Lower unit dissassembly 1971 Merc 800 (update with photos)

[Moody Blue]

Well so far so good. The hard part was easy and what I thought would be the easy part has me stumped

The lower unit on my 1971 Merc 800 had alot of metal shavings and chips in it when I drained it last fall.

My goal is two fold.
1) I need to find and repair the cause of the metal chips.
2) I want/need to replace the old carbon steel (corroded badly) driveshaft with a newer S/S type.

This is where I'm at. I have two complete lower units of the same vintage. One has the preload pin with angle teeth on the pinion and drive gears. The other does not have the preload pin and has straight teeth on the pinion and drive gears.

I have removed the carrier nuts and bearing carriers of both units without much trouble.

The damaged unit is the one without the preload pin (straight teeth). Turns out the reverse side of the clutch dog is badly worn as is the mating side of the reverse gear. Without a doubt this is where the metal is coming from.

The other unit looks in pristine condition. All edges of the clutch dog, drive gears and pinion gear look flawless to my untrained eye .

My plan is this. Simply replace the driveshaft in the "good" unit (preload pin with angled teeth) and bolt it up to the motor. Any issues with this approach?

Here is my problem. I can't get the old driveshaft out of the housing. I've removed the pinion gear nut, not an easy task. The driveshaft bearing is the non-tapered type.

What is my next step?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800

Re: Lower unit dissassembly 1971 Merc 800

I assume the pinion gear is still on the shaft, thats likely holding you back. Why not just use the good lower unit? Is the shaft on it badly corroded too? If not I would just use it. I know it is nice to have a SS one, but it took 40 years to rot the first one....I know where I will be in 40 years.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800

Re: Lower unit dissassembly 1971 Merc 800

Yes, pinion gear is still on the shaft.

Unfortunately the driveshafts in both lower units are pretty much shot. The area around the waterpump seals are pitted, grooved and rusted.

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800

Re: Lower unit dissassembly 1971 Merc 800

If you have the water pump housing off there is nothing else holding the shaft in except the pinion gear. They can be unbelievably difficult to remove. The good news is you are not trying to save the shaft, I would clamp the shaft in the vice and pry up on the housing, evenly from both sides to pop the pinion gear off. When you get your new shaft test fit the pinion gear on it before you try to assemble it. Any damage at all will prevent it from going on.

 

[Faztbullet]

Re: Lower unit dissassembly 1971 Merc 800

Re: Lower unit dissassembly 1971 Merc 800

Replacing the preload unit shaft is the way to go, the bad news is shafts will not interchange with each other as upper bearings are different designs and diameters. The non preload is a standard ball bearing held in place with clip and preload uses a cone bearing and race.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800

Re: Lower unit dissassembly 1971 Merc 800

Replacing the preload unit shaft is the way to go, the bad news is shafts will not interchange with each other as upper bearings are different designs and diameters. The non preload is a standard ball bearing held in place with clip and preload uses a cone bearing and race.

Hope I'm OK here. The replacement shaft is the pre-load pin type. The good unit with the old driveshaft also has the pre-load pin BUT has the one piece ball bearing. I'm hoping the shafts are interchangeable. I'm going to go out now and try to pound that pinion gear off the shaft so I can compare both driveshafts.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

I got the pinion gear off the driveshaft after some pretty enthusiastic pounding . I have a photo of the old and newer driveshaft for comparison.

I've also included a couple photos of the clutch dog and reverse gear.

As luck would have it, the newer driveshaft is an exact match to the old rusted one. I will be replacing the upper driveshaft bearing (found at my local auto supply store) and possibly the shift cam (see photo) but everything else looks very good.

WHat are your thoughts on this shift cam? Useable or replace it?

Will I be able to reuse the existing shim material and just reassemble the unit or do I need to set up the pinion depth and gear backlash after replacing the bearing?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

MB, I would not reuse the shift cam, its the reason the clutch dog wore the way it did, reverse was not fully engaged due to the wear on the cam. If you don,t have one I can shoot one in the mail to you, NC.

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Forgot to answer about the shim material, you can re use it no problem, however, there is no guarantee that it is the correct amount of shim. You have to put everything together and then measure the backlash then take it apart and shim accordingly. A pain I know. The condition of the gears will affect shim more than the replacement of the upper bearing. If you are using the original forward gear use the shims behind it that were there. If you are using the gears from the replacement unit use the shim that was behind that forward gear.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

MB, I would not reuse the shift cam, its the reason the clutch dog wore the way it did, reverse was not fully engaged due to the wear on the cam. If you don,t have one I can shoot one in the mail to you, NC.

Very good point, and one I completely overlooked.

I sent you a PM.

As for the backlash measurements, I'm not really clear on how that's done. I've read the procedure (a number of times) but not sure how to do the measurement. The spec for forward gear is 0.006-0.008 and reverse gear is 0.008-0.012. Where/how do you measure such small movements? Am I missing something?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Very good point, and one I completely overlooked.

I sent you a PM.

As for the backlash measurements, I'm not really clear on how that's done. I've read the procedure (a number of times) but not sure how to do the measurement. The spec for forward gear is 0.006-0.008 and reverse gear is 0.008-0.012. Where/how do you measure such small movements? Am I missing something?

Yup, its confusing. Try to think of reaching for a point in 3 dimensional space with your hand but the only inputs you are allowed is before you reach so you have to predict where you will land.

Since you have the standard ball bearing at the top, your measurements will be taken with a depth micrometer or vernier at the water pump base, or with a feeler gauge at the water pump base. The trick is that the gear mesh must be correct at the pinion gear/forward gear first. This is adjusted by the shims behind the forward gear and checked with the drive shaft pulled up. Machinists blue on the gear face will show you where the interface point is. It must be in the middle of the forward gear teeth, not the top or bottom. A good starting point for the shims behind the forward gear is .015 of shims. Blue the gear and rotate the drive shaft while pulling up on the shaft and check where the gears mesh. If it is too high on the forward gear you need more shim in behind it. If it is too low take some shim out.
Once you are happy with the gear mesh location you can clean everything up and re assemble. You know that with the drive shaft preloaded (pulled up) that the gears line up properly.
Now you shim the water pump base with the smaller diameter shims to get your (backlash clearance) The measurement takes into account the compression of the gasket when you install the wp base and torque to the proper spec.
The feeler gauge method is fine if you do not have a depth vernier or micrometer.

Let me know if this makes sense.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

I just can't get my head around the backlash measurement.

To be even more difficult, my stupid Seloc manual (yeah I know, but it came with the boat ) shows two different specs for reverse gear backlash. 0.008-0.012 AND 0.040-0.060. That's not just a typo

The unit I have has the preload pin on the driveshaft and tapered roller upper driveshaft bearing. The pinion and drive gears are helical (?) cut, not straight. I fully grasp the pinion/gear engagement height but not the backlash measurment.

Backlash from my understanding is rotary "slop" between meshed gears. In order to measure that, do you not have to measure rotary motion? If so, where is that measured taken, at the gear perimeter? Being that the gear is a round object, the measurement will differ depending on where you take the measurement. Measurements at the perimeter will be different than midway along the gear face for example. I must be over thinking this

One more thing. My "manual" says to use PERFECT SEAL when reassembling the unit. What is it and what can be used as a substitute?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Ok i probably screwed you up here, I thought you ended up with straight cut gears and a standard ball bearing under the pump housing. The procedure is completely different. Guaranteed to confuse.

Forget what I said about shimming the water pump base, you do not shim it with the tapered gears, the pinion gear pulls into the forward and reverse gears as it is turning, due to the taper. The preload pin keeps it engaged when not turning.
The specs that merc calls out for backlash are if you are using the special tool. The procedure shown in the seloc manual (and the merc manual) asks you to be subjective and determine if the backlash on the forward gear is acceptable using your fingers. After you have done a few you get the feel for it, however that does not help you out. I would suggest that you start off with a .015 shim behind the forward gear, complete your assembly and take your whole lower unit to a good merc shop that has the backlash measurement tool and have it checked. Based on the finding you can dis assemble the unit and add the correct shim to bring it into spec.

I will see if I can figure out an alternate way. I checked my lower end parts and I do not have a good shift cam, sorry bout that.

 

[rodbolt]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

back lash is measured with the little flag looking tool on the drive shaft and a dial indicator.
I also look at tooth contact pattern as its possible to have the lash correct but the tooth pattern incorrect.
after changing a drive shaft both fwd and rev lash and pattern needs to be carefully checked.
no guarantee that the new shaft was machined exactly like the old shaft,thats what the shims do.
remember on the lash,if you have to miss it go on the loose side,loose lives.

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

back lash is measured with the little flag looking tool on the drive shaft and a dial indicator.
I also look at tooth contact pattern as its possible to have the lash correct but the tooth pattern incorrect.
after changing a drive shaft both fwd and rev lash and pattern needs to be carefully checked.
no guarantee that the new shaft was machined exactly like the old shaft,thats what the shims do.
remember on the lash,if you have to miss it go on the loose side,loose lives.

I agree 100%, I was trying to think up a home brew way for him to measure it. The finite backlash amount will translate into degrees of rotation at the drive shaft, which can be measured with a degree wheel (lots of downloadable printouts on the net).

 

[rodbolt]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

that mark on the flag,best I remember is 1" from the shaft.
can be made with calipers,a hose clamp and a strip if faily stiff sheetmetal.

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

that mark on the flag,best I remember is 1" from the shaft.
can be made with calipers,a hose clamp and a strip if faily stiff sheetmetal.

Thats the detail we were missing, . Thanks Rodbolt. On a degree wheel that would translate to 2 degrees for a "safe" backlash of approx .017. to .018.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Thanks guys, that info helps alot. I've found a degree wheel on-line and I have an idea of how to use the dial indicator with a flag on the driveshaft. I've also found a reference that translates angular measurements to inches. The formula is

Backlash (degrees) = Arc Movement (inches) x 57.296 / Radius (inches)

Also found these two posts from another member here regarding backlash

Gary, if you are carefull and don't misplace any shims you do not have to shim a unit to change gears or bearings, they are all + or - .001 to .002 on any Mercury built lower unit, you are really shiming the case not the gears and bearing, most of the dealer talk about shimming is to scare people off. After 45 years at this you learn a couple of tricks, actually I had a freind that was a Mercury service instructer tell me that about 40 years ago and found it to be the truth in all but a couple of cases

As long as you are not changing cases if you replace all shims to the orginal positions the backlash will not change.

Do you happen to have the proper backlash specs ?

Ignoring for a moment the measurement of gear backlash, is it possible to get pretty close going just by "feel"? I'm assuming you want to get the mesh as close as possible without the gears binding. Is that correct?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Hey, MB, that formula should give you around 2 degrees for 17 to 18 thou of backlash. I just took the easy way since a 1 Inch diameter arc or circle circumference is Pi. 3.14159 etc. so converted that to degrees for you.
To be perfectly honest I set mine by feel. I agree that you will likely be OK if you just use the shims that were in the casing you are using.
We are way over thinking this. The important thing is that the gears do not bind and there is enough clearance for them to expand under load due to heat. The reality is that if your gear mesh is in the middle of the gear as shown by blueing or other method, and you can feel some slack or backlash with your fingers, you are good to go.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

OK so I've had a few minutes to think about this some more .

I attach a flag to the driveshaft. I can either printout a degree wheel OR use a dial indicator to measure the rotational movement, then translate that measurement (based on the radius of the measurement) to backlash. Yes?

Because this is an angular measurement, the radius of the measurement plays a role in the calculations. Now I'm lost again. Am I supposed to use the radius of the driven gear as the radius measurement to determine the backlash?

Arggghhhhhh. I'm really making this more complicated than it should be I'm sure.