New Piston for '87 90hp

[blimp]

So troubleshooting a leaking carb, i decide to take a compression test, and find the #3 hole is only 35lbs, take off head and find this:

The rest of the cyl all have 130-140 psi so i am hopefully just going to have to replace the piston and hone the cyl. The scoring is not too bad, barely can catch a fingernail on it. I am going to break it down and bring it to a machine shop to make sure all the holes are round and the scoring isn't too deep.

This'll be my first rebuild of anything bigger than a 2cyl...

I have a few questions, how should i remove the powerhead? Is it liftable by 2 people? I am going to remove carbs, heads, and flywheel to reduce weight a bit. I was thinking about using my garage rafters to fashion a pull style lift. The garage is a 2 car, has decent support, but i don't want to pull down any rafters on my head! Any suggestions from those who have rebuilt one of these v4 crossflows?

Should i re-ring the rest of the pistons? The compression is so good i'm leaning towards no.

Thanks for the help, this forum has been awesome and I have learned quite a bit.

 

[wilde1j]

Re: New Piston for '87 90hp

One person can carry a V4 powerhead, so you shouldn't have a problem with this one, after getting rid of the accessories (starter, etc.).

Get an OEM shop manual first.

Since the powerhead must be completely dis-assembled, why not replace all rings ... it's not that much compared to the labor. Also, try to determine why the hole failed.

Honing a bad cylinder is seldom an option.

I've used a come along tied to a garage beam to lift a powerhead. The problem isn't so much the weight, it's typically the adapter gasket doesn't want to release the powerhead. The powerhead probably doesn't weigh much over 150 lbs with accessories removed.

Use a quality marine machine shop and let them tell you what block work needs to be done.

 

[blimp]

Re: New Piston for '87 90hp

Thanks for the reply, I have an OEM manual and have been reading over the powerhead removal and rebuilding sections.

The machine shop I am going to use is reputable and used by several major marine repair shops in Austin, I am going to let them measure and diagnose any issues with the block. They charge $45/hole which seems very reasonable.

 

[bob johnson]

Re: New Piston for '87 90hp

Thanks for the reply, I have an OEM manual and have been reading over the powerhead removal and rebuilding sections.

The machine shop I am going to use is reputable and used by several major marine repair shops in Austin, I am going to let them measure and diagnose any issues with the block. They charge $45/hole which seems very reasonable.

if you can catch a fingernail it has to be .003-.005" minimum, double that for dia, and you would be way out of spec trying to clean that hole up...

since you are buying a new piston anyway...it make all the sense to just bore and hone that hole to the next size piction..( $45 seems like a price for both boring and honing)

the tricky question is wether you leave just stick the others back in...

rings are about $30 a hole...then you probably should hone a little.. so another 412 a hole..

but you feel better about going forward..and you get the most out of all those new gaskest you just bought!!

bob

 

[blimp]

Re: New Piston for '87 90hp

Some carnage pics, the #3 cyl had severe piston damage and a decent score, i'll find out what the machine shop says tomorrow:

#1 cyl had a broken ring as well, but no scoring or other damage:

I think the damage came about from lack of lubrication at idle on the lower cylinders, the float was getting stuck and they were running dry. #4 was very dry when i took it apart with some very very minor scoring.

Going to go with new rings for all cyl and hopefully just a bore and oversize piston for the #3, possible re-sleeve if the scoring is really bad.

 

[bob johnson]

Re: New Piston for '87 90hp

unless they have been bored before, cant imagine scoring, causing you to need a sleeve, unless you destroyed a port...

I am wondering if the holes will clean up(with honing) in the other cylinders that you can JUST install new rings...

good luck

bob

 

[blimp]

Re: New Piston for '87 90hp

So the machine shop says i need a .064 over piston, the only ones i can find are wiesco forged. is it ok to have 1 forged and 3 cast pistons? Is there a weight difference that can cause an imbalance?

 

[wilde1j]

Re: New Piston for '87 90hp

What does the machine shop say?

Personally, I wouldn't be too concerned.

 

[Dhadley]

Re: New Piston for '87 90hp

I doubt seriously if the other 3 measure within spec after being subjected to that much combustion temp. Assuming the other 3 are standard now you can go .020 on them and .064 (if needed) on the bad hole. No problem going that way.

 

[bob johnson]

Re: New Piston for '87 90hp

wow that is a huge amount...you must have really hammer the ring on the edge of the port.....

I didnt even know they went that far over and that they went on anything but .01" increments.

the specs are so tight for roundness and size, I also doubt that the others are in spec...especially if they get honed a little....

you have some hard choices i think.....

good luck

bob

 

[blimp]

Re: New Piston for '87 90hp

Other cylinders still have cross hatch marks, they are In really good shape. Still hoping to get away with one oversize and just rings for the rest.

 

[emdsapmgr]

Re: New Piston for '87 90hp

Looking at the middle picture, the bottom lip of the cylinder liner looks visibly damaged at the middle port-probably from the ringset. Any chance this will cause a long-term (ringset) running problem after overhaul? Is a new cylinder liner needed-or will this clean up with .064 overbore?

 

[blimp]

Re: New Piston for '87 90hp

Just heard back from the marine shop, he was able to clean up the cyl with a .030 oversize. Just ordered oversize piston kit and rings for the other 3, plus gasket set. I'm very excited to get this beast back on the water. I was looking at buying a new motor on craigslist, i'm glad i don't have to.

 

[emdsapmgr]

Re: New Piston for '87 90hp

.030 over is an good outcome.

 

[54fleetwin]

Re: New Piston for '87 90hp

I am amazed by any shop that would be willing to do a bore job before having the piston in hand.
You bore to maintain piston to wall clearance, not all pistons will measure exact.

 

[bob johnson]

Re: New Piston for '87 90hp

I am amazed by any shop that would be willing to do a bore job before having the piston in hand.
You bore to maintain piston to wall clearance, not all pistons will measure exact.

do you think that on the factory manufacturing line they have the exact piston at heand while they bore the block for each hole?? i seriously doubt it..

thats why you have measuring tools.. and tolerances...

they bore a hole to a spec and a tolerance...

then they make pistons to a spec and a tolerance...

thats how stuff fits...

when they make cranks they have way way tighter tolerances... but they dont have bearing sitting by...

the crank journals have to be within .0002"-.0005" usually

bores have a few .001" s, same for pistons....the rings take up the slack!!!

bob

 

[54fleetwin]

Re: New Piston for '87 90hp

The reputable shops around here wont do the bore until you order the piston.
.003 cylinder wall clearence does not leave much room, forged pistons expand differently than cast and require slightly different tolerences.
What if the shop sets you up for a cast piston and you order a forged?

Plus with the different manufactures of pistons, each brand can measure slightly different.

The factory only uses one brand.

 

[54fleetwin]

Re: New Piston for '87 90hp

usually

bores have a few .001" s, same for pistons....the rings take up the slack!!!

bob

You need enough room for piston expansion, this is why you have rings, but you dont want so much room that the piston skirts slap the walls.

Some 2 strokes use a tapered bore with ringless pistons

 

[blimp]

Re: New Piston for '87 90hp

Machine shop is waiting on piston to complete the job, I believe he just bored until the gouge was removed then measured. He then called me and told me what size piston to order so he can finish up the job.

 

[Dhadley]

Re: New Piston for '87 90hp

A 2 stroke outboard piston is tapered, cam ground and barrel shaped. Not many shops can accurately measure those. The clearance is built into the piston, not the cylinder as an automotive motor. This is the main reason to stay away from an automotive machine shop. A good marine machine shop does not need the piston in hand. They may ask what brand piston and what your set up rpm is but that's about all they need.