So I've Decided to rebuild 1971 Merc 800

[Moody Blue]

So, I've decided to go ahead and rebuild the old Merc. I have one running on a boat and two other identical motors in various stages of disassembly. I figure, with all those parts I should be able to put together an "almost new" motor for under $1000.

Thanks to Laddies, Faztbullet and sschefer for their knowledgeable information and direction on this earlier thread;
http://forums.iboats.com/showthread.php?t=288700. Decided to start this new thread just for the rebuild.

My plan is to bore the block and buy new oversized pistons and rings. Will reuse existing crank and rods but will replace all bearings with new. Will rebuild carbs and fuel pump at the same time. Electrics have already been rewired. The L/U has complete new waterpump assy w/seals and shift shaft bushing w/seals. A new gasket set is the last thing I should need, yes?

Now a couple questions if I may.
1) Can't find listings for pistons. Both Wiseco and ProMarine show nothing older than 1978 model year. The bores are the same but not sure if they are compatible.
2) The crank looks in amazing condition. Is there anything that is typically done to the crank for a rebuild?
3) What is the difference between top guided and bottom guided rods?
4) Faztbullet described modifications to the bleed restrictors. What is the purpose of this feature and why perform the mod?

Here we go, wish me luck.

 

[LOW-LIFE]

Re: So I've Decided to rebuild 1971 Merc 800

with the crank its never a bad idea to have it polished by the machine shop when doing a rebuild, also let the machine shop take a look at the conecting rods to ensure they are straight and true. just cause the motor they came from may not of seemed to have a problem it doesnt meant it didnt exsist.

 

[Laddies]

Re: So I've Decided to rebuild 1971 Merc 800

You looking at 3 cyl piston when they call fot top or bottom guided rods and tou do not want a mirror finish on a 2 stroke crank

 

[rtpassini]

Re: So I've Decided to rebuild 1971 Merc 800

take a million pics. i have that same year motor too. if i need to redo mine...it will be nice to have you do all the hard work.

p.s....want to get rid of a starter from one of your other motors for cheap? mine is bad.

 

[James R]

Re: So I've Decided to rebuild 1971 Merc 800

Finding pistons is very often a problem for older motors. Try Sea-way.com. These guys have a handle on vintage parts.

 

[Laddies]

Re: So I've Decided to rebuild 1971 Merc 800

Sierra/ NAPA lists pistons

 

[rtpassini]

Re: So I've Decided to rebuild 1971 Merc 800

quick side question, whats the max rpm's for this motor?

 

[Faztbullet]

Re: So I've Decided to rebuild 1971 Merc 800

Glad your going for it!!!
(1) Merc inline's 75/80/85/90/115/140/150 from 1969 to around 1988 are all the same bore (2.875). The piston differences where:High dome/low dome/2 rings/3 rings/ported and non ported. The suppliers all stock ported pistons in place of non-ported pistons(manufacturing & inventory)The pistons you can use are the following low domes:
Promarine#2315 series (.015) or 2330(.030) (3 rings)
Serria# 18-4515 (.015) or 18-4530 (.030) (2 rings on .015)
Wiseco(last option) 3123p2 (.020) and 3123p3 (.030)
(2) The only thing I do to cranks is replace the main bearings/upper and lower bearings and check for cracks(upper throw area). I also remove the reed blocks and check the reed stop heights and the labyrinth grooves. If grooves are somewhat flattened and edges are not sharply defined they are worn and discarded.When these are replaced i subsitute with the 10/10 blocks as these seem have better idle and midrange. Also check the rods on a piece of thick glass, they should lay flat on both sides( I have never seen a bent/warped rod from normal use)
(3)Top guided rods fit snug into the piston(casting boss is smaller).It uses flat washers, and shorter needle bearings,rod is also thinner than the bottom guide, but wider.The top guide does not touch the crank on the sides, thus top guide.Bottom is opposite, with rod being thicker(beam), but narrower
(4)The way the transfer port is designed/cast on a I-line, fuel will puddle up where the resticter is drilled into block.This puddled fuel will cause idle and off idle problems. It is bled into the next cylinder to be burnt(see attached)