1971 Merc 800 rebuild / refurbish

[Moody Blue]

A bit of background info first.

Have begun the task of refurbishing my 1971 Merc 800 motor. Problem began with cronic poor idle performance and stalling when put into gear. I also found signs of water intrusion on #4 cyl - rusted plug and steam cleaned piston head.

After pulling the powerhead I discovered that the lower crank bearing was shot (ball retainer had completely been destroyed). After pulling the lower retainer cap I found the one seal badly worn. Next I pulled the top retainer cap and found that the upper crank bearing was also shot. The ball retainer came out in about 8 pieces. Proceeded to split the crankcase and do a further inspection. Removed the rod caps and center main bearing and found all crankshaft bearing journals in pristine condition. No signs of rust inside the crankcase.

The center bearing race had some minor wear marks and I posted an earlier thread on that issue here
http://forums.iboats.com/mercury-ma...-800-center-main-bearing-question-589483.html

Noticed some minor scoring on the #4 cyl walls so I pulled the #4 piston. There was significant pitting/errosion on the piston top and the upper ring was seized in its groove. It's not reusable. All other cyls look perfect.

The rod caps/bearings/races all look great.

My plan is to replace what NEEDS to be replaced, clean it up as best I can and put it back together. I know at this point I need new upper and lower crank bearings, new upper and lower crank seals and a lower crankshaft wear sleeve. I also plan on replacing the bleed restrictors that were missing in the intake transfer ports.

Here are my questions to start. I'm sure there will be many to follow.

1) Can I take a piston/rod/bearings from a donor motor to replace the damaged #4 piston or must they be NEW?

2) Can I swap the piston/rod/bearings from one cyl to another or MUST they stay in their original cyl?

3) What may have caused the damage to the #4 piston? I want to prevent that from happening again.

4) Is it acceptable/recommended to replace the bearings (rod cap, center main, wrist pin) or resuse them if in good condition?

Thanks for your time and looking forward to your replies.

 

[Faztbullet]

Re: 1971 Merc 800 rebuild / refurbish

1) Can I take a piston/rod/bearings from a donor motor to replace the damaged #4 piston or must they be NEW?

They need not be new but be in spec..no since in putting a worn out part in service..

2) Can I swap the piston/rod/bearings from one cyl to another or MUST they stay in their original cyl?

Yes you can swap........see #4

3) What may have caused the damage to the #4 piston? I want to prevent that from happening again.

Dont remember see a pic of piston but if ring was intact and piston dome eroded it was water intrusion or a possible trigger failing. Usually if trigger has a problem it will get cylindesr #1 or #3 but have see them take #4

4) Is it acceptable/recommended to replace the bearings (rod cap, center main, wrist pin) or resuse them if in good condition?

For what your doing use the center main race and bearing from your donor, bearings and races need to stay together as a set. Just order you new rod bearings as they are cheap and reuse the spacer as they can be swapped around with no problem. Wrist pin bearing need to stay with the piston/pin they where removed from, I would just take the best piston from donor and use it in #4 as a assembly with new rod bearings and rings..

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

Same on mine #3 lowest on my 70 HP. Exhaust baffle leaking into the exhaust port, and into the engine wipeing alot of things out. Piston corroded, and some pitting on the #3 cylinder wall. Rented an hone and used some hydrolic oil and honned it a bit to deglaze it. Replacement (USED) Piston pin and new rings installed. Much more done, but the same deal. Ran good, but I used the old 70 rods that had a slight bit of nitching, and only took it out once so far. I will as stated be adding new wrist pin needles new rod to crank needles 3 replacement rods in good condition. I have the .015 oversized piston in case I decide to have the bad cylinder bored out for it. I don,t think it is nessecary as I have a good holeshot, and great WOT RPMS and speed. I will be redoing it for peice of mind as I have seen 2 catostrophic failures posted this week and don,t want to be the third. Fix the exaust baffle, and cover gaskets, and if the surfaces are good, and the plates have no cracks then unless like faztbullet say the trigger was the reason, it should be ok after rebuild. Match the baffle to the block after cleaning up the gaskets to see if it has a good mating surface without a gap when rocking it. If all looks good new gaskets (No sealer) tourque slowly and accordingly in a centered to outward motion from center, Be careful as the outter cover cracks fairly easily. JMO P.S If you do decide to do a clean up honing on the #4 cylinder than leave the exhaust cover off until it,s done and cleaned out. Then install new gaskets. I don,t have the funds to bore and replace the sleeve but maybe Faztbullet can give you the prefered iboats customer discount and get the job done right for you.

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

They need not be new but be in spec..no since in putting a worn out part in service..

Yes you can swap........see #4

Dont remember see a pic of piston but if ring was intact and piston dome eroded it was water intrusion or a possible trigger failing. Usually if trigger has a problem it will get cylindesr #1 or #3 but have see them take #4

For what your doing use the center main race and bearing from your donor, bearings and races need to stay together as a set. Just order you new rod bearings as they are cheap and reuse the spacer as they can be swapped around with no problem. Wrist pin bearing need to stay with the piston/pin they where removed from, I would just take the best piston from donor and use it in #4 as a assembly with new rod bearings and rings..

Here are photos of #4 piston top. Rings are intact but top ring is seized in groove.



#4 Cylinder diameters as follows measured at three points;
top (sparkplug side) - 2.878 (parallel with crank) and 2.877 (90*)
middle (just below ports) - 2.874 (parallel with crank) and 2.875 (90*)
bottom (crankshaft side) - 2.872 (parallel with crank) and 2.875 (90*)

The scoring in cyl #4 is mostly at the bottom of the cyl (not spark plug side). The region where the rings ride looks pretty good. Cause for concern, or just hone the cyl?

Here are photos of the "best" center main bearing. Is it useable or should I replace?



What do you mean by "rod bearings"? Are they the cap or wrist pin bearings?

What spacer are you referring to? I've got this taken apart and haven't found any spacers.

I think you've already answered this but to be clear, is it OK to install all new rings on existing pistons? Honing cyls would be required?

I think I almost have a clear picture of what has to be done.

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

I want to go ahead and measure the pistons from the donor motor to pick the best one. What is the correct way to measure them, ie where to take the measurement?

From my manual, the limits are

SKIRT 2.865 - is this the minimum skirt diameter?
ABOVE RINGS 2.857 - is this the minimum piston diameter at the crown of the piston?
CYL HONE FINISH 2.875 (this one is obvious)

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

That piston is trash as long as the cylinder is in decent to pretty good condition it may be workable. I see three pics of the main bearing two of which indicate water marking but no real wear so it can be cleaned up. New rings on the donor piston is good your description of cylinder indicates good. Spacers are on the wirst pin that seat the needles in the wrist pins to pistons. If your using a donor rod and piston disregaurd the spacer as you are not replacing it, but be sure to check for play side to side to insure it The rod play side to side is as good as the original rods to wristpins on the engine being fixed. No honing nessecary if there is not much scuffing or corrosion pitting. If you can see Knurling on the piston no need to measure it if it,s from the same HP model and type engine View attachment 179878 Remember this is a brand new piston and it may not look this good but no excessive wear or grooves are good to use Minor may be ok. I know 70 thru 115 are the same I,m even using an 80 HP 4 stroke rod on my 70 2 stroke, but the 80 HP piston is a different size. You have a 2 stroke correct?

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

Usually the top piston and rod are the best as the fuel, oil, trash etc. settle to the bottom cylinder. Any indication of wear on the center main bearing race is no good. Hard to see the pic unless dead on. It would ony wear in a certain part of the race not the whole race. I found another set today (Spares "Trash" Wear on a part of the race). Pic #1 race looks good and Pic 3 looks good hard to see pic 2 but see some what appears to be wear but not sure from the angle. Skirt wear is determind by the cylinder to the skirt. that would be your lower neasurment as the rings never get that far down on the full intake stroke. the amount of space between the piston and the cylinder wear to conform with the ring expansion to hold a proper seal is the middle and upper measurments you took. As long as the piston is semi pristine and the grooves for the ring are undamaged it is usable

 

[buzzm19]

Re: 1971 Merc 800 rebuild / refurbish

MB, I guess now with all the help and to think you just wanted to replace the crank bearings and seals? It is a good thing to look at everything now while the motor is out. All those worn out parts would of put that motor in a scrap yard, you caught all those problems early. My 140 l6 started with what sounded as wining bearing, I hope you are as happy as I am. There's a good feeling when you find something before the self destruct happens. Buz

 

[Faztbullet]

Re: 1971 Merc 800 rebuild / refurbish

That bearing/race is shot do not use!!! and check the lower bearing balls for spalling as this will throw particles into #4 cylinder.
Merc spec's from manual for that year:
OD above piston land......2.857
OD at skirt.....................2.865
Cylinder finish hone........2.875

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

MB, I guess now with all the help and to think you just wanted to replace the crank bearings and seals? It is a good thing to look at everything now while the motor is out. All those worn out parts would of put that motor in a scrap yard, you caught all those problems early. My 140 l6 started with what sounded as wining bearing, I hope you are as happy as I am. There's a good feeling when you find something before the self destruct happens. Buz

Yeah, I'm glad I followed everyones advice here. Like most projects that I embark on, they end up more complicated than I imagined. Gives me something to do in these cold winter months, and I have the satisfaction of learning something new. I'm looking forward to getting it done and running properly again.

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

That bearing/race is shot do not use!!! and check the lower bearing balls for spalling as this will throw particles into #4 cylinder.
Merc spec's from manual for that year:
OD above piston land......2.857 (sorry, what is that?)
OD at skirt.....................2.865 (in-line with pin or 90* ?)
Cylinder finish hone........2.875

Thanks for the heads up. New race is ordered but I still need to find new bearings for it.

The lower crank bearing was totally shot (ball cage was GONE). Could that explain the scoring on the lower part of #4 cyl walls ?

Can you clarify above and how to correctly measure the piston diameters?

Do you think the piston damage was caused by water intrusion?

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

Most defimitly water damage causes the pitting and bearing failure. The lower seal let go as the shaft was turnung out of round and the lack of oil due to the water mixed in the fuel. (Hence bad bearing due to water intrusion). I wish I could say for sure the exhaust cover was not leaking and you wouldn,t have to chance breaking bolts to get it off as well as more money for gaskets. But it cannot be ruled out as well as the age of the gaskets that are on there they may be descintagrating. You can try rebuil without it but then you risk water intrusion still after putting in all the new stuff. Up to you. Good luck with it Moody Blues. Piston land between the top and second ring. Piston skirt That part of a piston below the piston pin bore. Cylinder hone finnish is the total amount that can be honed before needing over sized parts. If you give me the S/N I can try to locate the correct Center Main needles for you

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

Thanks Maxz695. I am into it this far and will do a complete teardown. The exhaust covers and waterjacket cover come off today. Everything else is already apart. Will be installing all new bearings, seals, gaskets etc. Don't want to leave anything to chance.

I've got a lead on all the parts, just working out the shipping details.

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

It is the best thing to do. Then you can go fishing instead of refixing the engine. Time and money well spent beats being unsure and conatanly wondering Should i have done that and never nowing until it,s too late. My powerhead is on th back porch. I replaced two more rods as the 80 HP Same numbers as the replacements is better than the marked #3 rod I got in the set off a 65 HP. Using the best of what I have. I,m still waiting on the last set of rod bearings hopefully here tonight. I,m good to reassemble and get out on the water.

 

[Faztbullet]

Re: 1971 Merc 800 rebuild / refurbish

The lower crank bearing was totally shot (ball cage was GONE). Could that explain the scoring on the lower part of #4 cyl walls ?

Yes......

Can you clarify above and how to correctly measure the piston diameters?

Remove top ring and clean piston so there is no carbon and mic piston at the ring land, this is the part of piston above the ring , do this every 90?. Do the same for the skirt of piston about a 1/4" up from bottom edge of piston....

Do you think the piston damage was caused by water intrusion?

Its had some water in it but the bearing race when it failed did the most damage. That is one of the purposes of the centrifugal slinger down on the driveshaft as its keep water from climbing driveshaft at low to midrange RPM.

Cylinder hone finnish is the total amount that can be honed before needing over sized parts.

Close Max but just what is says, the finished size of the cylinder after it been honed, anything over .003-.004 will need boring. That why when I bore a block I cut it .001 under size and use hone to bring it out to finished size. On the bearings google 4037-56 to see if any of these suppliers are close to you as they are $18 and you have to buy 200 if you buy the sierra and that $80

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

Thanks for the tip on the bearings. I'll add that to my "must get" list which keeps getting bigger and bigger.

Here is another question/dilema I'm faced with. The #4 piston of course is garbage and I have pistons from a donor motor I could use but the rod and cap bearing surfaces are no good. I would like to take the rod AND pin from #4 garbage piston and install it on a "good" piston from the donor motor. Will use new bearings all around. Any reason this won't work?

The other thing I am considering is to use a different block. I have another block from the donor motor that appears to be in much better condition (yet to be measured). The original block has scoring on the #4 cyl walls, while the donor block looks great. Could I transplant the crank, pistons, end caps, center main bearings etc into the donor block? As mentioned earlier, I will be installing all new bearings and rings. A sound plan, or one destined to fail?

 

[Maxz695]

Re: 1971 Merc 800 rebuild / refurbish

Yes if they are the same type motor and from the same time perios (Year) You could use the good donor block. As for the rod. I don,t know that the condition of the#4 rod would be that good as it more than likely was a dammaged by the water intrusion. How did the needles taken off of it look. Are there any other rods off the donor motor you can swap with the best piston? I just wouldn,t think i would trust that #4 rod. Mine bottom cylinder wore down significantly and tore up the crank journal. The needles in the wirst pins where half the original size as well as the rod bearings. I remember you saying your crank looked pristine so it might be OK. Make sure you check the wrist pin play. (Side to side) Hold the piston in one hand and wobble the rod to check for excessive wear if you intend to swap it with the donor piston.

 

[Faztbullet]

Re: 1971 Merc 800 rebuild / refurbish

Yes you can swap all the parts into your donor block and you can swap rods also, just keep the matching wrist pin with the piston. You will want to check the crankshaft endplay when you swap to the other block.

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

Thanks guys for the info. That opens up a few more options for me. Key is to keep the pin with the piston.

I've read thru the section on measuring end play and will check it for sure. I have a series of shims that came out of the donor motors. Should be able to set it up properly.

 

[Moody Blue]

Re: 1971 Merc 800 rebuild / refurbish

Well I finished taking the crank assy apart. The reed blocks were shot as the labrynth seals were BADLY mushroomed. I did find a pair from another motor that are in much better condition with just slight mushrooming on the one half. Problem is the reed petals.

My assumption is that the reed petals should be sitting flush against the reed block. The "good" reed block has the majority of the petals lifted off the surface. On the "bad" reed blocks, the petals are resting on or much closer to the block. I'm hoping I can take the good petals from the bad blocks and install them on the good block.

Turns out that the crank from the donor motor is in even better condition than the original crank. Won't need the lower seal sleeve as the seal surface is perfect.

Making progress one small step at a time.