how to remove block - yamaha 2stroke 70hp

[mischa_70hp]

Hi boat people, due to zero compression on cylinder one I looked at the piston which indicates a particle went through damaging it, possible ring issue?? carb looks fine, sleeve isn't bad either ... next step I guess will be to remove the block. What is the procedure? Can the engine remain on the boat or do I need to separate it from the transum in order to be able to pull the block and piston?

I appreciate your help! stay dry!
mischa



I have a 1986 70hp yamaha 2stroke 6H300

[TownsendsFJR1300]

Block

You can pull the power head from the mid section leaving the transom mount attached to the boat. There should be numerous studs sticking down from the power head at the base of the power head.

You really should have a shop manual, but if non is available, bring up a parts fisch of your engine...

If the cylinder isn't torn up, IMHO, being you already have the head off, pull the plate that the carbs/reed valves attach to and unbolt the rod for that particular cylinder. Remove the piston and access if the block needs to come off for work... That plate will have to come off anyway to access the connecting rod..

[mischa_70hp]

Scott, thank you for the heads up!

[TownsendsFJR1300]

No problem, please post the results once you get it figured out.

[mischa_70hp]

pulled the carbs and the plate holding the reed valves and figured that the lower section of the block needs to go before getting access to the conn rod caps, isn't that right? will sleep over it ...

[TownsendsFJR1300]

pulled the carbs and the plate holding the reed valves and figured that the lower section of the block needs to go before getting access to the conn rod caps, isn't that right? will sleep over it ...

I just pulled up the parts fisch to your engine.

Apparently the connecting rods are one piece and the rotating assembly is pressed together as one unit. You cannot purchase a rod/lower bearing separatly, you must get the entire crank and rods($1,200.00+).

The piston pin and pistons do separate from the rotating assembly (and can be purchased separatly).

I was wrong believing the rods would un-bolt and you could pull an individual piston/rod.

It appears the power head will have to un-bolt from the mid section, then split the cases.

The entire assembly crank/rods/pistons will come out from the bottom.

A PIA, but at least, as long as the crank/rod isn't torn up you can replace the piston, block condition permitting... The piston IS AVAILABLE in .5mm oversize..

[mischa_70hp]

you're so right! found pic on ebay : Yamaha outboard motor 70 HP 90 crank crankshaft 3 cylinder rods 1984 | eBay it shows the pressed cap.
Thank you for looking it up, Scott. not sure if I'm gonna go there - should try though. wiil keep you posted

[rodbolt17]

I just had one rebuilt at carolina machine and tool in wilmington NC.
cost 500 including shipping.new rods,rod bearings and mains.
I think pistons from yamaha are std. .025mm and .05mm.
I think wisco goes as far as .040"
easy rebuild.
I currently have both a 50 and a 70 apart for cranks and pistons.

[TownsendsFJR1300]

I just had one rebuilt at carolina machine and tool in wilmington NC.
cost 500 including shipping.new rods,rod bearings and mains.
I think pistons from yamaha are std. .025mm and .05mm.
I think wisco goes as far as .040"
easy rebuild.
I currently have both a 50 and a 70 apart for cranks and pistons.

Those are one piece rods and the assembly is pressed togther, correct?

The parts fisch I found didn't show lower bearings/rods for sale separatly..

I know the old Yamaha 2 stroke motorcycles (that I grew up and learned to wrench/ride on) had one piece rods (closed on both ends) which were pressed all together with the crank. As a side note, we would stuff the hollow part of the crank with cork and epoxy as, at the time, was the thing to do to get get more air out from the bottom to the compression chamber...

Off topic a bit as michca_70hp has apparently a blown piston. Can I assume that IF THE BLOCK needs attention, you should bore all three cylinders? His crank/rods, are likely fine..

Are these motors known to have any issues with oiling making it to the #1 cylinder?

As for the Op, are you using pre-mix or is the oiling system (if equipped) still functioning?

[TownsendsFJR1300]

you're so right! found pic on ebay : Yamaha outboard motor 70 HP 90 crank crankshaft 3 cylinder rods 1984 | eBay it shows the pressed cap.
Thank you for looking it up, Scott. not sure if I'm gonna go there - should try though. wiil keep you posted

Yea, looking at the pic on your link, the rods are the same as the old 2 stroke motorcycles. They had caged roller bearings on the bottom end with a pressed in lower rod pin

The newer 4 strokes have rod caps.

If you can get the powerhead off, your halfways there. If the block doesn't need machining, a quick hone/piston and re-ring (I'd re-ring the entire engine as your already there). If not send the powerhead out for a rebuild or get a rebuilt powerhead..

Perhaps Rodbolt would like a project???

[rodbolt17]

rodbolt has to many projects currently.
however hat crankshaft is rebuildable.
google akermann marine,now carolina machine and tool, in wilmington NC.
talk to ken mcall.
they have done everything from a 225 suzuki crank to an 8hp tohtsu for me over the years.

[TownsendsFJR1300]

I don't think at this point the crank will be a problem, definitly the broke piston and perhaps the cylinder end of the block...