87 70hp Johnson or an older Johnson 85

[cesandroid]

Need some help on which direction to go. Contemplating getting an 87 70 HP Johnson complete but needs a new top end like my 85 70hp which I invested some new parts into it only to find out the block is in bad shape with water damage and needs a new top end as well. I like the water/ exhaust design on the 87 and can get this for 300 bucks including controls and use my 85 for parts. Or talking to a fellow who is selling a late 70s trihull with 85hp Johnson who he says runs very well for 1300. My boat is rated for a 100hp. Do I try to auction off what I have and go with a late 70s 85 that hopefully lasts a few years or do I stick with my initial investment? Would like to hear others opinions on this please, thanks!

 

[cesandroid]

Geez, now the seller phoned me today asking if I want it or it goes to someone else. I guess I'm leaning towards redoing the 70.

 

[flyingscott]

1300 is much for a 70s tri hull with an 85 hp motor. I would lean towards the 87 motor myself. the 87 is a significant power improvement over your 85 model. The ignition, lowerunit and some powerhead parts from your 85 interchange with the 87 motor. One thing to make sure of on the 87 is the bore size a lot of them have been overbored already.

 

[cesandroid]

Hmm thanks flyingScott. Don't know if this motor has ever been rebuilt as the seller doesn't know either. I guess what your saying if it has been bored to the max and the cylinders are pooched than the motor is pooched as there would be no pistons and rings that would fit. Is that correct?

 

[cesandroid]

Informed the seller of this but again he claims he has no history on the motor. I suppose other than measuring the cylinders, there's no way to tell if the walls have been over bored is there?

 

[flyingscott]

You would need to pull the head off and look at the pistons they would be marked. Does your 1985 motor have tilt trim. The 85 model powerhead does not.interchange with the 87 block. The 87 is a 56 cid motor and your 1985 is a 49 cid motor. If the 87 is pooched he should have no problem taking the head off. Otherwise powerheads for your 85 motor are plentiful.

 

[cesandroid]

Yes mine has tilt and trim, so does his. he has the head off and is looking at the cylinders. he figures honing is all it would need. he notices some lines on the walls but he says they dont look too bad. I asked him about scoring and he didnt think so. What are you referring to about the markings on pistons?

 

[flyingscott]

The pistons will have the part # on them and a size if it is overbored. May need to clean the carbon off to see them.

 

[cesandroid]

Ok flying theres a 0.50 on one side a 100-120-05 i believe on another and 080928 b33 i think as well. he had to scrape the carbon off. does this mean .5 over standard or am i hooped?

 

[Faztbullet]

It will fit if you use the correct adapter plate...only difference is the mounting bolts..3/8 vs 5/16. That piston is .020 oversize

 

[cesandroid]

Ok thanks Faztbullet, damn, learning fast here, this forum is great!! so that would mean theres only 1 size over left according to the parts i checked. the adapter plate??

 

[cesandroid]

So i guess my point here is is this powerhead still rebuildable with the info I have??

 

[flyingscott]

You only have .010 left. I would concentrate on your 85 motor

 

[cesandroid]

Ok flying I see. well this is priceless information here!! thanks so much as I will move on.

 

[cesandroid]

To finish off, is my 1985 70hp still a good fit for my boat. Even with a struggling motor I was still getting 34 with 2 adults in the boat. Thanks again

 

[flyingscott]

How is your motor damaged? What is the motor on? I have had that motor on a 16' aluminum boat and only hit 30-32 with a healthy motor

 

[cesandroid]

Water damage, scored pistons. Its on a 15 ft StarCraft semi tri hull. Boat weight around 1000lbs with the motor

 

[flyingscott]

Yes I.would.work on the 85 model.

 

[cesandroid]

Thanks flying. researching through this forum is indicating with the damage i have, boring and honing probably 2 sizes over is what i will have to do. any idea on rough cost of something like this?

 

[racerone]

I do my own boring at $ 25 a hole machine rental time + a set of honing stones.-----Pistons are $100 each.------Gasket set.------Having to pay a shop adds up quickly.to well over $2000.