'71 Merc 800 "center main" bearing question

[Moody Blue]

Powerhead is taken apart and I'm in the process of checking all bearings.

The center main bearing race "appears" to be cracked in half. Is this normal? The two halves fit together perfectly BUT the separation joints are not square/straight as I would have expected. It appears as thought a one-piece race had been cracked into two halves. Is it normal?

Hope this makes sense.

Good idea to replace the needle bearings or re-use if in good shape?

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

Yes and there should be a clip to hold the two pieces together. They are cut independantly and only the set can be matched to go back where the came from but on any journal Be sure to use the same needles with the same race. Using a needle from another set would not have the same wear and cause damage over time. It is said if one is bad change them all. Check the interior of the race to see if there is any wear. The bad bering may have caused an indented section of one of the races and the whole race would at that point need replacement. According to the manual I read the hole faces towards the bottom of the engine where the lower seal is. I will confirm that it would be the clip as well to the lower end of the crankshaft when reassembling it in a later post as my hole is centered and the description called for the clip to be facing the lower end.This is probably due to engine tourque when throttling.Edit > In looking at the crankshaft section it is confirmed that the retaining clip should face the bottom of the crankshaft when reassembling it. Clip facing the wear sleeve.

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

No replacement is nessecary if they are pristeen silvery and not excessive blue in color or nicking on even one needle. Blueing indicates and overheating has occured and replacement would then be advised. Same with the connecting rods. If no sign of overheating or chafeering nicking is appearent then they can be used. If there are line marks indicating water intrusion. 320 emery cloth is an option for cleaning it up with a spining motion described in the manual. No more than nessecary to get the surface polished up though.

 

[Moody Blue]

Re: '71 Merc 800 "center main" bearing question

Cool, thanks. There is a circular retaining ring that held the two halves together. The needle bearings looked good but the race shows signs of wear and I'm going to replace the whole thing. I have a parts motor in storage that I'll check the race condition on.

Crankshaft bearing surfaces look pristine. Yeah!!

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

I have a few extra sets but need some polishing . I changed the set on mine as I (stupid me) didn,t check them when changing pistons rods etc. Knocking after warm up. I replaced them with another set I had and much better although I used the old 70 HP rods on two replacement pistons. I at present have a .015 piston rings and pin New old stock for the bad sleeve. I honned them but have minor pitting on the lowest cylinder runs great though) as wel as 3 replacement rods I got cheap from a 65 HP 35 bucks . Take your time do it right or be like me and soon you,ll know how to do it in your sleep.

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

Heres what im working with. I have two replacement 115 pistons with new rings. 1 set of connecting rod bearings in the motor. I am useing 2 old affected 70 HP rods and 1 80 HP 4 stroke rod. The rod is the same,but the piston is smaller so i,m not using it. It,s cheap if you need it and in my shed in good condition. This is what I have so far when I redo it (LOL Again) and have the rest of what I need View attachment 179399View attachment 179400View attachment 179401 Pictured are the 3 65 HP rods a brand new carb connector and I have 2 sets of the weisco needle bearings for the wristpins and need 1 more. I have Loctite 815 but no primer for the case yet. I,m on a fixed budget but will wait until I have everything before completing this engine. The needle bearings are the ones I sent you the link for in the other post. Be sure you get the right ones for the 800 model or inquire if you need them they are less than 8 bucks for the set. PS Thats the old stock .015 piston with rings and wristpin total to my door 42 dollars

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

Moody Blue Before you try to reassemble the crank back into the block. I remembered you said you had some slight skirt wear. Not sure which cylinder. I think it was the lowest one. Test the wrist pins and needles by slowly moving the rod extended straight out from side to side. There is a chance the needles may be worn or the wrist pin may be affected. There should be just a little play but not excessive. You can use the two other piston rods for a guide to see if it is more play on the piston with the skirt wear. I know how much I didn,t want to remove the piston in my 40 HP first time doing it, and never did in tht engine which actually turned out not to be nessecary anyway. The piston should slide back and out of the sleeve fairly easily. Inspect the cylinder walls for deep digs or excessive skirt wear. A quick honing can usually fix this issue. ( Rent a tool HONE from autozone) Use Hydrolic oil and soak the hone blades as well as the entire cylinder. then use a slow speed adjustable drill. (Best if reversable for cross hatching) and then slowly hone in and out a few times maybe 4 strokes. This should not be to much and cause you to need an oversized piston and rings. The problem lies in after the honing. Aluminum will be present in the cylinder. Remove the plug if you havn,t yet and then thoroughlly rinse the engine. I used a hose and then dryed it out real good so rust did not accumulate and then coated it with 2 stroke oil. (This is just in case you need to). The wash will send the debris into the transfer cover and the exhaust port so rinse real well. Take the piston and remove the wrist pin clips. I used a 7 /16th deep socket on an extenstion after removing the rings to tap out the wristpin. A press would be great if i had one. Do not leave the rings in as they can be damaged when taping the wrist pin.The needles will fall out as well as the spacers. If the 800 has them. There are 2 different types of rods that are used on engines and one will work with the other but the other will not work on a certain type. Use a non fiberous assembly greese (I used this) or even 2 stroke oil to hold the pins inside the rod, and then tap the wrist pin back into the piston checking to see all pins are holding in place and not binding as you go. It is recommended to use new clips. I didn,t but stretched them a tad as well as after inserting made sure the open ends where within the solid ring grove and not exposed. Put your rings back on the same way they came off OR Chrome side facing up. Some rings have a dot or letter indicating that side goes facing UP^ but not all. Re install the piston into the cylinder using a ring compressor or algin the ring to the ring lock pin on the side of the piston. Top pin > middle pin < bottom pin > and one at a time squeeze the ring to the lockpin and gently insert the piston lining up the rings to the lockpin as you go. Take your time until all are inside the piston, Work the piston all the way up and an check back and fourth to be sure there is no binding. Now you are ready to re asemble the carnkshaft. Removing the exhaust cover con be a night mare and should not be done unless absolutely nessecary. This could ruin your day ad open a whole new can of worms. Rinse well. it should clear it enough as not to cause damage. I have done this three times with No ill effects. Proper tool or proceedures are reccommended. Inside micrometer slide micrometer original cylinder bore read the cylinder find the Max honing measument without need oversized piston are all suggested. I,m suggesting what they call a de galzeing. The middle of the cylinder is where the most wea will be. De burring is reccomended after honing. The home might not even come close the he diamiter of the worst wear. in the description I have laid out. If you are not confortable ask question. Faztbullet is superior in rebuilding techniues and would probably shoot me for giving this info. You may just be fine to replace the wrist pin needles and move on. Check the wristpin for any type of wear. If any encountered replace the wristpin. LAST Suggestion Take the best piston and rod from the parts donor motor (Usually the top Piston) and relace it into the cylinder with skirt wear. and then slap it back together. It sholuld be fine.

 

[Moody Blue]

Re: '71 Merc 800 "center main" bearing question

Max thanks again for all your suggestions and tips. I really appreciate you taking the time. Hope to pull the parts motor parts out today to see what I have to work with, then order the needed parts.

I don't plan on a "complete" professional rebuild, just replace obvious broken and worn parts then slap it back together. When I look at the condition of the two broken main bearings I am amazed that the motor ran as well as it did. The only issue I had with it was stalling when going into gear and poor idle. I pulled skiers and wakeboarders for hours and hours with it without issue.

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

The stalling was more than likely due to crankcase pressure loss at the lower end cap seals. I hear what your saying. I was suprized as well to find severe water damage by water intrusion and have it run as far and as good as it did before I discovered the engine I bought was in that bad of shape. I agree the less money spent the better off you,ll be.

 

[Faztbullet]

Re: '71 Merc 800 "center main" bearing question

Inspect the cylinder walls for deep digs or excessive skirt wear.

You want to inspect for out of roundness as the inlines wear around the port area, if .004 out of round it really needs to bored...

A quick honing can usually fix this issue. ( Rent a tool HONE from autozone) Use Hydrolic oil and soak the hone blades as well as the entire cylinder. then use a slow speed adjustable drill. (Best if reversable for cross hatching)

If cylinder are worn honeing will not fix this and you never reverse the hone to cross hatch!!!!

I used a 7 /16th deep socket on an extenstion after removing the rings to tap out the wristpin. A press would be great if i had one.

You never press in a wrist pin with press (unless old OMC or Chrysler) you heat piston so it slides in.....

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

Ty for the tips. I t will definitely help alot

 

[Faztbullet]

Re: '71 Merc 800 "center main" bearing question

The only issue I had with it was stalling when going into gear and poor idle.

Two things can cause this....worn cylinders(around ports) and labyrinth seals worn on reed blocks(cylinders"talk")

 

[Moody Blue]

Re: '71 Merc 800 "center main" bearing question

Very good info Fazt. Is it possible to have good compression numbers (150psi) AND have cyls out of round?

 

[Faztbullet]

Re: '71 Merc 800 "center main" bearing question

Yes.... Engines can be worn out internally and still have good compression. Compression really tells you nothing except when a cylinder is low it a basic check to see if combustion will occur..

 

[Moody Blue]

Re: '71 Merc 800 "center main" bearing question

Only tools I have to measure internal diameter are telescopic bore gauges. Is that good enough to determine wear or is something more precise required?

I understand measuring the cyl at three levels and 90* apart.

 

[mercuk]

Re: '71 Merc 800 "center main" bearing question

Two things can cause this....worn cylinders(around ports) and labyrinth seals worn on reed blocks(cylinders"talk")

Sorry Faz, I'm not trying to undermine your knowledge here but, in my personal experience this statement just isn't true. I had the exact same symptons on my 4cyl Kiekhaefer block and the route cause of it came down to worn crankshaft seals. These are often overlooked and like any rubber compound, will go brittle and will no longer form an air tight seal. I have seen this several times on many older engines, you also tend to hear the crank bearings starting to get noisey when the seals are getting really bad.
Don't bite my head off on this one, its just my personal opinions/experiences.

 

[Maxz695]

Re: '71 Merc 800 "center main" bearing question

He is fully aware of the shot lower bearing eating the lower seal up and I made a suggetion for him to check the center mains that indeed did show wear. He is now checking bore and out of round as there seemed to be some slight skirt wear. He just wants to measure the circumference to insure honing or boring isn,t nessecary. He has a donor engine and just wants it to be as good as possible before re assembly. Your assumption of the seal is correct. It was shot as well and will be replaced.

 

[mercuk]

Re: '71 Merc 800 "center main" bearing question

Jolly good...

 

[Moody Blue]

Re: '71 Merc 800 "center main" bearing question

the route cause of it came down to worn crankshaft seals. These are often overlooked and like any rubber compound, will go brittle and will no longer form an air tight seal. I have seen this several times on many older engines, you also tend to hear the crank bearings starting to get noisey when the seals are getting really bad.

Thanks for the insight mercuk. Good to get a second opinion. Faztbullet make the same diagnosis back in the fall when I originally posted my symptoms. I was surprised to see his response here pointing to other possible causes.

Based on the fact I had water in the #4cyl and the bad idle performance I decided to dig into this motor and see what was up. I was surprised to see the extent of damage to the main bearings. The lower seals were completely shot. The bearings must have been making one hell of a noise but I can't say that I noticed. These old Mercs seem bullet proof.

 

[mercuk]

Re: '71 Merc 800 "center main" bearing question

Thanks for the insight mercuk. Good to get a second opinion. Faztbullet make the same diagnosis back in the fall when I originally posted my symptoms. I was surprised to see his response here pointing to other possible causes.

Based on the fact I had water in the #4cyl and the bad idle performance I decided to dig into this motor and see what was up. I was surprised to see the extent of damage to the main bearings. The lower seals were completely shot. The bearings must have been making one hell of a noise but I can't say that I noticed. These old Mercs seem bullet proof.

Yes they certainly are pretty tough old motors...
When mine was getting worse and worse, I bought it to several local marine guys and it cost me like nearly ?300 and they NEVER actaully accurately diagnosed it as failing crank seals.

Any way I carried on using it and the crankbearings when getting louder and louder until one day I thought, right, this isnt going to last much longer until it goes BANG so I plucked up the courage and tore into the motor.....

I replaced the crank seals and bearings and what a difference it made!!! Engine was smooth as anything and it instanly regained all its low end power and didn't stall when putting it into gear.. PLUS it made a big difference to the overall MPG as well.

Its not always going to be this straight forward, but from what you had describe on your thread it sounded very similar to the way mine was behaving.....

Anyway best of luck and just take your time over the rebuild, be methodical in your approach and keep your seperate needle bearings, con rod bolts etc etc for each cylinder and not to mix them up...

That is the best advice I could give anyone doing a powerhead rebuild, be carefully, keep your work surface as clean as possible and don't rush anything.....

Before you know it, she'll be purring like a kitten...... You just got to love these old kiekhaefer engines, nothing else quite like em'

Good luck!!