70 hp powerhead problems and questions

[bob johnson]

decided since i had the power head off on this non stop refurbish of my 1988 70 evinrude...Id just pop the head ...worst that could happen is I got to replace the gasket...but it gives me a look and see!

well the lower cyliner looked alike a bomb went off in it..shrapnel embedded in the cycliner and the piston...and it was metal and hard...

so I had no choice but to completely tear it down so i could see if it was the ring or rings...

the motor had obviously been rebuilt...seeing all the extra gasket sealer and the pistons were marked .040. which I found means .040 over size and the bore are 3.229.

well the picton on the underside looks brand spanking new!! the rings look new and both intact on that piston.

the needle bearing on the wrist pin are all there..and look brand new.. the rod journal bearings look new and are all accounted for.

so i am wondering what could have let go to come flying into pieces in this cylinder???

it would have to be small in dia but hard!!..the pieces broke apart...they didnt get bent like soft metal..they broke like a hardened piece of metal.

Also the piston measures about 3.218..is it supposed to be that much smaller than the bore??? i was thinkign it should be within maybe .004 or .005 " of the bore dia!!!

the piston actuall comes up over the deck..but it does so less than the thickness of the head gasket...

there is just a slight bit of scouring on the cyliner walls.. and I can still see a lot of cross hatch from the honing operation...

so what could have come loose in this motor that is available to the combustion chamber?

and it a .01- .011" dia ok for the difference between the piston and the bore???

thanks

hope to hear from you guys

head...

bob

 

[da83moon]

Re: 70 hp powerhead problems and questions

If the piston is coming up to far in the cylinder wall, a apark plug could have possibly broken the top off, or the cylinder could have overheated.

 

[bob johnson]

Re: 70 hp powerhead problems and questions

doesnt seem to have been the problem...the spark plug would have hit the center of the piston...there is too much shrapnel in there for the end of a spark plug.

I took the pistons out of the block today...

they really looked new..one has damage...and the scouring goes down the side of the piston to the firt ring groove...

but the other pistons show no wear and the bearings all look new...

I think it was rebuilt and something let go or flew down through the carbs..

i have accounted for all the needle bearings on all the psitons and journals...

the clips for the wrist pins are all there...

the edges of the rings are like mirrors!!!

here is the piston top...

bob

 

[bob johnson]

Re: 70 hp powerhead problems and questions

here is a photo of the underside of the piston...

does it look clean and new???

anyone want to take a stab at what came apart that might have been hardened steel???


also in the seloc book...they say up to 1988 the ring end gap was half of what it is from 1989 and on

mine 1988 seems to be out of spec...but then again it is an over size piston..does that matter?

I have a gap of .024"

the seloc book says .07-.17" for up to 1988
and .19-.30" for 1989 and on

i am sure. that they missed a decimal point( they did use inches")
the 1989 Johnson manual I have lists the ring gap at
the .007- .017" spec.....

i guess you cant trust SELOC


the johnson manual has the piston dia and from .004-.007 under the bore dia.

the pistons in my motor seem to be about .008 - .01" undersize.

is this splitting hairs???

plus the extra ring gap???

I have to figure if i need to buy a new SET of pistons, or just one !

thanks

bob



bob

 

[HybridMX6]

Re: 70 hp powerhead problems and questions

I'm just thinking it looks like a ring broke, and they rebuilt it and used to old pistons with new rings. Crappy, but I wouldn't put it past some shops these days, or some people who are trying to save a buck and get rid of it. If it looks that new and yet this one is all chewed up, I'd say that is what happened. I am sure others will follow with their opinions as well.

 

[bob johnson]

Re: 70 hp powerhead problems and questions

but the pistons are .04 over!

I understand your thought...they would have had to already rebuild it...THEN bust a ring on the new rebuild... and then just stick a new set of rings in the once cylinder..

I guess the bottom line really isnt what caused it( although Id love to have an idea..maybe watch out for it)

my hard decisions are if this thing is close enough to tolerance specs to let go with 1 new piston...( if i can find a .040 over )

if i re hone. I might loose another .002"

then my bore is .012" over my piston..

is that acceptable?

how about the ring gap..at .024"

is that ok to let go??

can I buy three new .046" over size pistons anmd matching rings??? to get everything back in spec??

I am here I might as well put it back together to give me the most confidence.

also I had the carbs rebuilt with standard rebuild kits

not knowing this thing was so over bore.

what size jets should I have?

and is there anythign else that needs to change with the over bore condition?

thanks guys

bob

 

[HybridMX6]

Re: 70 hp powerhead problems and questions

Not sure about the over bore aspect of it, I am sure others who are more experienced with that will give their advice, but I do believe you can get it re-sleeved and bring it back into spec. I think .040" over is as big as they go. Hopefully others with more experience will chime in shortly and help you out more than I can at this time. I'd see about re-sleeving it though in the meantime, so see if it can be done and how much it would be.

 

[bob johnson]

Re: 70 hp powerhead problems and questions

no way am I resleeving this motor.

I bought it and have gone from one issue to the next and althoguh I have sandblasted and painted every single peice of paintable motor part(not block yet) and I have re gone through the lower unit( lots of new stuff already in there)... and id even go for a set of piston and rings new..( $135 a set= $400 ) and all the gaskets I still need to get.....I wont dunb another $600 into it!!!

i read here were someone found it cost about $200 per cylinder to resleeve.. PLUS the pistons and everything else!!

Id just as soon put here back together as is, if i can't find bigger pistons( although at least one new .040 over slug)

and get what i get.

heck the compression with the way the motor is now is 135-148 ish..

we squirted some wd-40 into the bore before we cranked her over..

and although we let her crank a dozen times over to get the compression seeling liquid out..there probably was a little residue left to help.


we had no water , that is why i went right to a new water pump kit( although YOU CANNOT tell the difference between the new one i bought and the one that was in there( I think it was also brand new...) the dry cranking did NOTHING to that impeller...it is flawless!!!!

we cranked the motor to check spark and to do each cylinders compression

bob

 

[bob johnson]

Re: 70 hp powerhead problems and questions

here is a shot of the bore:

I think I am buying three new pistons and rings...

I called Wiseco and the new pistons are 3.225" dia...much closer than the 3.218" dia i have now...even though they look new

little better fit..maybe the ring gap will go down also

bob