on them........No go..........

 

[F_R]

Re: OMC 4 hp pistons

Piston part number 384651 fits JW-10 thru JW-21, and 4R thru 1977. So, that would be what you need for your 4R75 (1975). 1978-up was different.