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Bore out merc800

haventaclue

Outstanding Contributor
"I was considering a new to me

"I was considering a new to me motor but due to the credit crunch,ok recession,I was considering either having my beautiful
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merc800 bored out or at the very least honed,if that's possible.
I have read on previous posts that a rebuild will leave the motor as new? What does a re build entail?i.e. new pistons etc? would I be better boring it out a little putting in oversize rings leaving it at that? Or am I just wasting my time money and my very patient wife's patience?"
 
"Noel, a number of different t

"Noel, a number of different terms will mean different things to different rebuilders.

You may hear "recondition" which may simply be, hone the cylinders, new rings, clean up the pistons, inspect and/or replace any bearings that are worn. That would also include new gaskets/seals.

A rebuild may not be a whole lot different depending on the "builder".

I would consider a rebuild to be - bore the cylinder(s) as required. New piston/rings/bearings/seals/gaskets.

I'm not a huge fan of simply boring one or two holes. I look at this way. If you are going to crack her open, why not do everything while you are in there.

On a 4 or 6 cylinder for example, a couple extra pistons/rings plus a couple extra holes punched may only add up to 300 or 400 bucks. Then you have everything done and you have a "like new" powerhead. Nothing worse that skimping and only doing one or two pistons only to have another one chew a ring 12 months later.

The greatest cost is having it torn down, plus the cost of gaskets/seals. A rebuild on 2 cylinders may run 1500-2000 at a shop a total rebuild on 4 or 6 would probably run 2000-2500.

If you have the ability to do it mostly yourself you can usually pick up a rebuild kit (pistons/rings/seal/gaskets etc) for somewhat less than 1000 bucks and then count on 50-75 bucks a hole to have them bored or honed.

If the cylinders are not scored and the pistons are good, new rings/gaskets/seals and a hone could be done for well under 500 bucks with you supplying most of the labour..

It is always possible as well to find a rebuilt powerhead. There are a number of reputable builders that may or may not give you a core rebate for your current powerhead and quite a few give a warranty with their "new" powerheads..."
 
"Hi Graham, I'll split her

"Hi Graham, I'll split her and have a look at the cylinders,if they aren't scored I'll just have it honed, decarb the pistons,replace the rings,seals and have a look at the bearings. Would I need to get a different size rings if I only hone it? If the pistons are scored I'll get them all bored and renew rings.
As I live in Ireland everything has to be shipped from the States as it seems to be the only place I can get the parts for this age of a motor.
I have split her before to do the reeds,so I'll chance it again.I didn't do everything then because I didn't know and I hadn't found this site at that time and I'm very greatfull for the help I get here and thanks again."
 
"No, if it only needs a hone,

"No, if it only needs a hone, all they do is remove any glazing on the cylinders and give the walls a good hatch patern. You then use "standard" rings."
 
"Thanks again Graham, I'll

"Thanks again Graham, I'll split her first opprtunity I get. I'm going to give up work ,it's interfering with my hobby
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Hey JB - Na - you haven't

Hey JB - Na - you haven't missed much. We were just discussing reconditioning/rebuilding in general.

You get any deer hunting in this year??
 
"I bow hunt, been out a bunch,

"I bow hunt, been out a bunch, let a couple bambi's go. Nothing big has jumped in front of my arrow.
I thought maybe Noel's engine expired."
 
"No - just kinda going over re

"No - just kinda going over reconditioning etc to compare against a "new" motor.. Haven't had "new" one hangin' off the back of any boat I have owned since about 20 years ago - I must be really cheap


The early bow season up here sucked - way too warm - late bow has been open for two weeks now but I haven't got out yet at all - finally have some snow on the ground so I'm running out of excuses (although I have been totally reconditioning a 140 Johnson that I picked up) - just have to decide - do I take out the recurve or the cross-bow?"
 
"I have a couple of those deer

"I have a couple of those deer mowing my lawn for me,pity they mow everything else as well.Bigger pity gun control here is tighter than the proverbial egg in a ducks a...,and a permit for a crossbow costs the same as for a rifle,renewable every year,an as its past midnight I'm of to bed"
 
"morning I went of at a tangen

"morning I went of at a tangent last night.No she hasn't kicked the bucket yeeeet!What I was wondering,as I wait for my lifting eye to arrive,as my beautiful 1969 vintage merc800,ok not vintage, just old,no clipart as it closes the window,as the hp is rated at the block at 80hp,I'm told that the hp at the prop is about 60 -63,would I be under powered.Boat being of unknown origin,the only one I can find like it is a maxum 1700,I'm claiming the 5th here,is supposed to be fitted with either a 70hp force with a 10-1/8-15 or a mercury 75 with a 12 3/4. 21.
I have read in other posts that 80% of max rating is good,so as my beautiful merc8oo which has a lot of $ spent on it might need a smaller prop as well as I currently have a 13 1/4 17" on.
I'm still going to split her to have a look, take it to a fella to have a look,hone,bore,rebuild as need be.Good hunting"
 
"Sorry - response got a little

"Sorry - response got a little long here...

Noel, your 80 "brake horse power" rated motor would still be at least 70 "shaft horsepower" under the current rating system - and I would think probably closer to 72 or 73 horses actual.

And yes, an "optimized" rigging would usually call for the outboard to be about 80% of the maximum rating for the particular hull you are hangin' it on. And that's a fair way to figure with most utility/fish-ski/bowrider style hulls up to about the 20 foot length range.

That is a changing concept though. Lund, recently changed the ratings on their hulls. A 17 footer which was rated for 150 horse in 2005 dropped to 115 for 2007. When I questioned why? because it looked to be the exact same hull to me, the answer came back "because a 115 4 stroke weighs in at about the same as a 150 2 stroke. We had to either lower the rating or redesign the hull to account for the additional weight at the transom which would have caused handing issues".

If you have an older boat or home made etc where there is no "rating", or one can not be found, you can use the US Coast Guards formula to figure one out.

The formula is pretty simple -
Measure the length of the boat from bow to stern in total feet (so 17'4" would be 17.33 feet)

Multiply that by the width of the transom at it's widest point the same way. So 5'6" would be 5.5 feet.

In this example the answer would be 95.3 (17.33x5.5), rounded down to "95". (if this factor is less than 52 there is a different set of rules).

If that boat had a 15" transom, the max horsepower would be 95 horse.

With a 20" transom, you double your answer (95x2 = 190) and subtract 90 (their formula - I don't know where the 90 comes from). So with a 20 inch transom the same boat would be rated at 100 horse (190-90).

I have simplified it a bit here, but those are general rules of thumb. I do have the complete work sheets with all the instructions to calculate horsepower based on the above, plus hull shape etc."
 
"Hey Graham, Did you measure m

"Hey Graham, Did you measure my boat? Those are the measurements to the inch,20" transom.
I've hit 40mph by speedo in perfect conditions,if I hung 100 horses on the back of her I would probably give myself heart failure,I'll stick with the black beauty,for some reason she kinda appeals to me.
I hope to split her next weekend and hopefully I'll know before Christmas whether I'm reconditioning or rebuilding.Should I change the bearings as well while I'm at it?
I spotted a rebuild kit $710,plus p&p,plus customs duty,plus VAT @ 21.5% .Thats if they spot it.
Enough of crying poverty, thank you for the info,I'll get my very patient wife to print it off the page."
 
"This is like watching one of

"This is like watching one of your kids grow up. I remember when Noel was just a little guy, now he's going to crack his first case open. Sad to see em grow up so fast.
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"I really would go with the wh

"I really would go with the whole kit if you plan on keeping the boat for years to come. Nothing worse than doing "some" work and having something else fail next season.

700 bucks or so is really quite reasonable. That build could last you 10, 20 or even 30 more years - and you would have no regrets - no worries about that bearing that you should have changed while you were in there...

And what really makes it worthwhile... "DAD" is proud
"
 
"Ye're giving me white hai

"Ye're giving me white hair,I split her over the last couple of evenings,One cyl. scored very slightly, rest ok.Gum or burn marks down each piston,I'm assuming the rings are badly worn. Wrung every nut on the block cover
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.Why was I taking it off? The corner of the cover was broken off,NOT GUILTY. I have managed to locate a cover but nuts and bolts,nope,plus a couple for the crank case,so it looks like I'm going for the bore and rebuild,and if I get 30 more years out of it I'll be well into my three score and ten
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Thanks for the help and advice Graham and everybody else who takes the time to answer my posts"
 
"I know its to late but, 6 poi

"I know its to late but, 6 point sockets, not 12, 1/4 inch impact gun on all to remove. Grade 8 nuts and bolts at the hardware store. Light scoring cleans up with a good hone. Measure bore to see if it's in tolarance. Check wear on crankshaft splines before investing $."
 
"Hi JB ,thanks for the quick r

"Hi JB ,thanks for the quick response.It's at times like this I wish we had Home Depot or Lowes here,there's not the varity in B & Q.Special order.Splines look to be A1 definitely need bearings at both ends.So I think I'll change them all
Amazingly I found a guy who works on outboards about 10 mile over the road but will not touch my beautiful merc800,but is prepared to advise and check out the block,that doesn't mean I'm going to leave you guys in peace
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"Hi folks Here we go again.

"Hi folks Here we go again.
Honed, had her measured ,replaced all bearings,rings and gaskets and she is turning out 120psi down the line,which is a rise of 30psi.So thank you all for the advice and encouragement.
I have purchased a tacho with the wiring in situ and different color codes.So I'm guessing,mostly, brown = sig.> gray, black > black, white > purple with a loop to dark blue for panel lights?
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me this morning when she fired up.Thanks again folks."
 
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